seismological society of america annual meeting fieldtrip ...community fault model (cfm). san...

Seismological Society of America Annual Meeting Fieldtrip 2015 Faults, vistas, and earthquakes in the northern Los Angeles and Pasadena Region

April 24, 2015 Kate Scharer (USGS), James Dolan (USC), Jerry Treiman

and Janis Hernandez (CGS) and Doug Yule (CSUN)

Route (black line), stops (black stars), on map adapted from Geologic Map of Los Angeles Quad (Yerkes and Campbell, 2005). Main faults discussed on the trip (red lines) are generalized. Accurate information on fault location and activity can be found on the CGS and USGS websites. CGS: http://www.consrv.ca.gov/cgs/rghm/ap/Pages/main.aspx USGS: http://earthquake.usgs.gov/hazards/qfaults/

2

THE PLAN 8:00-8:15 Load and depart! 9:00-10:30 Griffith Observatory (Dolan) 10:30-12:00 Hollywood Blvd/Argyle St (Treiman, Hernandez) 12:00-1:15 Wattles Park for lunch (Dolan) 1:15-1:45 Autry Overlook 2:15-4:00 Northridge Recreation Center and CSU Northridge (Yule) 4:00-5:00 Loma Alta Park (Scharer, Dolan) START: Pasadena Convention Center Route travels west out of Pasadena on Highway 134. Accessing the highway, the bus will cross Arroyo Seco, a drainage that forms the western border of Pasadena (the Rose Bowl is about a mile to our north). The highway cuts the southern tip of the Verdugo Mountains, along the Verdugo-Eagle Rock Fault system, a northeast-dipping, northwest-trending fault with estimated < 1 mm/yr slip rate. No neotectonic studies with slip rates exist for Verdugo Fault, but Weber et al., (1980) report south facing scarps in Holocene alluvium, and deformation models by Cooke and Marshall (2006) predict it is oblique, with ~0.6 mm/yr right-lateral and 0.5 mm/yr dip slip rates. The Verdugo Mountains are largely Cretaceous gneissic quartz diorite; low terrain to the south is Miocene Topanga Group conglomerates and siltstones (Yerkes and Campbell, 2005). Head south on Interstate 5. Here Interstate 5 parallels the Los Angeles River for about 4 km on the western edge of the Santa Monica Mountains; at this location they are known as the Hollywood Hills. Rocks immediately west of I-5 are Miocene Monterey Formation shale; these overlie the granodiorite basement rocks exposed to west at Griffith Observatory (Dibblee and Ehrenspeck, 1991).

This low oblique Google Earth view looks westward along Los Feliz Blvd. in our direction of travel. An aerial photo of the area from 1927 is draped over the landscape (vertical exaggeration x2). Our route follows this eroded and breached fault-line trough along which remnants of the old topography are still visible. The magenta lines indicate interpreted fault traces. (1927 photo by Fairchild Aerial Surveys).

3

Composite map of the Burbank (1926) and Glendale (1928) 6-minute topographic maps that show the Los Feliz Blvd. portion of our route. This map series nicely depicts the topography with 5-foot contours.

STOP 1: GRIFFITH OBSERVATORY Our first stop is at the famed Griffith Park Observatory, notable for the nice view of the Hollywood sign atop the westernmost end of the Elysian Park anticlinorium (hanging wall fold above Elysian Park blind thrust fault), and for providing the backdrop for numerous movies, including perhaps most notably Rebel Without a Cause with James Dean (sorry, Charlie's Angels: Full Throttle!). The observatory is an ideal spot to introduce and discuss LA's urban fault network, as it provides expansive views of many of these structures.

4

At this stop we will discuss what is known about the major fault systems that underlie metropolitan Los Angeles, with a particular focus on the Sierra Madre-Cucamonga fault system, the Raymond-Hollywood-Santa Monica fault system, and the Puente Hills and Compton blind thrust faults, the two largest active blind thrust systems that underlie most of the area you can see from Griffith Park.

Map of major faults in southern California from the Southern California Earthquake Center’s Community Fault Model (CFM). San Andreas fault (red) and Puente Hills and Compton blind thrust faults (orange) are highlighted. Thin red liens show surface traces of faults in the CFM. White lines denote coastline and California State boundaries. Image created by Andreas Plesch and John Shaw (Harvard University). As we look southward over what is known as the LA basin, in addition to noticing the density of development, including the downtown LA high-rise district (which keeps rising higher every year), it is important to note that the "LA basin" isn't just a geographic term used by traffic reporters. Rather, the Los Angeles basin is also a deep (10 km at its deepest point), narrow, 50-km-long sedimentary trough that extends northwest-southeast beneath much of the urban core of LA. You can't see it because it has been filled with sediments eroded off the San Gabriel Mountains and deposited on the floodplains of the Los Angeles and San Gabriel Rivers. The LA basin, which has been the subject of extensive modeling efforts to understand the seismic hazard ramifications, will trap and amplify seismic waves during future earthquakes. Of particular importance in this regard is the proximity of Puente Hills and Compton blind thrust faults to the basin. Indeed, the location and geometry of the PHT render it perhaps a "worst-case-scenario" fault for metropolitan LA, as any upward rupture directivity will funnel energy directly into the downtown region and the deep sedimentary basin.

5

Map of major blind thrust faults in the Los Angeles basin. Note segmented nature of Puente Hills blind thrust fault (orange) and Compton blind thrust ramp (yellow) extending beneath much of much of the western LA metropolitan area. Both blind thrust ramps dip gently northward (Shaw et al., 2002; Leon et al., 2009).

3D cutaway image showing geometry of the central, Santa Fe Springs segment of the Puente Hills blind thrust fault through the site of the 1987 Mw 6.0 Whittier Narrows focus. Geometry of the uppermost 7 km is constrained by high-quality petroleum-industry seismic reflection data, whereas the deeper fault geometry is constrained by the location of the 1987 mainshock focus and relocated aftershocks. Data from Shaw and Shearer (1999) and Shaw et al. (2002).

6

Northeastward-looking oblique view of depth to basement surface highlighting the presence of the deep Los Angeles basin (darker colors). Orange surfaces are the three main structural segments of the Puente Hills blind thrust fault. Blue surface to west of PHT is the CFM representation of the Newport-Inglewood fault, and the blue surface to the east is the CFM representation of the Whittier fault extending along the eastern end of the PHT. Red lines show surface traces of SCEC CFM faults, and the white line denotes coastline. Image created by Andreas Plesch and John Shaw (Harvard University). NEXT: STOP 2: HOLLYWOOD BLVD/ARGYLE STOP (PANTAGES THEATRE) Exit Griffith Observatory, south onto Vermont Ave, right onto Los Feliz Boulevard, left onto Western Ave, then right at Hollywood Boulevard. From Griffith Park we will drop down the southern side of the Hollywood Hills, and drive along Hollywood Boulevard. The saddle in the hill occupied by the 101 Freeway exposes marine clastic rocks of the Miocene Upper Topanga Formation.

7

This vintage oblique aerial photo from 1938 is looking northeast across an elevated alluvial fan, north of Los Feliz Blvd. There are at least two fault scarps visible across this fan (yellow annotations). The area was developed by mass grading in 1962, before faulting was well understood or considered active in this area. (photo by Fairchild Aerial Surveys).

The south-flowing drainage at Fern Dell Drive appears to have been offset at least 130 meters in a left-lateral sense (locality marked S4e in this figure) with a younger fan starting to develop where Fern Dell crosses Los Feliz. (Figure modified from Figure 14 of Hernandez & Treiman, 2014).

Seismological Society of America Field Trip Stop – Hollywood/Argyle

April 24, 2015

Fault Rupture Hazard Hollywood Fault Zone, Hollywood, California

Jerry Treiman, Senior Engineering Geologist Janis Hernandez, Engineering Geologist

California Geological Survey This stop will provide an overview of fault rupture hazard recognition in the Hollywood area under the Alquist-Priolo Earthquake Fault Zoning Act (the "Act"). The Act was passed in 1972 as a direct result of surface rupture damage to buildings in the 1971 M 6.6 San Fernando Earthquake, damage that led to the recognition that surface fault rupture is a readily avoidable hazard. The intent of the Act is to reduce the risk from surface fault rupture by prohibiting the location of developments and structures for human occupancy across the trace of active faults. This goal is achieved through a division of responsibilities between the State, local government and landowners/developers: • It is the responsibility of the State Geologist to issue maps depicting zones of required investigation

for the hazard of surface fault rupture. These Earthquake Fault Zones (EFZ) are typically 1000 feet wide or greater and are established to encompass identified active faults and an area around those faults where secondary fault traces might lie.

• It is the responsibility of the local permitting agency (city or county) to require and approve a geologic report defining and delineating any hazard of surface fault rupture for specified projects within the EFZ.

• It is the responsibility of the property owner or developer to cause the property to be investigated by a licensed geologist who will identify the location of any active fault traces so that they may be avoided.

For more information on the Alquist-Priolo Earthquake Fault Zoning Act refer to Bryant and Hart (2007). For our zoning evaluation of the Hollywood Fault within the Hollywood Quadrangle we made use of all available data. This included analysis of vintage aerial photography and topographic maps as well as the compilation and reference to all available geotechnical studies, subsurface data and mapping along the fault zone. Vintage aerial photographs and maps are a tremendous resource for interpreting the tectonic geomorphology along a fault zone, which is often a key to identifying active fault strands. In areas such as Hollywood this can be a particular challenge because most of the area was already developed, with a grid of streets and structures obscuring the original landform, before the first stereo aerial imagery in 1927. Geologic mapping over the decades had identified some (probably inactive) faults within uplifted bedrock areas, but the more recently active fault traces are mapped as concealed near the interface between the uplifted Santa Monica Mountains and the alluvial fans to the south (Hoots, 1930; Dibblee, 1991). Dolan and others (1997) made great strides in appreciating the tectonic geomorphology of the fault zone.

Text Box

8

Fault expression and bedrock geology in the Hollywood area. (Modified from Dolan and others, 2000, Fig.2) Within the western part of the study area we had access to a number of detailed site studies to help constrain age of faulting as well as fault location. These data tended to corroborate geomorphic assessments, but with some minor adjustments needed. In contrast, the eastern part of the study area

contained relatively few site-specific studies, but perhaps a more telling geomorphic signature. We were fortunate to have a remarkable topographic survey of the area conducted in 1924 & 1925 that captured the landform with 5-foot contours (USGS, 1926 & 1928). Based on these maps, 1927 and 1928 vintage vertical aerial photography and some historic oblique images from the 1920s and 1930s, we were able to interpret a relatively continuous zone of faults across this eastern area.

Geomorphic features in the Hollywood area with faults that are basis for APEFZ. Local anticlinal fold (green) based on geotechnical studies. Capitol Records Tower indicated by yellow dot. This field trip stop indicated by yellow star. See previous figure for explanation of symbols.

Text Box

9

North-south section along Cahuenga Blvd. showing subsurface interpretation of faults based on studies for Los Angeles subway (Crook and Proctor, 1992). Northern and southern strands are indicated on previous figures. Fault location is only one factor of concern, the other being recency of the latest fault displacement. Several studies in the western part of the map demonstrated that the Hollywood Fault had experienced displacement during the Holocene.

Fault influences on drainage and deposition patterns in the eastern part of the map appear to also be in accord with relatively young (although currently unquantified) fault activity. Holocene displacement does not need to be documented on all elements of a fault zone for those traces to be included in a zone of required investigation. Our evaluation of regional and detailed data indicated that the Hollywood Fault is active and comprises a sometimes complex zone of faults that extends in a generally east-west direction across the Hollywood area (California Geological Survey, 2014). Our findings and conclusions were summarized in a final Fault Evaluation Report after public review of our preliminary recommendations (Hernandez and Treiman, 2014; Hernandez, 2014). References

Bryant, W.A., and Hart, E.W., 2007, Fault-Rupture Hazard Zones in California, Alquist-Priolo Earthquake Fault Zoning Act with Index to Earthquake Fault Zones Maps: California Geological Survey. Special Publication 42, 42 p. (digital version only, electronic document available at ftp://ftp.consrv.ca.gov/pub/dmg/pubs/sp/Sp42.pdf ).

California Geological Survey, 2014, Earthquake Zones of Required Investigation, Hollywood 7.5 Minute Quadrangle: California Geological Survey, 1:24,000.

Crook, R., Jr., and Proctor, R.J., 1992, The Santa Monica and Hollywood faults and the southern boundary of the Transverse Ranges Province, in Pipkin, B.W. and Proctor, R.J., eds., Engineering Geology Practice in Southern California: Association of Engineering Geologists, Southern California Section, Special Publication No.4, p.233-246.

Dibblee Jr., T.W., 1991, Geologic map of the Hollywood and Burbank (south ½) Quadrangles, Los Angeles County, California: Dibblee Geological Foundation, Map DF-30, 1:24,000.

Dolan, J.F., Sieh, K., Rockwell, T.K., Guptill, P. and Miller, G., 1997, Active tectonics, paleoseismology, and seismic hazards of the Hollywood fault, northern Los Angeles basin, California: Geological Society of America Bulletin, v.109, p.1595-1616.

Dolan, J.F., Stevens, D., and Rockwell, T.K., 2000, Paleoseismologic evidence for an early to mid-Holocene age of the most recent surface rupture on the Hollywood Fault, Los Angeles, California: Bulletin of the Seismological Society of America, v.90, p.334-344.

Hernandez, J.L., 2014, The Hollywood Fault in the Hollywood 7.5’ quadrangle, Los Angeles County, California: California Geological Survey Fault Evaluation Report FER-253, Supplement No.1, November 5, 2014, 35p.

Hernandez, J.L., and Treiman, J.A., 2014, The Hollywood Fault in the Hollywood 7.5’ quadrangle, Los Angeles County, California: California Geological Survey, Fault Evaluation Report FER-253, February 14, 2014, 35p.

Hoots, H.W., 1930, Geology of the eastern part of the Santa Monica Mountains, Los Angeles County: U.S. Geological Survey Professional Paper 165, p. 83 -134, map scale 1:24,000.

USGS, 1926, Burbank, Calif.: United States Geological Survey, 6-minute topographic map series for Los Angeles County, surveyed 1924, 1:24,000.

USGS, 1928, Glendale, Calif.: United States Geological Survey, 6-minute topographic map series for Los Angeles County, surveyed 1925, 1:24,000.

Text Box

10

Strip

map

show

ing

feat

ures

rela

ted

to fa

ult i

nter

pret

atio

n in

the

Holly

woo

d, C

A Q

uadr

angl

e. Y

ello

w st

ar in

dica

tes t

his f

ield

trip

stop

loca

tion.

(ba

se m

ap is

fr

om 1

923-

1925

topo

grap

hic

surv

ey)

This

map

show

s the

inte

rpre

ted

and

loca

lly v

erifi

ed a

ctiv

e tr

aces

of t

he H

olly

woo

d Fa

ult Z

one

acro

ss th

e Ho

llyw

ood

7.5’

qua

dran

gle

(Her

nand

ez a

nd

Trei

man

, 201

4; H

erna

ndez

, 201

4).

The

yello

w sh

aded

zone

surr

ound

ing

the

faul

ts is

the

Alqu

ist-P

riolo

Ear

thqu

ake

Faul

t Zon

e fo

r req

uire

d in

vest

igat

ion

(Cal

iforn

ia G

eolo

gica

l Sur

vey,

201

4).

Text Box

11

12

STOP 3: LUNCH AT WATTLES MANSION / RUNYON CANYON PARK From Pantages, drive west on Hollywood Boulevard, and then turn north onto Curson Ave. Our lunch stop takes us back into the Santa Monica Mountains, here a quartz diorite crystalline basement rocks that intruded the Santa Monica Slate exposed to our west. At this stop we will discuss what is known about the earthquake history of the Hollywood fault and its extensions to the west (Santa Monica fault) and East (Raymond fault). This section of the Hollywood fault was the subject of intensive study during the mid-1990’s as part of the investigation of the subsurface conditions for the LA subway Red Line extension through the Santa Monica Mountains to the San Fernando Valley.

Map of Hollywood fault and environs (Dolan et al., 1997). Scarps associated with recently active traces of the Hollywood fault shown in reddish-orange. Young alluvial fans (bright colors) are actively being deposited along much of the mountain front (or at least they were being deposited before everything got paved over). Note ~1.5-km-wide left step at west end of the Hollywood fault onto its continuation farther west – the Santa Monica fault. Also note zone of uplifted older alluvium south of the active alluvial front along the Hollywood fault. This uplift is a surface manifestation of uplift above the western, Los Angeles segment of the PHT.

13

Geologic map of young features within detailed study area west of downtown Hollywood. Runyon Canyon, Vista Street, and Outpost Drive fans are shown in shades of gray. Narrow, dark gray horizontal swath shows location of Hollywood fault inferred from subsurface data. Medium gray shading shows fault scarps inferred from topography. No scarps are discernible across the recently active parts of the fans. Thick black north-south lines show locations of trenches and borehole transects discussed in text. Secondary strand of Hollywood fault encountered in Fuller Avenue trench is shown by short black line immediately south of borehole OW-34A. Location of Metropolitan Transit Authority subway tunnel excavated as of July 1995 is shown as a dashed line. Triangular facets in northeast corner of figure show possible northeast-trending fault strand. CP-MT—Camino Palmero–Martel Avenue Transect; HA— Hillside Avenue; NLBT—North La Brea transect; WP—Wattles Park; Q shows location of near-surface (<1 m depth) quartz diorite from Crook and Proctor (1992). Topography redrafted from Burbank and Hollywood 1:24 000 6¢ USGS quadrangles (~1926). Contour interval is 1.5 m (5 ft) up to the 500 ft contour, above which the interval is 7.6 m (25 ft). Dolan et al. (1997) detailed the results of two north-south (i.e., fault-perpendicular) transects of continuously cored hollow-stem boreholes. Twenty-five boreholes comprised the longer, western transect, which extended southward from the mountain front along Camino Palmero and Martell venue for 525 m. Correlation of numerous laterally continuous soils showed that the recently active trace of the Hollywood fault is located just south of the southernmost surface exposures of the quartz diorite that makes up this part of the Santa Monica Mountains.

14

Cross section of the northern half of Camino Palmero–Martel Avenue borehole transect. Thick vertical lines denote continuously cored boreholes; thinner lines show sections of B-8 that were not cored. Sub-horizontal black lines denote A and Bt horizons of buried soil horizons of unit C. White zones between these buried soils denote C horizons of buried soils and unaltered sedimentary strata that do not exhibit any soil development. Small triangles and gray lines denote ground-water level in boreholes during 1992. Open circles show locations of two accelerator mass spectrometry–dated detrital charcoal samples discussed in text. Locations of boreholes B-17 and B-16 are projected due east to the line of the cross section. Because of uncertainty of the fault strike, only water-level data from these two boreholes are shown in the figure; stratigraphic data from these holes are not shown in the figure. Subsequent investigations of the main Hollywood fault crossing by Dolan et al. (2000b) using adjacent large-diameter “bucket-auger” boreholes revealed the details of the alluvial stratigraphy and allowed direct examination of the most recent surface rupture at the site. Bulk-soil radiocarbon ages from both unfaulted and faulted strata demonstrate that the MRE occurred between ~6 ka and 11 ka, with a preferred age range of 7 ka to 9.5 ka.

15

Cross section of large-diameter borehole transect discussed in this article. Calibrated 14C ages are all from bulk-soil samples except the two shallowest samples, which are from detrital charcoal fragments. Note the clear fault displacements of the Unit 3/Unit 4 contact in CP-D, CP-C, and CP-F, as well as the lack of displacement of the Unit 2/Unit 3 contact. Also note the 1.3-m-tall, north-side-down step in the Unit 4/Unit 5 contact between CP-G and CP-A. The inferred fault (dashed) shown at this step is drawn at the same dip as that of fault strands observed in CP-D (see text for discussion). The dip of the Unit 4/Unit 5 contacts in CP-C and CP-A was not mapped in detail due to very wet conditions. The average depth of these contacts, however, is accurate to within a few cm. These contacts are shown as dipping the same as the average dip of the Unit 4/Unit 5 contact between CP-H and CP-C. Zigzag lines denote numerous closely spaced fractures not logged in detail due locally unsafe hole conditions. Wavy horizontal lines denote the shallowest groundwater encountered in the boreholes; no groundwater was encountered in boreholes CP-D, CP-G, and CP-I. The irregular dotted line at 5 m depth between CP-G and CP-A shows the area that was hand-excavated from borehole CP-G in order to confirm that the Unit 2/Unit 3 contact was not faulted between the two boreholes. Small-diameter boreholes B-10 and B-12 are from Dolan and others (1997). Only the upper 15 m of these boreholes is shown. They extend to total depths of 73.2 m and 29.4 m, respectively.

16

STOP 4: AUTRY OVERLOOK ON MULLHOLAND DRIVE Drive west on Hollywood Boulevard to Laurel Canyon, head north. Near the top turn west onto Mulholland Drive. Bus will cross up and over the Santa Monica Mountains, stopping on Mulholland Drive to overlook the San Fernando Valley (like, totally!) and a view of the mountain ranges (and their faults) to the north.

https://www.flickr.com/photos/cityprojectca/4480269277/ From this the overlook you can see the broad expanse of the San Fernando Valley. The Simi Hills and Santa Susana Mts form the western and northwestern margins of the valley, respectively, with Oat Mt forming the high point to the northwest (1142 m). The low saddle to the east of Oat Mt (Santa Susana Pass) separates the Santa Susana Mts (west) from the western San Gabriel Mts (east) capped by San Gabriel Pk as the high point (1878 m). Overpasses connecting the Golden State and Antelope Valley freeways (I-5 and Hwy 14) in Santa Susana Pass partially collapsed during both the 1971 and 1994 earthquakes. The Verdugo Mountains (where we started today) are the nearest range to the northeast, in front of the San Gabriel Mts. The north-dipping Sierra Madre Fault System is the longest fault seen from this vantage, including the Santa Susana Section within the Santa Susana Mountains, the San Fernando Section in the middle, and the Sierra Madre section under the San Gabriel Mountains. Splays of this fault system also include the north-dipping Mission Hills fault zone and Northridge Hills (blind) thrust, the subject of our next stop. The 1971 Sylmar earthquake (Mw6.7) ruptured about 15 km of the San Fernando Fault. The 1994 Northridge earthquake (Mw6.7) occurred on a south-dipping blind thrust that projects beneath and terminates against the overriding Northridge Hills fault in the northwest part of the valley.

17

Perspective view of terrain from Hollywood Hills to the Garlock Fault, from Q-fault database (http://earthquake.usgs.gov/hazards/qfaults/). Note that the unnamed blind fault responsible for the 1994 Northridge earthquake is not the Northridge Hills Fault! (The 1994 rupture plane terminates against the overriding Northridge Hills fault) SMF=Sierra Madre Fault System. STOP 5: NORTHRIDGE RECREATION CENTER Drive west along Mulholland Drive, turn onto Coldwater Canyon Ave headed north, and get on the 101 freeway headed west. Then take I-405 N about 8 miles to Nordhoff Street exit, drive west 0.7 miles to Woodley Avenue, turn right. Drive 1.0 mile north on Woodley to Lassen Street, turn left. Drive 2.9 miles west to Reseda Boulevard, turn right. Drive north on Reseda 0.4 miles to Lemarsh Street, turn right. Drive 0.2 miles east on Lemarsh to the Northridge Recreation Center and park in the parking lot to the east of the center. Entrances to public restrooms are located at the west side of the building. En route to this site we crossed the Northridge Hills fault while heading north on Woodley Avenue (just east of Bull Canyon wash on Figure 1, below). Here, the structure forms an impressive south-facing topographic escarpment. This hill is part of a series of aligned, WNW-trending hills that form the north ridge of the San Fernando Valley, hence the name of the city, the university, and the 1994 earthquake! It is important to note that the Northridge Hills fault does not reach the earth’s surface. Rather, the Northridge hills are the expression of a fault-propagation fold above the tip of the blind Northridge Hills thrust, located ~1.5 km below this location (see cross-section E-E’ below, from Figure in Tsutsumi and Yeats, 1999). The purpose of the stop at Lemarsh Street is to visit the Northridge Park site of Baldwin et al. (2000) (Figure 1). Walk southeast from the parking area along a dirt road until you are standing in beneath LA Department of Water and Power’s high-tension power lines. The power lines follow Aliso Canyon where young alluvium fills a late Pleistocene incision into Plio-Pleistocene Saugus Formation (Figure 4). Baldwin et al. (2000) chose to conduct their study at this site because it represents the only place in the Northridge Hills that has not seen significant cultural modification. Here they excavated six test pits, nine boreholes, and one trench (Figure 5). The excavations provided no evidence of surface faulting but do

18

show Holocene growth strata with 13 ± 2 m of vertical relief across a monocline, and a dip-slip rate for the Northridge Hills fault of 1.0 ± 0.7 mm/yr (Figure 6). This study therefore provides direct evidence that the structure deforms Holocene strata, confirming previous interpretations of the faults activity (e.g., Barnhart and Slosson, 1973; Saul, 1979; Dibblee, 1992; Johnson et al., 1996; Hitchock and Kelson, 1996).

Figure 7 from Tsutsumi and Yeats, 1999. [Note: see Figure 1 of this field guide (Baldwin et al., 2000) for location of cross section line]. Figures 1, 4, 5, and 6 below are from Baldwin et al. (2000).

20

STOP 6: CALIFORNIA STATE UNIVERSITY NORTHRIDGE, PARKING LOT F10 Return to the bus/vehicles. Drive 0.2 miles east on Lemarsh Street to Lindley Avenue, turn right. Drive 0.2 miles south and turn left into the northwest entrance to CSUN parking lot F10. Park at base of hill adjacent to dirt path. Walk up the path to reach the top of the hill. Note that the hill that you climbed has been modified extensively by grading. Resting on top of this hill are ‘relics’ of damage to CSUN’s campus during the 1994 earthquake. The relics are damaged support columns from the Oviatt Library. Rumor has it that the University has stored these ‘ruins’ with plans to use them one day to construct an earthquake memorial on campus. From this vantage point one can see the five mountain ranges that ring the San Fernando Valley (clockwise from north, the San Gabriel, Verdugo, Santa Monica (Autry overlook stop), Simi Hills/Chatsworth and Santa Susana ranges). The saddle at the eastern end of the Santa Monica Mts is Cahuenga Pass, traversed by the Hollywood Freeway (Hwy 101). In 2005-6 California State University Northridge (CSUN) hired William Lettis and Associates, Inc. to excavate a series of trenches across this parking lot and hill. The trenches total length exceeded 300 m! At the time CSUN was planning to develop the hill with affordable housing units for faculty and staff. However, the economic downturn of 2007-2009 put a halt to these plans. If you look carefully you can still see the patched areas of the parking lot where the trenches were cut. The dirt path that you followed to the hilltop is a scar from one of these trenches. The trenches exposed minor faulting with <10 cm of offset per fault zone, suggesting the site does not experience large ground ruptures during earthquakes (John Baldwin, oral communication, 2006). The main purpose of this stop is to discuss the tectonic setting of the 1971 Sylmar and 1994 Northridge earthquakes. The epicenters of these earthquakes are 22 km and 4 km to the north of this stop, respectively. The Northridge Hills overlie the north-dipping 1971 Sylmar and south-dipping 1994 Northridge ruptures. Aftershocks define the ruptures in cross section (Figure 7) and map view (Figure 8).

21

The 1971 earthquake ruptured the San Fernando section of the fault (see figure above). Southwestern splays of the San Fernando section, including the Mission Hills fault zone and the Northridge Hills fault (this stop), did not slip during this earthquake. However, small surface breaks did occur on the north-limb of the Sylmar syncline (Figure 7) interpreted to accommodate bedding-parallel flexural slip (Tsutsumi and Yeats, 1999). The 1994 earthquake ruptured an un-named south-dipping fault that intersects the Northridge Hills thrust at 5-7 km depth beneath the Sylmar basin (~10 km to the north of this stop). Uplift at this location of ~0.25 m following the 1994 earthquake (Johnson et al., 1996) can be explained by slip on the un-named blind fault triggering slip on the overriding Northridge Hills fault. Opposite-verging thrust systems like this one are common in the region. For example, the blind Northridge/un-named thrust system here is very similar in geometry and style to the opposite-verging Oak Ridge/San Cayetano thrust system 35 km to the northwest of this location, just 7 km deeper.

Cross section E-E’ from Figure 4, Tsutsumi and Yeats, 1999.

22

Figure 8 from Baldwin et al., 2000. STOP 7: LOMA ALTA PARK Return to Reseda Boulevard and take Hwy 118 E to I-210 E. Take Arroyo Boulevard Exit, and turn north on Windsor Ave, East on Woodbury Road, north on Lincoln Ave and East on Loma Alta Dr. The parking lot is just to north on Sunset Ridge Rd. This part of the tour takes us between the San Gabriel Mountains (to our north) and the Verdugo Mountains (to our south). The purpose of this stop is to provide a quick overview and description of the Sierra Madre Fault system. We are standing about half way along the Sierra Madre Fault System – recall it has several sections. From east to west, the main fault sections are the Santa Susana Fault (30 km), the San Fernando Fault (20 km), the Sierra Madre Fault (60 km), and the Cucamonga Fault (25 km). The complexity of the fault system leads to a wide range of magnitudes that could rupture on this fault; earthquakes could be moderate (like the M6.7 Sylmar earthquake) or if multiple fault sections are connected, as large as ~M8 (Field et al., 2013).

Map of Sierra Madre Fault System. BLUE: Santa Susana Fault, 6 (0.5-10) mm/yr; YELLOW: San Fernando Fault, 2 (1-3) mm/yr; RED: Central Sierra Madre Fault, 2 (1-3) mm/yr, PURPLE: Cucamonga Fault 1.5 (1-2) mm/yr. Slip rates used in UCERF3. From Burgette et al., 2014.

23

Quaternary geology of the JPL area (Crook et al., 1987), draped over lidar-derived shaded relief. R98 is location of Loma Alta Park. Profiles across mapped traces of the SMF show significant late Quaternary offset (e.g., A-A’). A cross-fan profile (B-B’) illustrates likely miscorrelation of mapped terrace surfaces in earlier work. From Burgette et al. (2014). Slip rates are variable across this system, and limited in number. For example, the Santa Susana Fault slip rate could be as high as 5.9±3.9 mm/yr (based on offset Plio-Pleistocene units; Huftile and Yeats, 1996) or as low as 2.5 mm/yr based on BEM modeling (Marshall et al., 2013). Lindvall and Rubin (2007) completed cosmogenic dating on the Wilson fan on the San Fernando section; summing offsets on the three strands at that location provides an upper rate of 3 mm/yr. At this location on the Sierra Madre Fault, Crook et al., (1987) estimated slip rates from soil chronosequences of 1.2-3 mm/yr. To the east, Lindvall and Rubin (2007) determined a slip rate of 1.9±0.4 mm/yr across the Cucamonga Fault zone. There have been a few paleoseismic studies of this system. Here at Loma Alta Park, Rubin et al. (1998) trenched the ~2 m high scarp to our north and see evidence for two paleoearthquakes. They estimate that slip of ~4 m and 6.5 m occurred in the last two earthquakes, which happened since about 18,000 years ago, producing a slip rate of about 0.6 mm/yr (although this is just for one strand of system). Just west of the intersection of the Sierra Madre and Cucamonga Faults, Tucker and Dolan (2001) completed a boring and trench at Horsethief Canyon. They found evidence for one event over 8000 years ago, and determined a slip rate of 0.6-0.9 mm/yr.

24

Figures from Rubin et al. (1998)

Figures from Rubin et al. (1998)

25

1930’s photo of the San Gabriel Mountains with pre-development and relatively tree-free view of uplifted fan surfaces (arrow) above what is now Arroyo Seco. RETURN TO PASADENA CONVENTION CENTER References by stop Verdugo Fault: Weber, F. H., Bennett, J.J., Chapman, R.H., Chase, G.W., and Saul, R.B., 1980. Earthquake hazards associated with the Verugo-Eagle Rock and Benedict Canyon Fault Zones, Los Angeles County, Califorinia, USGS Open File Report 81-296. Griffith Observatory and Wattles Mansion Stop (Dolan) Buwalda, J. P., 1940, Geology of the Raymond Basin: Report to the City of Pasadena Water Dept., 9 plates, 131 p. Chapman, R. H. and Chase, G. W., 1979, Geophysical investigations of the Santa Monica-Raymond fault zone, Los Angeles County, California, Calif. Div. Mines and Geology, Open-File Report 79-16, p. E-1 to E-30. Crook, R., and Proctor, R., 1992, The Hollywood and Santa Monica faults and the southern boundary of the Transverse Ranges province, in Pipkin, B., and Proctor, R., eds., Engineering geology practice in southern California: Belmont, California, Star Publishing, p. 233–246. Dolan, J. F., Sieh, K., Rockwell, T. K., Guptill, P., and Miller, G., 1997, Active tectonics, paleoseismology, and seismic hazards of the Hollywood fault, northern Los Angeles basin, California: Bulletin of the Geological Society of America, v. 109, p. 1595-1616. Dolan, J. F., Stevens, D., and Rockwell, T. K., 2000, Paleoseismologic evidence for an early to mid-Holocene age of the most recent surface rupture on the Hollywood fault, Los Angeles, California: Bulletin of the Seismological Society of America, v. 90, p. 334-344.

26

Leon, L. A., Dolan, J. F., Shaw, J. H., and Pratt, T. L., 2009, Evidence for large-magnitude Holocene earthquakes on the Compton blind thrust fault, Los Angeles, California: Jour. Geophys. Res., doi: 10.1029/2008JB006129. Shaw, J.H., and P. Shearer, (1999). An elusive blind-thrust fault beneath metropolitan Los Angeles, Science, 283, 1516-1518. Shaw, J. H., Plesch, A., Dolan, J. F., Pratt, T., and Fiore, P., 2002, Puente Hills blind-thrust system, Los Angeles basin, California: Bulletin of the Seismological Society of America, v. 92, p. 2946-2960. Northridge Stops (Yule) Baldwin, J., N., Kelson, K.I., and Randolph, C.E., (2000). Late Quaternary fold deformation along the Northridge Hills fault, Northridge, California: Deformation coincident with past Northridge blind-thrust earthquakes and other nearby structures?, Bull. Seism. Soc. Am. 90, 629-642. Barnhart, J., and J. Slosson (1973). The Northridge Hills and associated faults: a zone of high seismic probability? Assoc. Eng. Geol. Spec. Pubs. 253. Dibblee, T. W., Jr. (1992). Geologic map of the Oat Mountain and Canoga Park (north 1/2) quadrangles, Los Angeles County, California, Dibblee Geol. Foundation Map DF-36, Santa Barbara, California, scale1:24,000. Earthquake Engineering Research Institute (1994). Northridge earthquake, January 17, 1994, Preliminary reconnaissance report, J. F. Hall (Editor), California Institute of Technology, Pasadena, California, 96 pp. Hitchcock, C., and K. Kelson (1999). Growth of late Quaternary folds in southwest Santa Clara Valley, San Francisco Bay area, California:implications of triggered slip for seismic hazard and earthquake recurrence, Geology 27, 5, 391–394. Johnson, J. A., R. J. Shlemon, and J. E. Slosson (1996). Permanent ground deformation associated with the 17 January 1994 Northridge, California Earthquake; and Geomorphic indicators of potential future zones of deformation, in 1996 Progress Reports from SCEC Scientists, March 1997, Vol II. Levi, S., and R. S. Yeats (1993). Paleomagnetic constraints on the initiation of uplift on the Santa Susana fault, western Transverse Ranges, California, Tectonics 12, 688–702. Mori, J., D. J. Wald, and R. L. Wesson (1995). Overlapping fault planes of the 1971 San Fernando and 1994 Northridge, California earthquakes, Geophy. Res. Let. 22, 1033–1036. Saul, R. (1979). Geology of the southeast quarter of the Oat Mountain Quadrangle, Los Angeles County, California, Div. of Mines and Geology Map Sheet 30, scale 1:12,000, 19 pp. Tsutsumi, H., and R. S. Yeats (1999). Tectonic setting of the 1971 Sylmar and 1994 Northridge earthquake in the San Fernando Valley, California, Bull. Seism. Soc. Am. 89, 1232–1249. Wald, D. J., T. H. Heaton, and K. W. Hudnut (1996). The slip history of the 1994 Northridge, California, earthquake determined from strong ground motion, teleseismic, GPS and leveling data, Bull. Seism. Soc. Amer. 86, S49–70. Wills, C. J., and C. S. Hitchcock (1999). Late Quaternary sedimentationand liquefaction hazard in San Fernando Valley, Los Angeles County,California, Environmental and Engineering Geoscience V., No. 4, 441–458. Loma Alta Park (Scharer) Burgette, R., Scharer, K., and Hanson, A. (2014). Late Quaternary offset along the Sierra Madre Fault revealed by high-resolution topographic data, GSA Annual Meeting, 323-10. Lindvall, S.C., & Rubin, C.M., (2007). Slip Rate Studies Along The Sierra Madre-Cucamonga Fault System Using Geomorphic and Cosmogenic Surface Exposure Age Constraints. NEHRP Final Technical Report, award 03HQGR0084, 13 p. Marshall, S. T., Funning, G. J., & Owen, S. E. (2013). Fault slip rates and interseismic deformation in the western Transverse Ranges, California. Journal of Geophysical Research: Solid Earth, 118(8), 4511-4534. Rubin, C. M., Lindvall, S. C., & Rockwell, T. K. (1998). Evidence for large earthquakes in metropolitan Los Angeles. Science, 281(5375), 398-402. Tucker, A. Z., & Dolan, J. F. (2001). Paleoseismologic evidence for a> 8 ka age of the most recent surface rupture on the eastern Sierra Madre fault, northern Los Angeles metropolitan region, California. Bulletin of the Seismological Society of America, 91(2), 232-249. Yerkes, R. F., and Campbell, R. H., comp (2005). Preliminary Geologic Map of the Los Angeles 30” x 60’ Quadrangle, Southern California, USGS Open File Report 2005-0019, http://pubs.usgs.gov/of/2005/1019/.

2946

Bulletin of the Seismological Society of America, Vol. 92, No. 8, pp. 2946–2960, December 2002

Puente Hills Blind-Thrust System, Los Angeles, California

by John H. Shaw, Andreas Plesch, James F. Dolan, Thomas L. Pratt, and Patricia Fiore

Abstract We describe the three-dimensional geometry and Quaternary slip his-tory of the Puente Hills blind-thrust system (PHT) using seismic reflection profiles,petroleum well data, and precisely located seismicity. The PHT generated the 1987Whittier Narrows (moment magnitude [Mw] 6.0) earthquake and extends for morethan 40 km along strike beneath the northern Los Angeles basin. The PHT comprisesthree, north-dipping ramp segments that are overlain by contractional fault-relatedfolds. Based on an analysis of these folds, we produce Quaternary slip profiles alongeach ramp segment. The fault geometry and slip patterns indicate that segments ofthe PHT are related by soft-linkage boundaries, where the fault ramps are en echelonand displacements are gradually transferred from one segment to the next. AverageQuaternary slip rates on the ramp segments range from 0.44 to 1.7 mm/yr, withpreferred rates between 0.62 and 1.28 mm/yr. Using empirical relations among rup-ture area, magnitude, and coseismic displacement, we estimate the magnitude andfrequency of single (Mw 6.5–6.6) and multisegment (Mw 7.1) rupture scenarios forthe PHT.

Introduction

The Los Angeles basin lies along the southern Califor-nia coast at the junction of the Transverse and PeninsularRanges. The basin is currently being deformed by severalblind-thrust and strike-slip faults that have generated his-toric, moderate-size earthquakes (Hauksson, 1990; Dolan etal., 1995). Geodetic studies suggest that the northern basinis shortening at a rate of 4.4–5 mm/yr in a north–south tonortheast–southwest direction (Walls et al., 1998; Argus etal., 1999; Bawden et al., 2001). Part of this shortening isaccommodated on recognized fault systems, and two com-peting models have been proposed to explain which faultsaccount for the remainder. Walls et al. (1998) suggested thatthe remaining shortening is accommodated by higher ratesof slip on conjugate strike-slip systems in the northern basin.Motion on these faults produces north–south shortening bylateral crustal extrusion. In contrast, Argus et al. (1999) andBawden et al. (2001) proposed that the shortening is accom-modated primarily by activity on thrust and reverse faults.In this article, we document an active blind-thrust system inthe northern Los Angeles basin, termed the Puente Hillsblind thrust (PHT), that accommodates a component of theunresolved basin shortening and poses substantial earth-quake hazards to metropolitan Los Angeles.

The PHT extends for more than 40 km along strike inthe northern Los Angeles basin from downtown Los Angeleseast to Brea in northern Orange County (Fig. 1). The faultconsists of at least three distinct geometric segments, termedLos Angeles, Santa Fe Springs, and Coyote Hills, from westto east. Shaw and Shearer (1999) defined the central portion

of the PHT with seismic reflection profiles and petroleumwell data. Using precisely relocated seismicity, these authorsproposed that the Santa Fe Springs segment of the PHT gen-erated the 1987 (Mw 6) Whittier Narrows earthquake. In thisarticle, we define the size and three-dimensional geometryof the PHT using additional seismic reflection data. We de-scribe the systematic behavior of the segments that composethe PHT and consider the geometric and kinematic relationsof the PHT to other blind-thrust and strike-slip systems. Fi-nally, we map the distribution of slip on the PHT and discussthe implications of this study for earthquake hazard assess-ment in metropolitan Los Angeles.

Puente Hills Blind Thrust

Displacements on the PHT contribute to the growth ofa series of en echelon anticlines in the northern Los Angelesbasin that involve Miocene through Quaternary strata. Theeasternmost fold forms the Coyote Hills and provides struc-tural closure for the east and west Coyote Hills oil fields(Yerkes, 1972). The central fold has only subtle surface ex-pression and provides structural closure for the Santa FeSprings oil field (Fig. 1). The westernmost fold is containedwithin a broad, south-dipping monocline that forms thenorthern boundary of the Los Angeles basin (Davis et al.,1989; Shaw and Suppe, 1996; Schneider et al., 1996). Allof these folds are bounded on their southern margins bynarrow forelimbs (Fig. 2). These forelimbs, or kink bands,consist of concordantly folded, south-dipping Pliocene

!

Tom Pratt (USGS), James Dolan (USC) and John Shaw (Harvard) collecting high-resolution seismic re�ection data above the central, Santa Fe Springs segment of the Puente Hills thrust (see Pratt et al., 2002 (BSSA); Shaw et al. 2002 (BSSA); and Dolan et al., 2004 (Science).

16 boreholes drilled to �11 to 50 m depths (Figure 5).Initially interpreted by Dolan et al. [2003], this boreholetransect was excavated along the same alignment as the high-resolution hammer seismic reflection profile (Figure 3c).These boreholes thus provide direct ground truth on theyoungest reflectors imaged by seismic reflection data(Figure 6). Seven of these boreholes (18 through 24) werecontinuously sampled hollow stem boreholes, whereaseight of the boreholes (10 through 17) were large-diameter(70 cm) bucket auger holes. The bucket augers were sampleddownhole directly by a geologist to the depth of theshallowest aquifer. Below this depth, one foot bucket drives

were used for sampling. The newly acquired Gardenlandborehole transect, which consisted of seven, continuouslycored hollow stem boreholes, was drilled parallel to and�100 m west of, the Carfax Avenue transect (Figure 5). TheSCE transect, which was much shorter (80 m) than the othertwo transects, was collected�50 m east of and parallel to thenorthern part of the Carfax transect. The SCE transectconsisted of seven, large-diameter (70 cm) bucket augerholes excavated to 12 m in depth (Figure 5). These results arerecorded in the auxiliary material1.

Figure 5. Borehole results from the Carfax Avenue transect, Gardenland-Greenhurst transect, and SCEtransect showing major stratigraphic units (8X vertical exaggeration). Thin vertical lines denoteboreholes. Numbers show key stratigraphic units discussed in the text. Thin red lines mark the tops ofmajor sand- and gravel-filled units. Double-headed red vertical arrows along the left side of the figureshow the stratigraphic range within which discrete uplift events occurred (see text for discussion). Solidgreen lines between red arrows on left side of figure show stratigraphic intervals that do not changethickness across the transect, reflecting periods of structural quiescence. Proposed correlations betweenCarfax boreholes and a water well (1589S) located 175 m north of the transect are shown by dashedstratigraphic contacts along the right side of the figure. Contacts at 12, 17, and 20 m depth are correlativebetween borehole 22 and the water well.

1Auxiliary materials are available in the HTML. doi:10.1029/2006JB004461.

B03S03 LEON ET AL.: PUENTE HILLS THRUST EARTHQUAKE

7 of 18

B03S03






7 of 18

B03S03






7 of 18

B03S03

Earthquake-by-earthquake fold growth above the Puente Hills blind

thrust fault, Los Angeles, California: Implications for fold

kinematics and seismic hazard

Lorraine A. Leon,1 Shari A. Christofferson,1 James F. Dolan,1

John H. Shaw,2 and Thomas L. Pratt3

Received 22 April 2006; revised 7 November 2006; accepted 27 February 2007; published 29 March 2007.

[1] Boreholes and high-resolution seismic reflection data collected across the forelimbgrowth triangle above the central segment of the Puente Hills thrust fault (PHT) beneathLos Angeles, California, provide a detailed record of incremental fold growth duringlarge earthquakes on this major blind thrust fault. These data document fold growthwithin a discrete kink band that narrows upward from �460 m at the base of theQuaternary section (200–250 m depth) to <150 m at 2.5 m depth, with most growthduring the most recent folding event occurring within a zone only �60 m wide. Theseobservations, coupled with evidence from petroleum industry seismic reflection data,demonstrate that most (>82% at 250 m depth) folding and uplift occur within discrete kinkbands, thereby enabling us to develop a paleoseismic history of the underlying blind thrustfault. The borehole data reveal that the youngest part of the growth triangle in theuppermost 20 m comprises three stratigraphically discrete growth intervals marked bysouthward thickening sedimentary strata that are separated by intervals in which sedimentsdo not change thickness across the site. We interpret the intervals of growth as occurringafter the formation of now-buried paleofold scarps during three large PHT earthquakes inthe past 8 kyr. The intervening intervals of no growth record periods of structuralquiescence and deposition at the regional, near-horizontal stream gradient at the study site.Minimum uplift in each of the scarp-forming events, which occurred at 0.2–2.2 ka (eventY), 3.0–6.3 ka (event X), and 6.6–8.1 ka (event W), ranged from �1.1 to �1.6 m,indicating minimum thrust displacements of �2.5 to 4.5 m. Such large displacements areconsistent with the occurrence of large-magnitude earthquakes (Mw > 7). Cumulative,minimum uplift in the past three events was 3.3 to 4.7 m, suggesting cumulativethrust displacement of �7 to 10.5 m. These values yield a minimum Holocene sliprate for the PHT of �0.9 to 1.6 mm/yr. The borehole and seismic reflection datademonstrate that dip within the kink band is acquired incrementally, such that olderstrata that have been deformed by more earthquakes dip more steeply than youngerstrata. Specifically, strata dip 0.4� at 4 m depth, 0.7� at 20 m depth, 8� at 90 m, 16�at 110 m, and 17� at 200 m. Moreover, structural restorations of the borehole datashow that the locus of active folding (the anticlinal active axial surface) does notextend to the surface in exactly the same location from earthquake to earthquake.Rather, that the axial surfaces migrate from earthquake to earthquake, reflecting acomponent of fold growth by kink band migration. The incremental acquisition of beddip in the growth triangle may reflect some combination of fold growth by limbrotation in addition to kink band migration, possibly through a component of trishearor shear fault bend folding. Alternatively, the component of limb rotation mayresult from curved hinge fault bend folding, and/or the mechanical response of looselyconsolidated granular sediments in the shallow subsurface to folding at depth.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B03S03, doi:10.1029/2006JB004461, 2007ClickHere

for

FullArticle

1Department of Earth Sciences, University of Southern California, LosAngeles, California, USA.

2Department of Earth and Planetary Sciences, Harvard University,Cambridge, Massachusetts, USA.

3U. S. Geological Survey, Seattle, Washington, USA.

Copyright 2007 by the American Geophysical Union.0148-0227/07/2006JB004461$09.00

B03S03 1 of 18

Earthquake-by-earthquake fold growth above the Puente Hills blind

thrust fault, Los Angeles, California: Implications for fold

kinematics and seismic hazard

Lorraine A. Leon,1 Shari A. Christofferson,1 James F. Dolan,1

John H. Shaw,2 and Thomas L. Pratt3

Received 22 April 2006; revised 7 November 2006; accepted 27 February 2007; published 29 March 2007.

[1] Boreholes and high-resolution seismic reflection data collected across the forelimbgrowth triangle above the central segment of the Puente Hills thrust fault (PHT) beneathLos Angeles, California, provide a detailed record of incremental fold growth duringlarge earthquakes on this major blind thrust fault. These data document fold growthwithin a discrete kink band that narrows upward from �460 m at the base of theQuaternary section (200–250 m depth) to <150 m at 2.5 m depth, with most growthduring the most recent folding event occurring within a zone only �60 m wide. Theseobservations, coupled with evidence from petroleum industry seismic reflection data,demonstrate that most (>82% at 250 m depth) folding and uplift occur within discrete kinkbands, thereby enabling us to develop a paleoseismic history of the underlying blind thrustfault. The borehole data reveal that the youngest part of the growth triangle in theuppermost 20 m comprises three stratigraphically discrete growth intervals marked bysouthward thickening sedimentary strata that are separated by intervals in which sedimentsdo not change thickness across the site. We interpret the intervals of growth as occurringafter the formation of now-buried paleofold scarps during three large PHT earthquakes inthe past 8 kyr. The intervening intervals of no growth record periods of structuralquiescence and deposition at the regional, near-horizontal stream gradient at the study site.Minimum uplift in each of the scarp-forming events, which occurred at 0.2–2.2 ka (eventY), 3.0–6.3 ka (event X), and 6.6–8.1 ka (event W), ranged from �1.1 to �1.6 m,indicating minimum thrust displacements of �2.5 to 4.5 m. Such large displacements areconsistent with the occurrence of large-magnitude earthquakes (Mw > 7). Cumulative,minimum uplift in the past three events was 3.3 to 4.7 m, suggesting cumulativethrust displacement of �7 to 10.5 m. These values yield a minimum Holocene sliprate for the PHT of �0.9 to 1.6 mm/yr. The borehole and seismic reflection datademonstrate that dip within the kink band is acquired incrementally, such that olderstrata that have been deformed by more earthquakes dip more steeply than youngerstrata. Specifically, strata dip 0.4� at 4 m depth, 0.7� at 20 m depth, 8� at 90 m, 16�at 110 m, and 17� at 200 m. Moreover, structural restorations of the borehole datashow that the locus of active folding (the anticlinal active axial surface) does notextend to the surface in exactly the same location from earthquake to earthquake.Rather, that the axial surfaces migrate from earthquake to earthquake, reflecting acomponent of fold growth by kink band migration. The incremental acquisition of beddip in the growth triangle may reflect some combination of fold growth by limbrotation in addition to kink band migration, possibly through a component of trishearor shear fault bend folding. Alternatively, the component of limb rotation mayresult from curved hinge fault bend folding, and/or the mechanical response of looselyconsolidated granular sediments in the shallow subsurface to folding at depth.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B03S03, doi:10.1029/2006JB004461, 2007ClickHere

for

FullArticle



3U. S. Geological Survey, Seattle, Washington, USA.


B03S03 1 of 18

from additional tension in the wires. With areasonable reduction of resonator lengths to 0.25�m, it should be possible to fabricate resonatorswith �res � 2.5 GHz.

As a final demonstration of the flexibility ofthe SNAP process, the process was repeated tofabricate ultradense crossbar circuit elements. Asdiscussed above, one intrinsic advantage of theSNAP process is that it is a direct transfer ofprefabricated nanowires, enabling many patternsto be deposited on top of one another. A secondadvantage of the SNAP process is related to theextremely high aspect ratio of the nanowire ar-rays. Because the arrays are so long (mmlengths), the only critical alignment between atop and bottom pattern is the angle between thetwo patterns. In other words, no particular spatialalignment is needed even to fabricate crossbarstructures at densities in excess of 1011 junctionscm�2. Figure 4C is an electron micrograph ofseveral crossbar circuits fabricated by repeatingthe SNAP process at 90° to each other. Anobvious limitation of the SNAP process is that itcan only produce crossbar circuits (or other rel-atively simple structures). However, the crossbarhas emerged as the circuit of choice for a numberof nanoelectronic applications (15), such as mo-lecular switch–based random access memory(16) or nanowire-based logic circuits (17, 18).

Many applications are possible with the com-bination of nanowire materials, diameters, pitch-es, and aspect ratios that the SNAP approachyields. Possibilities include one-dimensional su-perconductors, high-density semiconductornanowire sensor arrays, gigahertz nanomechani-cal resonators, and high-density molecular elec-tronics circuits. However, one critical issue thatmust be faced if this approach is to be fullyexploited is the task of individually addressingeach nanowire within a small pitch array. TheSNAP technique can generate wiring networksthat are well beyond the resolution of evenEBL. Thus, this approach brings into focus theproblem of interfacing the micrometer or sub-micrometer world of lithography and the mac-romolecular-type densities achievable withthis technique. This is one of the outstandingproblems in nanotechnology, but potential so-lutions, such as binary tree demultiplexers,have been proposed (19). Building such de-multiplexers, with less stringent (�50 nm)requirements in terms of the pitch and align-ment of the electrical connections (an essentialrequirement), will be a major challenge.

References and Notes1. C. Vieu et al., Appl. Surf. Sci. 164, 111 (2000).2. Y. Xia et al., Science 273, 347 (1996).3. S. Y. Chou, P. R. Krauss, P. J. Renstrom, Science 272,

85 (1996).4. We have reproduced this technique on more than a

dozen commercially purchased and grown-in-housesuperlattices, and we have found that there is appre-ciable chemical variability from wafer to wafer, witha corresponding variability in etch chemistry.

5. Currently the superlattice is not being reused becauseof the small amount necessary for each imprint.

However, polishing of GaAs/AlGaAs superlattices iswell established for laser devices and may be usefulhere.

6. The brightness/contrast of the SEM images wasadjusted, but no other image processing was used.

7. Y. Huang, X. F. Duan, Q. Q. Wei, C. M. Lieber, Science291, 630 (2001).

8. M. R. Diehl et al., Angew. Chem. Int. Ed. 41, 353(2001).

9. Typical Si and poly-Si thin films were 30 to 50 nmthick and were etched using Cl2 RIE at low power (80W ) and pressure (5 mT ) to carefully control the etchrate.

10. Contacts were made by using EBL to define �100-nm wires placed on either side of the etched portionof the wire array. Accurate placement of the contactwires was achieved with the use of predepositedalignment markers. Wires with narrow pitch (�80nm) could generally not be individually addressedthis way because of the resolution limits of EBL.Contact of a single wire was achieved by depositingelectrodes onto all the wires in parallel, then using afocused ion beam to cut all but one wire between theelectrodes.

11. The metal nanowires (�15 nm wide) exhibited con-sistent bulk metallic conductivity to 4 K; smallerwires could not be measured individually. The con-ductivity of the SOI nanowires fluctuated much morewidely, depending on the processing conditions andnanowire crystallographic orientation.

12. A. N. Cleland, M. L. Roukes, J. Appl. Phys. 92, 2758(2002).

13. In a strong magnetic field, current flowing through thewires creates a Lorentz force orthogonal to the mag-netic field and current directions, in turn creating anoscillatory force on the wire with the frequency of theapplied ac current. At resonance, the wire oscillatesthough the magnetic field with appreciable amplitude,producing an induction current that can be measuredwith a network analyzer. To enhance sensitivity, weused a current-sensitive bridge circuit to reduce back-ground and reflected intensity.

14. P. Mohanty et al., Phys. Rev. B 66, 085416 (2002).15. J. R. Heath, P. J. Kuekes, G. Snider, R. S. Williams,

Science 280, 1716 (1998).16. Yi Luo et al., ChemPhysChem 2002, 519 (2002).17. A. Bachtold, P. Hadley, T. Nakanishi, C. Dekker,

Science 294, 1317 (2001).18. Y. Huang et al., Science 294, 1313 (2001).19. P. J. Kuekes, R. S. Williams, U.S. Patent 6,256,767

(2001).20. Supported by the Office of Naval Research and the

Defense Advanced Research Projects Agency. Wethank M. Roukes and his group for teaching us how toperform high-frequency nanomechanical resonatormeasurements.

27 December 2002; accepted 4 March 2003Published online 13 March 2003;10.1126/science.1081940Include this information when citing this paper.

Recognition of Paleoearthquakeson the Puente Hills Blind Thrust

Fault, CaliforniaJames F. Dolan,1* Shari A. Christofferson,1 John H. Shaw2

Borehole data from young sediments folded above the Puente Hills blind thrustfault beneath Los Angeles reveal that the folding extends to the surface as adiscrete zone (�145 meters wide). Buried fold scarps within an upward-narrowing zone of deformation, which extends from the upward terminationof the thrust ramp at 3 kilometers depth to the surface, document the occur-rence of at least four large (moment-magnitude 7.2 to 7.5) earthquakes on thisfault during the past 11,000 years. Future events of this type pose a seismichazard to metropolitan Los Angeles. Moreover, the methods developed in thisstudy can be used to refine seismic hazard assessments of blind thrusts in othermetropolitan regions.

During the past 30 years, paleoseismology (thestudy of ancient earthquakes) has provided awealth of information on the locations, magni-tudes, and dates of earthquakes (1, 2). These dataare critical for probabilistic seismic hazard as-sessments, which dictate modern hazard zoning,emergency response, and risk mitigation strate-gies. Previous paleoseismological studies, how-ever, have largely neglected an entire class offaults—the so-called blind thrust faults. Thesefaults do not extend all the way to the surface,

and shortening in the near-surface is accommo-dated by folding rather than by fault slip (3–6).The absence of paleoearthquake age and dis-placement data from blind thrust faults creates amajor gap both in our understanding of thecollective behavior of regional fault systemsand in our ability to construct accurate seismichazard models. Here we document the agesand displacements of individual paleoearth-quakes on a major, active blind thrust fault.

Our study focused on the Puente Hills thrust(PHT), a major blind fault beneath metropolitanLos Angeles, California (7). Extensive petro-leum-industry seismic reflection and well data,relocated earthquakes, and geomorphologic ob-servations demonstrate that the PHT, which dipsgently northeastward and terminates upward at adepth of �3 km, extends for 40 to 50 km fromnorthern Orange County, beneath downtown Los

1Department of Earth Sciences, University of South-ern California, Los Angeles, CA 90089–0740, USA.2Department of Earth and Planetary Sciences, HarvardUniversity, 20 Oxford Street, Cambridge, MA 02138,USA.

*To whom correspondence should be addressed. E-mail: [email protected]

R E P O R T S

www.sciencemag.org SCIENCE VOL 300 4 APRIL 2003 115

on

Oct

ober

13,

200

8 w

ww

.sci

ence

mag

.org

Dow

nloa

ded

from

from additional tension in the wires. With areasonable reduction of resonator lengths to 0.25�m, it should be possible to fabricate resonatorswith �res � 2.5 GHz.

As a final demonstration of the flexibility ofthe SNAP process, the process was repeated tofabricate ultradense crossbar circuit elements. Asdiscussed above, one intrinsic advantage of theSNAP process is that it is a direct transfer ofprefabricated nanowires, enabling many patternsto be deposited on top of one another. A secondadvantage of the SNAP process is related to theextremely high aspect ratio of the nanowire ar-rays. Because the arrays are so long (mmlengths), the only critical alignment between atop and bottom pattern is the angle between thetwo patterns. In other words, no particular spatialalignment is needed even to fabricate crossbarstructures at densities in excess of 1011 junctionscm�2. Figure 4C is an electron micrograph ofseveral crossbar circuits fabricated by repeatingthe SNAP process at 90° to each other. Anobvious limitation of the SNAP process is that itcan only produce crossbar circuits (or other rel-atively simple structures). However, the crossbarhas emerged as the circuit of choice for a numberof nanoelectronic applications (15), such as mo-lecular switch–based random access memory(16) or nanowire-based logic circuits (17, 18).

Many applications are possible with the com-bination of nanowire materials, diameters, pitch-es, and aspect ratios that the SNAP approachyields. Possibilities include one-dimensional su-perconductors, high-density semiconductornanowire sensor arrays, gigahertz nanomechani-cal resonators, and high-density molecular elec-tronics circuits. However, one critical issue thatmust be faced if this approach is to be fullyexploited is the task of individually addressingeach nanowire within a small pitch array. TheSNAP technique can generate wiring networksthat are well beyond the resolution of evenEBL. Thus, this approach brings into focus theproblem of interfacing the micrometer or sub-micrometer world of lithography and the mac-romolecular-type densities achievable withthis technique. This is one of the outstandingproblems in nanotechnology, but potential so-lutions, such as binary tree demultiplexers,have been proposed (19). Building such de-multiplexers, with less stringent (�50 nm)requirements in terms of the pitch and align-ment of the electrical connections (an essentialrequirement), will be a major challenge.

References and Notes1. C. Vieu et al., Appl. Surf. Sci. 164, 111 (2000).2. Y. Xia et al., Science 273, 347 (1996).3. S. Y. Chou, P. R. Krauss, P. J. Renstrom, Science 272,

85 (1996).4. We have reproduced this technique on more than a

dozen commercially purchased and grown-in-housesuperlattices, and we have found that there is appre-ciable chemical variability from wafer to wafer, witha corresponding variability in etch chemistry.

5. Currently the superlattice is not being reused becauseof the small amount necessary for each imprint.

However, polishing of GaAs/AlGaAs superlattices iswell established for laser devices and may be usefulhere.

6. The brightness/contrast of the SEM images wasadjusted, but no other image processing was used.

7. Y. Huang, X. F. Duan, Q. Q. Wei, C. M. Lieber, Science291, 630 (2001).

8. M. R. Diehl et al., Angew. Chem. Int. Ed. 41, 353(2001).

9. Typical Si and poly-Si thin films were 30 to 50 nmthick and were etched using Cl2 RIE at low power (80W ) and pressure (5 mT ) to carefully control the etchrate.

10. Contacts were made by using EBL to define �100-nm wires placed on either side of the etched portionof the wire array. Accurate placement of the contactwires was achieved with the use of predepositedalignment markers. Wires with narrow pitch (�80nm) could generally not be individually addressedthis way because of the resolution limits of EBL.Contact of a single wire was achieved by depositingelectrodes onto all the wires in parallel, then using afocused ion beam to cut all but one wire between theelectrodes.

11. The metal nanowires (�15 nm wide) exhibited con-sistent bulk metallic conductivity to 4 K; smallerwires could not be measured individually. The con-ductivity of the SOI nanowires fluctuated much morewidely, depending on the processing conditions andnanowire crystallographic orientation.

12. A. N. Cleland, M. L. Roukes, J. Appl. Phys. 92, 2758(2002).

13. In a strong magnetic field, current flowing through thewires creates a Lorentz force orthogonal to the mag-netic field and current directions, in turn creating anoscillatory force on the wire with the frequency of theapplied ac current. At resonance, the wire oscillatesthough the magnetic field with appreciable amplitude,producing an induction current that can be measuredwith a network analyzer. To enhance sensitivity, weused a current-sensitive bridge circuit to reduce back-ground and reflected intensity.

14. P. Mohanty et al., Phys. Rev. B 66, 085416 (2002).15. J. R. Heath, P. J. Kuekes, G. Snider, R. S. Williams,

Science 280, 1716 (1998).16. Yi Luo et al., ChemPhysChem 2002, 519 (2002).17. A. Bachtold, P. Hadley, T. Nakanishi, C. Dekker,

Science 294, 1317 (2001).18. Y. Huang et al., Science 294, 1313 (2001).19. P. J. Kuekes, R. S. Williams, U.S. Patent 6,256,767

(2001).20. Supported by the Office of Naval Research and the

Defense Advanced Research Projects Agency. Wethank M. Roukes and his group for teaching us how toperform high-frequency nanomechanical resonatormeasurements.

27 December 2002; accepted 4 March 2003Published online 13 March 2003;10.1126/science.1081940Include this information when citing this paper.

Recognition of Paleoearthquakeson the Puente Hills Blind Thrust

Fault, CaliforniaJames F. Dolan,1* Shari A. Christofferson,1 John H. Shaw2

Borehole data from young sediments folded above the Puente Hills blind thrustfault beneath Los Angeles reveal that the folding extends to the surface as adiscrete zone (�145 meters wide). Buried fold scarps within an upward-narrowing zone of deformation, which extends from the upward terminationof the thrust ramp at 3 kilometers depth to the surface, document the occur-rence of at least four large (moment-magnitude 7.2 to 7.5) earthquakes on thisfault during the past 11,000 years. Future events of this type pose a seismichazard to metropolitan Los Angeles. Moreover, the methods developed in thisstudy can be used to refine seismic hazard assessments of blind thrusts in othermetropolitan regions.

During the past 30 years, paleoseismology (thestudy of ancient earthquakes) has provided awealth of information on the locations, magni-tudes, and dates of earthquakes (1, 2). These dataare critical for probabilistic seismic hazard as-sessments, which dictate modern hazard zoning,emergency response, and risk mitigation strate-gies. Previous paleoseismological studies, how-ever, have largely neglected an entire class offaults—the so-called blind thrust faults. Thesefaults do not extend all the way to the surface,

and shortening in the near-surface is accommo-dated by folding rather than by fault slip (3–6).The absence of paleoearthquake age and dis-placement data from blind thrust faults creates amajor gap both in our understanding of thecollective behavior of regional fault systemsand in our ability to construct accurate seismichazard models. Here we document the agesand displacements of individual paleoearth-quakes on a major, active blind thrust fault.

Our study focused on the Puente Hills thrust(PHT), a major blind fault beneath metropolitanLos Angeles, California (7). Extensive petro-leum-industry seismic reflection and well data,relocated earthquakes, and geomorphologic ob-servations demonstrate that the PHT, which dipsgently northeastward and terminates upward at adepth of �3 km, extends for 40 to 50 km fromnorthern Orange County, beneath downtown Los

1Department of Earth Sciences, University of South-ern California, Los Angeles, CA 90089–0740, USA.2Department of Earth and Planetary Sciences, HarvardUniversity, 20 Oxford Street, Cambridge, MA 02138,USA.

*To whom correspondence should be addressed. E-mail: [email protected]

R E P O R T S


on

Oct

ober

13,

200

8 w

ww

.sci

ence

mag

.org

Dow

nloa

ded

from

near-horizontal paleostream gradient. This infer-ence is supported by the laterally constant thick-ness of Units 46 and 47, which indicates thatthese sediments were deposited near-horizontal-ly. Units 46 and 47 record a period of structuralquiescence, with no discernible uplift occurringbetween 7.8 thousand years ago (ka) and �9.5 to10.0 ka (14). In contrast, the overlying intervalcomprising the Unit 40 sand and the Unit 45overbank muds thickens southward by 2 macross the zone of active folding. The absence ofgrowth in the Units 30 to 35 section supports ourinference that the top of the Unit 40 channel sandwas deposited at the near-horizontal paleostreamgradient. Thus, the sedimentary growth withinthe Units 40 to 45 section records onlap of anow-buried southfacing fold scarp that developedsometime after the deposition of Unit 46 at 7.8 to8.2 ka and before the completion of deposition ofUnit 40 before 6.6 to 6.9 ka (the age of the middleof overlying Unit 35). We refer to this strati-graphically discrete uplift episode as Event W.

Using similar observations, we identifiedthree other temporally discrete periods of upliftduring the past �11,000 years (Table 1, EventsY, X, and V) (14), as well as evidence foradditional older uplift and sedimentary thicken-ing (15). We attribute these four uplift events tocoseismic fold growth during large earthquakeson the PHT (16), on the basis of the episodicnature of the uplift and the spatial associationof the zone of uplift with the active axialsurface (the locus of active folding) of the foldforming above the tip of the deep PHT ramp(7, 9, 10). These discoveries obviate concernsthat near-surface deformation above blindthrust faults may be too diffuse to recorduseful paleoseismological information anddemonstrate that earthquake ages and displace-ments can be extracted from blind thrust faultswith precision comparable to that of tradition-al paleoseismologic studies.

The well-constrained geometry and kinemat-ics of the PHT and its associated folds (7, 9, 10)allow us to convert uplift in Events V through Yto estimates of coseismic displacement on thePHT. We have conservatively assumed simplerigid-block motion of the hanging wall up thePHT ramp (i.e., that no strain is consumed bydeformation of the hanging wall block), yieldingminimum measurements of reverse slip on thePHT ramp in past earthquakes (Table 1). Theminimum 13.7 � 1.7 m of slip in these earth-quakes indicates a minimum slip rate of 1.1 to1.6 mm/year since the 9.5 to 10.7 ka age ofEvent V. We estimate a conservative range ofpaleomagnitudes for these earthquakes by as-suming that our minimum slip estimates repre-sent either average slip or maximum slip duringpast PHT ruptures (17). Comparison with re-gressions of maximum and average slip versusMw (18) indicates that Events Y and X were onthe order of Mw 7.2 � 0.2/–0.4, whereas EventW was Mw �7.5 � 0.3/–0.6 and Event V wasMw �7.4 � 0.3/–0.4 (19).

The large displacements associated withEvents V through Y suggest that the PHTtypically ruptures in large earthquakes (Mw�7). These PHT earthquakes would have

been larger than any earthquakes that haveoccurred on Los Angeles–region faults (ex-cluding the San Andreas fault) during the200-year-long historic period (20). The rec-

Fig. 2. Borehole results from the Carfax Avenue transect. Cross section of major stratigraphic units(eight times vertical exaggeration). The thin vertical lines are boreholes. Colors denote differentsedimentary units. Numbers show key units discussed in the text. Thin red lines mark the tops of majorsand- and gravel-filled channels. Double-headed red vertical arrows along the left side of the figure showthe stratigraphic range of sedimentary growth after uplift events; green boxes show sections withconstant sedimentary thickness across the transect (see text for discussion). Proposed correlationsbetween Carfax boreholes and a water well (1589S) located 175 m north of the transect are shown bydashed stratigraphic contacts along the right side of the figure. Contacts at 12, 17, and 20 m arecorrelative between borehole 22 and the water well. G, ground; S, surface.

Fig. 3. Schematic reconstructions ofgrowth strata showing the kinematicrelation between development of foldscarps and slip on the PHT. Colors ofyoung growth strata are the same as forCarfax Units 30 through 50 shown inFig. 2. (A) Deposition of Units 47 and 46(and probably the lower part of 45) at anear-horizontal stream gradient afterburial of the Event V fold scarp by Unit50 sand. (B) Coseismic development ofa 2-m–high fold scarp in earthquake W.(C) Onlap of the Event W fold scarp byUnit 40 sand. (D) Deposition of Units30 to 35 at a near-horizontal streamgradient after Event W. The steeply dip-ping black dashed line shows the activeanticlinal axial surface; the gray dashedline shows the inactive synclinal axialsurface (7, 9, 10).

R E P O R T S


on

Oct

ober

13,

200

8 w

ww

.sci

ence

mag

.org

Dow

nloa

ded

from

Puente Hills Blind-Thrust System, Los Angeles, California 2959

Figure 14. Perspective view of a three-dimen-sional model describing the PHT in relation to othermajor blind-thrust systems in the northern Los An-geles basin. PHT segments: CH, Coyote Hills; SFS,Santa Fe Springs; LA, Los Angeles segment; SV, SanVicente fault; LC, Las Cienegas fault (Schneider etal., 1996); uEP, upper Elysian Park thrust (Oskin etal., 2000); lEP, lower Elysian Park (Davis et al.,1989; Shaw and Suppe, 1996); CP, Compton ramp(Shaw and Suppe, 1996).

7.1) earthquakes could occur with repeat times of 780–2600years. As shown by the 1994 Northridge (Mw 6.7) event,earthquakes of these magnitudes would cause severe damagein the LA metropolitan area.

Acknowledgments

This research was supported by the National Earthquake Hazards Re-duction Program (Grant 01HQGR0035), SCEC, and the National ScienceFoundation (EAR-0087648). The authors thank Michelle Cooke, KarlMueller, Tom Rockwell, and Tom Wright for their helpful insights andJohn Suppe and Robert S. Yeats for constructive reviews.

References

Allmendinger, R. W. (1998). Inverse and forward numerical modeling oftrishear fault-propagation folds, Tectonics 17, 640–656.

Allmendinger, R., and J. H. Shaw (2002). Estimation of fault-propagationdistance from fold shape: implications for earthquake hazards assess-ment, Geology 28, no. 12, 1099–1102.

Argus, D. F., M. D. Heflin, A. Donnellan, F. H. Webb, D. Dong, K. J.Hurst, D. C. Jefferson, G. A. Lyzenga, M. M. Watkins, and J. F.Zumberge (1999). Shortening and thickening of metropolitan Los An-geles measured and inferred by using geodesy, Geology 27, 703–706.

Bawden, G. W., W. Thatcher, R. S. Stein, K. W. Hudnut, and G. Peltzer(2001). Tectonic contraction across Los Angeles after removal ofgroundwater pumping effects, Nature 412, 812–815.

Barbier, M. G. (1983). The Mini-Sosie Method, International Human Re-sources Development Corporation, Boston, 186 pp.

Blake, G. H. (1991). Review of Neogene biostratigraphy and stratigraphyof the Los Angeles basin and implications for basin evolution, inActive Margin Basins, K. T. Biddle (Ed.), American Association ofPetroleum Geologists Memoir 52, 135–184.

Davis, T. L., J. Namson, and R. F. Yerkes (1989). A cross section of theLos Angeles area: seismically active fold-and-thrust belt, the 1987Whittier Narrows Earthquake and earthquake hazard, J. Geophys.Res. 94, 9644–9664.

Dolan, J. F., K. Sieh, T. K. Rockwell, R. S. Yeats, J. Shaw, J. Suppe, G.Huftile, and E. Gath (1995). Prospects for larger or more frequentearthquakes in the greater Los Angeles metropolitan region, Califor-nia, Science 267, 199–205.

Dolan, J. F., K. Sieh, T. K. Rockwell, P. Guptill, and G. Miller (1997).Active tectonics, paleoseismology, and seismic hazards of the Hol-lywood fault, northern Los Angeles basin, California, Geol. Soc. Am.Bull. 109, 1595–1616.

Dolan, J. F., K. Sieh, and T. K. Rockwell, (2000). Late Quaternary activityand seismic potential of the Santa Monica fault system, Los Angeles,California, Geol. Soc. Am. Bull. 112, 1559–1581.

Eguchi, R. T., J. D. Goltz, C. E. Taylor, S. E. Chang, P. J. Flores, L. A.Johnson, H. A. Seligson, and N. C. Blais (1998). Direct economiclosses in the Northridge earthquake: a three-year post-event perspec-tive, Earthquake Spectra 14, 245–264.

Erslev, E. A. (1991). Trishear fault-propagation folding, Geology 19, 617–620.

Hauksson, E. (1990). Earthquakes, faulting and stress in the Los AngelesBasin, J. Geophys. Res. 95, 15365–15394.

Hauksson, E., and L. M. Jones (1989). The 1987 Whittier Narrows Earth-quake sequence in Los Angeles, southern California: seismologicaland tectonic analysis, J. Geophys. Res. 94, 9569–9589.

Heaton, T. H., J. F. Hall, D. J. Wald, and M. W. Halling (1995). Responseof high-rise and base-isolated buildings to a hypothetical Mw 7.0blind-thrust earthquake, Science 267, 206–210.

King, G. C. P., and C. Vita-Finzi (1981). Active folding in the Algerianearthquake of 10 October 1980, Nature 292, 22–26.

Medwedeff, D. A. (1992). Geometry and kinematics of an active, laterallypropagating wedge thrust, Wheeler Ridge, California, in StructuralGeology of Fold and Thrust Belts, S. Mitra and G. Fisher (Eds.) JohnHopkins University Press, Baltimore, 3–28.

Myers, D. J. (2001). Structural geology and dislocation modeling of theeast Coyote anticline, Eastern Los Angeles basin, M.S. Thesis,Oregon State University, 49 pp.

Oskin, M., K Sieh, T. Rockwell, G. Miller, P. Guptill, M. Curtis, S. McAr-dle, and P. Elliot (2000). Active parasitic folds on the Elysian Parkanticline: Implications for seismic hazard in central Los Angeles,California, Geol. Soc. Am. Bull. 112, no. 5, 693–707.

Pratt, T. L., J. H. Shaw, J. F. Dolan, S. Christofferson, R. A. Williams, J. K.Odum, and A. Plesch (2002). Shallow folding imaged above the Pu-ente Hills blind-thrust fault, Los Angeles, California, Geophys. Res.Lett. (in review).

Schneider, C. L., C. Hummon, R. S. Yeats, and G. Huftile (1996). Structuralevolution of the northern Los Angeles basin, California, based ongrowth strata, Tectonics 15, 341–355.

Scientists of the U.S. Geological Survey and the Southern California Earth-quake Center (1994). The magnitude 6.7 Northridge, California,Earthquake of 17 January 1994, Science 266, 389–397.

Shaw, J. H., and P. M. Shearer (1999). An elusive blind-thrust fault beneathmetropolitan Los Angeles, Science 283, 1516–1518.

Shaw, J. H., and J. Suppe (1994). Active faulting and growth folding in theeastern Santa Barbara Channel, California, Geol. Soc. Am. Bull. 106,607–626.

Shaw J. H., and J. Suppe (1996). Earthquake hazards of active blind-thrustfaults under the central Los Angeles basin, California, J. Geophys.Res. 101, 8623–8642.

Shearer, P. M. (1997). Improving local earthquake locations using the L1norm and waveform cross correlation: Application to the WhittierNarrows, California, aftershock sequence, J. Geophys. Res. 102,8269–8283.

Stein, R. S., and G. Ekstrom (1992). Seismicity and geometry of a 110-km-long blind thrust fault, 2, Synthesis of the 1982–85 California Earth-quake Sequence, J. Geophys. Res. 97, 4865–4883.

N5 km

B

405

600

500

San Gabriel Fault

Pacific Ocean900

LA

downtownLos Angeles

Brea

400000 420000

= upper tip line of PHT ramp

California

37800003760000

B1

B2

S.l.

5

10

15

2 km

Tp

Qt

Tfl

TfuA T

middle Miocene& older

B2B1B

MFWF

depth(km

)

PHT

PHT

1987 M6.0

1987 M6.0

15

55

10 km

5

10

15

15

5

10 km

34°

118°

Shaw et al. (2002)(A) Map of three main structural segments of the Puente Hills Thrust (PHT). From west to east these are the Los Angeles segment, the Santa Fe Springs segment, and the Coyote Hills segment, which itself comprises two distinct sub-segments.(B) Cross section based on petroleum-industry seismic relfection data (to 7 km depth) and mainshock and relocated aftershocks of the 1987 Mw 6.0 Whittier Narrows earthquake (Shaw & Shearer, 1999 Science) from 11–15 km depth).

Evidence for large Holocene earthquakes on the Compton thrust

fault, Los Angeles, California

Lorraine A. Leon,1 James F. Dolan,1 John H. Shaw,2 and Thomas L. Pratt3

Received 30 September 2008; revised 28 May 2009; accepted 21 July 2009; published 16 December 2009.

[1] We demonstrate that the Compton blind thrust fault is active and has generated at leastsix large-magnitude earthquakes (Mw 7.0–7.4) during the past 14,000 years. This large,concealed fault underlies the Los Angeles metropolitan area and thus poses one of thelargest deterministic seismic risk in the United States. We employ a methodology that usesa combination of high-resolution seismic reflection profiles and borehole excavations tolink blind faulting at seismogenic depths directly to near-surface fault-related folding.Deformed Holocene strata record recent activity on the Compton thrust and are marked bydiscrete sequences that thicken repeatedly across a series of buried fold scarps. Weinterpret the intervals of growth as occurring after the formation of now-buried paleofoldscarps that formed during uplift events on the underlying Compton thrust ramp. Minimumuplift in each of the scarp-forming events, which occurred at 0.7–1.75 ka (event 1),0.7–3.4 ka or 1.9–3.4 (event 2), 5.6–7.2 ka (event 3), 5.4–8.4 ka (event 4), 10.3–12.5 ka(event 5), and 10.3–13.7 ka (event 6), ranged from �0.6 to �1.9 m, indicating minimumthrust displacements of �1.3 to 4.2 m. Such large displacements are consistent with theoccurrence of large-magnitude earthquakes (Mw � 7). This multidisciplinary methodologyprovides a means of defining the recent seismic behavior, and therefore the hazard, forblind thrust faults that underlie other major metropolitan regions around the world.

Citation: Leon, L. A., J. F. Dolan, J. H. Shaw, and T. L. Pratt (2009), Evidence for large Holocene earthquakes on the Compton thrust

fault, Los Angeles, California, J. Geophys. Res., 114, B12305, doi:10.1029/2008JB006129.

1. Introduction

[2] The recent occurrence of several highly destructiveearthquakes on thrust faults (e.g., 1994 Mw 6.7 Northridge,California, 1999 Mw 7.6 Chi-Chi, Taiwan, 2005 Mw 7.5Kashmir, Pakistan, and 2008 Mw 7.9 Wenchuan, China)highlights the need to better understand the seismic hazardsposed by these structures to an increasingly urbanizedglobal population. The keys to this understanding are therecognition of the rate at which these faults store and releaseseismic energy, and the acquisition of detailed paleoseismo-logical data on the ages and displacements of ancientearthquakes that they have generated. Although such dataare routinely documented for many faults around the world,one class of faults, the so-called ‘‘blind thrust faults,’’ inwhich the fault plane does not extend all the way to theEarth’s surface, has proved particularly difficult to study.[3] Unlike most faults, which rupture to the surface in

large earthquakes, near-surface deformation above blindthrust faults is accommodated by folding, rather than fault-ing. In many thrust fault earthquakes, coseismic fold growth

has been documented [King and Vita-Finzi, 1981; Stein andKing, 1984; Stein and Yeats, 1989; Lin and Stein, 1989; Laiet al., 2006; Chen et al., 2007; Streig et al., 2007] and invery large thrust fault earthquakes, discrete coseismic foldscarps have developed above bends and at the tips ofpropagating faults [Stein and Yeats, 1989; Ishiyama et al.,2004, 2007; Lai et al., 2006; Chen et al., 2007; Streig et al.,2007]. The youngest folded strata thus contain the informa-tion necessary to understand the recent seismic behavior ofthese faults. However, extracting paleoseismological datafrom blind thrusts has proved to be challenging, althoughthe development over the past several years of new inter-disciplinary methodologies for documenting the sizes andages of past events is beginning to yield a new understand-ing of the behavior of this problematic class of faults at thetime scales of individual earthquakes [e.g., Dolan et al.,2003; Sugiyama et al., 2003; Ishiyama et al., 2004, 2007;Leon et al., 2007]. The difficulties in studying the paleo-seismology of blind thrusts are particularly well illustratedby the Compton thrust, a major blind thrust fault beneathmetropolitan Los Angeles. Although this fault was declaredto be inactive on the basis of earlier studies [Mueller, 1997],our investigations demonstrate that the Compton fault isindeed active and capable of generating large-magnitude(Mw � 7) earthquakes. In this article we discuss theimplications of these results for seismic hazard assessmentin Los Angeles and describe a standard, multidisciplinarymethodology that can be used to define seismic hazards

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, B12305, doi:10.1029/2008JB006129, 2009ClickHere

for

FullArticle



3U.S. Geological Survey, University ofWashington, Seattle,Washington,USA.


B12305 1 of 14

Compton ramp

Act ive axialsurface

LA

PuenteHills

thrust

N. Compton study site

PHTLA segment

study site

PHT SFSsegment study site

ical withdrawal of water and petroleum, the ground surfacenow dips slightly (0.06�) to the north. The gentle, near-horizontal regional slope at the study site has resulted in thedeposition of laterally extensive sedimentary layers that canbe correlated over large distances.[8] As is important with all paleoseismic studies, we

chose a site characterized by continuous, relatively rapidsediment accumulation, leading to stratigraphic separationof event horizons and preservation of event stratigraphy. Wespecifically chose a study site in which the uplifted hangingwall exhibited no topographic expression in order to ensurethat our site was located in an area of continuous aggradation.The random occurrence within any floodplain of topographicfeatures would serve to decrease the consistency of strati-graphic growth patterns. Moreover, historical evidencereveals that the floodplain was so low gradient, that duringrainy winters, the entire Los Angeles Basin (>20 km wide)was a marsh or shallow lake with standing surface water[Gumprecht, 2001; Linton, 2005].[9] We excavated a north-south transect of ten 25 to 35 m

deep, continuously cored boreholes across the zone ofyoung folding imaged in the high-resolution seismic reflec-tion profiles. Excavation of boreholes allows us to docu-ment the details of the most recent folding events, as well asto sample the sediments for age control. The cores revealsequences of fluvial sand and gravel, interbedded withcohesive overbank silt and clay layers (Figure 4; auxiliary

material1 Figure S1). Grain size was visually identified usinga handheld GSA grain size card, and we assigned soil colorson wet material using a Munsell color chart.[10] Deposition of these units in a low-gradient floodplain

has resulted in remarkable lateral continuity; more than25 units are traceable continuously across the entire 760 mlength of the borehole transect. Calibrated radiocarbon datesfrom 30 humic, charcoal, and bulk soil samples indicate thatsediment accumulation has been relatively continuous at�2 mm/yr over the past 14,000 years (Figure 5). Thesediment accumulation reveals no major hiatuses and thefew soils observed in the section consist of weakly devel-oped A–C profiles that record brief periods of pedogenesis.We see exceptionally well defined growth in exactly thesame location throughout the stratigraphic section. Thisargues strongly that we are seeing a tectonic signal and thatany natural variation of the floodplain is not a significantsource of error. We can also directly demonstrate throughoverlapping radiocarbon dates that at least some of theseunits are synchronous along the length of the transect. Forexample, three dates from the shallowest of several con-tinuous marsh deposits (unit 11, organic rich soil beneathunit 10) yield overlapping radiocarbon dates that confirmsimultaneous development across the transect of this layer.

Figure 4. (a) Borehole results from the Stanford Avenue transect. Cross section of major stratigraphicunits (16� vertical exaggeration) showing the details of the most recent uplift event (event 1). Thinvertical lines are boreholes. Green horizontal line is the present ground surface. (b) Cross section of majorstratigraphic units (8� vertical exaggeration). Colors denote different sedimentary units. Thin red linesmark the tops of major sand and gravel units. Double-headed red vertical arrows along the right sideshow the stratigraphic ranges of intervals of sedimentary thickening across the transect, with the uplift ineach event shown to the right of each arrow. Double-headed green vertical arrows show intervals of nosedimentary growth.


B12305 LEON ET AL.: HOLOCENE EARTHQUAKES ON COMPTON THRUST

5 of 14

B12305

petroleum industry seismic reflection profiles provide anexceptionally clear image of the folding above the Comptonthrust ramp beneath the site. In particular, these data showthat the back-limb axial surface, the locus of foldingassociated with fault slip across the base of the Comptonramp at depth, exhibits an upward narrowing growthtriangle that extends to within �200 m of the surface [Shawand Suppe, 1996]. This fault bend fold, referred to as theCompton–Los Alamitos trend, marks the base and north-eastern extent of the Compton ramp. Our study site is

located along this trend on a previously marshy, extremelylow-gradient floodplain, facilitating reconstruction of orig-inally near-horizontal floodplain surfaces and allowingcollection of numerous organic-rich samples for radiocar-bon dating. It is also far removed from structural interferenceassociated with other active faults, such as the Newport-Inglewood right-lateral strike-slip fault (Figure 2). Twentiethcentury topographic maps of the site show the groundsurface as near horizontal, with an extremely gentle dip of0.2� to the south. However, presumably because of histor-

Figure 3. Compton fault seismic reflection data illustrating the multidisciplinary methodology utilizedin this study [Pratt et al., 2002; Shaw et al., 2002; Dolan et al., 2003] (see also similar studies bySugiyama et al. [2003] and Ishiyama et al. [2004, 2007]). (a) Hammer source seismic reflection profile(migrated; 8� vertical exaggeration). The prominent reflector at 10–17 m depth shows the kink bandclearly. Thin, vertical, white lines show borehole locations. Dashed white lines represent active, synclinaland inactive, anticlinal axial surfaces. Black box indicates location of borehole transect cross section inFigure 4. A–A0 marks the trace of the seismic reflection profile in Figure 2. (b) Weight drop sourceseismic reflection profile (migrated). The prominent reflector at 120–200 m depth shows the kink bandwell. The subhorizontal reflector at about 100 m depth is likely the water table, as velocities above thereflector are consistent with unsaturated strata. The reflector also cuts across the geologic structureconsistently defined by the underlying reflectors and overlying drill hole data, as would be expected forthe water table. Thin, vertical, white lines show borehole locations. Dashed white lines represent active,synclinal and inactive, anticlinal axial surfaces. Black box indicates location of hammer source profileshown in part A. B–B0 marks the trace of the seismic reflection profile in Figure 2. (c) Seismic reflectionimage of the back-limb fold structure showing upward narrowing zone of active folding (growth triangle)delimited by sharply defined axial surfaces [Shaw and Suppe, 1996]. Dashed white lines represent active,synclinal and inactive, anticlinal axial surfaces. Black box indicates location of weight drop source profileshown in part B. C–C0 marks the trace of the seismic reflection profile in Figures 1 and 2. (d) Fault bendfold model of back-limb fold structure of the Compton fault with discrete axial surface yielding anupward narrowing fold limb in growth strata (modified after Shaw and Suppe [1996]).

B12305 LEON ET AL.: HOLOCENE EARTHQUAKES ON COMPTON THRUST

4 of 14

B12305

seismological society of america annual meeting fieldtrip ...community fault model (cfm). san...

Documents