earthquake hazards of southwestern utahgeode.colorado.edu/~structure/teaching_geol4721/5....

Utah Geological Association Publication 21 123

EARTHQUAKE HAZARDS OF SOUTHWESTERN UTAH

GARY E. CHRISTENSON1 AND SUSAN J. NAVA2

ABSTRACT

Parts of southwestern Utah lie within the southern Intermountain seismic belt (ISB), a zone of shallow, diffuse seismicity which in southwestern Utah trends from Scipio southwest through Richfield, Cedar City, and St. George. Historical earthquakes have reached magnitude 6 to 6'A, mostly near Richfield and Elsinore. Geologic studies indicate that late Quaternary deformation (faults, folds, uplift) has occurred in southwestern Utah, and many potentially seismogenic geologic structures are present. Prehistoric earthquakes in the magnitude 7 to 7'/4 range have occurred throughout the area and could occur in the future.

The principal earthquake hazards in southwestern Utah are ground shaking, surface fault rupture, tectonic subsidence, liquefaction, slope failure, and flooding. Strong ground shaking is the most widespread and frequently occurring of these hazards. The strongest shaking is expected within the ISB in the eastern and southern parts of the study area. Faults potentially capable of rupturing the surface, with accompanying tectonic subsidence and strong ground shaking, are found throughout the area, but are concentrated in the ISB. Areas of highest liquefaction potential are along stream-valley bottoms, particularly along the Sevier and Virgin Rivers. Slope failures, including rock falls and landslides, and flooding of various types, may also accompany moderate to large earthquakes in the area.

INTRODUCTION

Southwestern Utah, particularly the Richfield-Elsinore area, has historically been one of the most seismically active parts of the state. However, because many of the larger historical earthquakes occurred in the early 1900s, prior to most urbanization, cumulative damage to date has been relatively slight. Although the potential for large, damaging earthquakes in the area is well known, much work is still needed to further define earthquake sources and to quantify earthquake probabilities.

The purpose of this paper is to summarize our present knowledge of earthquake occurrence and hazards in southwestern Utah, based on a compilation and review of existing information. Relevant information on Quaternary faulting and earthquake hazards has recently been compiled for the central Sevier Valley (Anderson and Barnhard, 1987), western Utah (Christenson and others, 1987), extreme southwestern Utah (Anderson and Christenson, 1989), and for all of Utah (Hecker, in preparation; Christenson, in preparation). These studies have led to a more complete understanding of faulting and earthquake hazards in southwestern Utah and have also identified the need for much more detailed work.

'Utah Geological Survey, 2363 South Foothill Drive, Salt Lake City, UT 84109-1491

2University of Utah Seismograph Stations, Department of Geology and Geophysics, 705 WBB, University of Utah, Salt Lake City, UT, 84112

SEISMICITY

The southwestern Utah (SWU) region (defined here as the area lying between 36° 45' and 40° North latitude and between 112° and 114° 15' West longitude; figure 1) includes part of the southern portion of the Intermountain seismic belt (ISB), a 100-200-km- (60-120-mi-) wide zone of mostly shallow ( < 20 km [12 mi]) seismicity, which extends from southern Nevada and northern Arizona to northwestern Montana (Smith and Arabasz, 1991). Although the overall trend of the ISB is north-south, locally in the SWU region the ISB trends northeast-southwest, following the tectonic transition zone between the Basin and Range province on the northwest and the Colorado Plateau province on the southeast (figure 1).

Figure 2, based on data from the University of Utah Seismograph Stations earthquake catalog, shows the epicenters of approximately 2,300 earthquakes of magnitude 2.0 and greater which have occurred in the SWU region from 1850 through 1991. The locations and magnitudes of the so-called "historical" earthquakes—earthquakes occurring before July 1962 (squares, figure 2)—are based primarily on felt reports rather than instrumental data. Non-instrumental locations of historical earthquakes were usually assigned arbitrarily to the city or town where the felt effects were strongest, and they have an estimated epicentral accuracy of ± 25-50 km (16-31 mi) (Arabasz and McKee, 1979). Beginning in July 1962, modern instrumental earthquake locations (circles, figure 2) and magnitudes were calculated using data collected by a sparse statewide seismograph

Engineering and Environmental Geology of Southwestern Utah, 1992Copyright © 2012 Utah Geological Association

124 Engineering and Environmental Geology of Southwestern Utah

Figure 1 . M a p of U t a h s h o w i n g t h e s o u t h w e s t e r n U t a h region a n d phys iographic prov inces ( f r o m S t o k e s , 1 9 7 7 ) .

network (see Arabasz and others, 1979, 1987; Smith and Arabasz, 1991). The location accuracy improved in the late 1970s and early 1980s with the deployment of a more extensive statewide network of telemetered seismic stations (Arabasz and others, 1979, 1987) in the SWU region. Most of the earthquakes that occurred after densification of the seismograph network in the SWU region probably have a location accuracy of + 3 km (2 mi), but errors as large as ± 10 km (6 mi) cannot be ruled out for those events occurring at the fringes of the main seismic belt (Arabasz and Julander, 1986). Currently, the University of Utah Seismograph Stations operates ten seismograph stations in the SWU region, eight of which are located in the Basin and Range—Colorado Plateau transition zone. Station spacing ranges from about 30 to 120 km (19 to 75 mi). The earthquake catalog for the SWU region is estimated to be systematically complete (after 1981) above magnitude 2.5 (Nava and others, 1990).

Table 1 lists earthquakes with measured or estimated magnitudes of 4 and larger that have occurred in the SWU region between 1850 and 1991. Since 1850, the SWU region has been shaken by four earthquakes with estimated magnitudes of 6 to 6V4, three of which were located in or

near the Sevier Valley. Smith and Arabasz (1991) judged the 1901 Richfield earthquake (magnitude 6'A) to be the second largest historical earthquake in the southern ISB (south of 42° 15' North latitude). The exact location and magnitude of the 1901 event are not well known. The earthquake was felt over 130,000 km2 (50,200 mi2), and had a maximum Modified Mercalli intensity (MMI) of IX. The shock caused substantial damage in several towns, and ground cracks (no surface faulting), liquefaction, and extensive rock slides were observed locally (Williams and Tapper, 1953). In 1902, an earthquake centered in Pine Valley (33 km [20.5 mi] north of St. George) caused substantial damage in southwestern Utah. Williams and Tapper (1953) estimated a maximum MMI of VIII for the Pine Valley earthquake. In late 1921, after two and a half weeks of foreshock activity, the Elsinore area (10 km [6 mi] southwest of Richfield) was shaken by two damaging earthquakes each of about magnitude 6 '4, separated by 50 hours, with an intervening shock of 5 (Pack, 1921; table 1). Arabasz and Julander (1986) observed that a similar pattern of paired main shocks has occurred in two other instances close to Elsinore: in 1910 (two shocks of magnitude 5.0 [MMI VI] within 38 hours) and in 1972 (shocks of magnitude 4.4 and 4.0 within six months).

The map pattern of figure 2 suggests locally dense clustering of small- to moderate-sized earthquakes superimposed on a background of diffusely scattered seismicity lying mostly within the Basin and Range—Colorado Plateau transition zone. Earthquake swarm activity, the clustering of earthquakes of similar size in space and time without an outstanding mainshock, has been commonly observed in the SWU region (see Williams and Tapper, 1953; Smith and Sbar, 1974; Olson, 1976; Arabasz and McKee, 1979; Arabasz and Smith, 1979; Richins and others, 1981b; Zandt and others, 1982). These earthquake swarms typically have maximum magnitudes in the 3 to 4 range and tend to occur in or near areas of Quaternary volcanism (Arabasz and Julander, 1986).

On a regional scale, both the Quaternary faults and the earthquake epicenters trend northeast-southwest and are primarily concentrated within the Basin and Range—Colorado Plateau transition zone. Inadequate focal-depth resolution prohibits drawing any conclusions about the relationship between Quaternary faulting and localized seismicity patterns. Arabasz and Julander (1986), using data obtained through special portable-seismograph studies in central and southwestern Utah, were able to discriminate depth-vaiying patterns of locally intense upper-crustal seismicity (6-8 km [4-6 mi] deep), perhaps separated by low-angle structural discontinuities from deeper, but less frequent background earthquakes (down to about 15 km [9 mi]). According to Arabasz and Julander (1986), rupture pathways have yet to be identified between existing surface fault scarps in the region and deep nucleation points at about 15 km (9 mi) depth for large surface-faulting earthquakes (magnitude 6.5 and larger) based on observations elsewhere in the ISB (Doser, 1985;


• • 1 I I I I • I i I • I • I

3 9 ° -

3 8 ° -

1966 M, 5.7

37

N

114 113 112

Figure 2 . Earthquakes in the southwestern Utah region since 1 8 5 0 . Circles indicate instrumental locations for the period July 1 9 6 2 through December 1 9 9 1 . based on data from the University of Utah Seismograph Stations (Arabasz and others, 1 9 7 9 ; Richins and others, 1 9 8 1 a ; Richins and others, 1 9 8 4 ; Brown and others, 1 9 8 6 ; Nava and others, 1 9 9 0 ; Nava and others, in preparation). Squares indicate primarily non-instrumental locations for earthquakes occurring during the time period 1 8 5 0 through June 1 9 6 2 (Arabasz and others, 1 9 7 9 ) . Epicenters for all shocks wi th an estimated magnitude of 5 . 5 or larger occurring since 1 8 5 0 are labeled.


Table 1. Magnitude 4.0 and larger earthquakes in the southwest Utah region1. 1850-1991

Date (UTC) Location2 Latitude (N)2 Longitude (W)2 Magnitude3 MMI4

1873 Jul 31 Beaver 38° 16.74' 112° 38.38 ' (4.3) 5-6

1878 Aug 14 16 miles SSW of Kanosh 38° 36 ' 112° 34.80 ' (4.3) 5

1881 Mar 26 Hebron, 5 miles W of Enterprise 37° 35 ' 113° 48 ' (4.3) 5

1887 Dec 05 Kanab 37° 2 .84 ' 112° 31.34 ' (5.7) 6-7

1891 Apr 20 St. George 37° 6 .38 ' 113° 34.41 ' (5.0) 6

1894 Jan 08 44 miles WSW of Delta 39° 45.6 ' 113° 23.4 ' (4.3) 5

1900 Aug 01 Eureka 39° 57.15 ' 112° 6.84 (5.7) 7

1901 Nov 14 Richfield 38° 46.15 ' 112° 5 .02 ' 6V4* 8-9

1902 Jul 31 Beaver 38° 16.74' 112° 38.38 ' (4.3) 5

1902 Nov 17 Pine Valley 37° 23.58 ' 113° 31.20 ' (6.3) 7-8

1902 Dec 05 Pine Valley 37° 23.68 ' 113° 31.20 ' (5.0) 6

1903 Nov 23 St. George 37° 6 .38 ' 113° 34.41 ' (4.3) 5

1908 Apr 15 Milford 38° 23.58 ' 113° 00.44 ' (5-0) 6

1910 Jan 10 Elsinore 38° 40.97' 112° 8.98' (5.0) 6-7

1910 Jan 12 Elsinore 38° 40.97' 112° 8 .98 ' (5.0) 6

1914 Dec 14 Enterprise 37° 34.37' 113° 42.79' (4.3) 5

1920 Nov 26 St. George 37° 6 .38 ' 113° 34.41 ' (4.3) 5-6 1921 Sep 12 Richfield 38° 46.15 ' 112° 5 .02 ' (4.3) 4-5

1921 Sep 13 Richfield 38° 46.15' 112° 5 .02 ' (4.3) 5 1921 Sep 29 Elsinore 38° 40.97 ' 112° 8.98' (6.3) 8 1921 Sep 30 Elsinore 38° 40.97 ' 112° 8.98' (5.7) 7 1921 Oct 01 Elsinore (second main shock) 38° 40.97 ' 112° 8.98' (6.3) 8 1921 Oct 27 Elsinore 38° 40.97 ' 112° 8.98' (4.3) 5-7 1923 May 14 Nada, 17 miles WSW of Minersville 38° 10' 113° 14' (4-3) 5 1933 Jan 20 Parowan 37° 50.52 ' 112° 49.63 ' (5.0) 6 1936 May 09 5 miles NNE of Springdale 37° 15' 112° 57.48 ' (4.3) 5-6 1936 Sep 21 23 miles NW of Enoch 38° 00 ' 113° 18' 4 .7 5 1937 Feb 18 Panguitch (swarm) 37° 49.36 ' 112° 26.02' (4.3) 4-5 1942 Aug 30 Cedar City (swarm) 37° 40.97' 113° 3.95' (5.0) 6 1942 Sep 26 Cedar City (swarm) 37° 40.97' 113° 3.95' (4-3) 5 1942 Sep 26 Cedar City (swarm) 37° 40.97' 113° 3 .95 ' (5.0) 6 1943 Jan 16 Cedar City (swarm) (IL) 37° 40.97 ' 113° 3 .95 ' (4.3) 4-5 1943 Nov 03 4 miles SSW of Joseph 38° 34.8 ' 112° 15.6' (4.3) 5 1949 Nov 02 St. George 37° 6 .38 ' 113° 34.41 ' 4 .7 6 1953 Oct 22 Panguitch 37° 49.36 ' 112° 26.02 ' (4.3) 5 1959 Feb 27 13 miles NNW of Panguitch (IL) 38° 00 ' 112° 30 ' (5-0) 6 1959 Jul 21 3 miles SSE of Kanab (IL) 37° 00 ' 112° 30 ' M L 5.7** 6 1962 Feb 15 12 miles SE of Kanab (IL) 36° 54 ' 112° 24 ' 4 .5 5 1966 Aug 16 25 miles WSW of Enteiprise 37° 27.81 ' 114° 9.07' M l 5 .7 5-6 1966 Aug 19 28 miles WSW of Enterprise 37° 26.33 ' 114° 11.48' M l 4 .7 F 1966 Oct 21 13 miles WSW of Minersville 38° 11.74' 113° 9 .45 ' M l 4 .2 1967 Oct 04 7 miles S of Monroe 38° 32.59 ' 112° 9 .39 ' M l 5 .2 7 1972 Jan 03 2 miles W of Monroe 38° 38.81 ' 112° 9.91' M l 4 .4 6 1972 Jun 02 3 miles S of Annabella 38° 40.27 ' 112° 4 .32 ' M l 4 .0 5 1981 Apr 05 8 miles W N W of Kanarraville 37° 35.49 ' 113° 17.87' M l 4.6 5 1982 May 24 1 mile ENE of Annabella 38° 42.50 ' 112° 02.19 ' M l 4 .0 6 1986 May 24 6 miles E of Scipio 39° 14.04' 112° 00.37 ' M l 4 .4 5***

1 Defined as the area between latitude 36° 45 ' and 40° N, and between longitude 112° and 114 ° 15' W. 2 Locations for earthquakes occurring between 1850 and June 1962 are approximate. All but four are based upon felt information and assigned to cities and towns where the felt intensity was strongest (Arabasz and McKee, 1979). The locations of the other four earthquakes occurring during this time period, identified by (IL), were based upon instrumental data. Locations for earthquakes occurring after July 1962 are based upon data from the University of Utah regional seismograph network. 3 Magnitudes enclosed in parenthesis are based upon a conversion from felt area to local magnitude (Arabasz and McKee, 1979). T h e magnitude for this earthquake is listed as 7.0 in the Arabasz and McKee (1979) earthquake catalog, however, based on the judgment of Arabasz and others (1987), the value was revised to 6'A. ** Value is from Smith and Arabasz (1991). is local magnitude. 4 Maximum Modified Mercalli intensity (MMI-shown in Arabic rather than Roman numerals) values for events through 1949 are f rom Williams and Tapper (1953). For earthquakes occurring after 1949, MMI values are from the annual publication, U.S. Earthquakes (U.S. Department of Commerce). F: this earthquake is listed in U.S. Earthquakes, 1966 as felt, but was not assigned an intensity. *** Value f rom U.S. Geological Survey (1986).


Smith and Arabasz, 1991). Focal-mechanism studies by Arabasz and Julander (1986) and Bjarnason and Pechmann (1989), for areas of Utah that include portions of the SWU region, indicate that seismic slip predominates on fault segments of moderate to steep dip ( > 30°). Despite geologic observations of low-angle faults in the study area, no convincing evidence has yet been found for seismic slip on either a downward-flattening or low-angle normal fault.

QUATERNARY FAULTS AND FOLDS

Many Quaternary-age faults and folds, possibly capable of generating magnitude 6.0 to 6.5 or larger earthquakes, are found in southwestern Utah. Figure 3 is modified from a compilation of Quaternary-age geologic structures in Utah by Hecker (in preparation), and shows faults and folds with evidence of most recent movement in either (1) Holocene-latest Pleistocene time (0-30,000 years ago) or (2) Pleistocene and suspected Quaternary time (10,000 years-1.6 million years ago). Detailed paleoseismic studies have generally not been performed for most faults in southwestern Utah to characterize the time of most recent surface rupture, amount of offset per event, average recurrence interval, slip rate, and segmentation. Data that are available are summarized by Anderson and Christenson (1989) and by Hecker (in preparation).

No surface-faulting earthquakes have occurred in historical time in southwestern Utah, although some faults have evidence for late Holocene rupture. The youngest scarps appear to be those about 2,000-3,000 years old along the Scipio Valley fault (Bucknam and Anderson, 1979a; Oviatt, 1990). Scarps of probable early Holocene age, , perhaps about 9,000 years old (Pierce and Colman, 1986), are found on the Drum Mountains faults (Bucknam and Anderson, 1979b; Sterr, 1985) (figure 4). Other principal faults with evidence for probable post-Bonneville (Holocene-latest Pleistocene) surface rupture are the Cricket Mountains, House Range, Snake Valley, Clear Lake, Tabernacle, Little Valley, Parowan Valley, Washington, and some Beaver Basin faults (figure 3; Hecker, in preparation). However, there is little conclusive evidence for multiple Holocene-latest Pleistocene surface rupture on any faults in southwestern Utah.

Some major range-front faults such as the Hurricane, Paragonah, and Sevier faults lack evidence for Holocene surface rupture, but have clear evidence for recurrent surface rupture during late Quaternary time. Relatively high long-term (late Quaternary) slip rates on these faults of 0.30 to 0.47 mm/year (0.012 to 0.019 in/year) seem to contradict the lack of evidence for Holocene-latest Pleistocene surface rupture (Anderson and Christenson, 1989). Assuming an average displacement of 1-2 m (3-6 ft) per event, an average recurrence interval of less than 10,000 years is implied by slip rates on these faults, yet no firm evidence for Holocene-

latest Pleistocene offset has yet been found. Little is known of average recurrence intervals for most

faults in southwestern Utah, but the lack of evidence for Holocene-latest Pleistocene movement on many of the faults indicates that recurrence intervals are greater than 10,000-30,000 years for most major structures. Studies of segmentation on these faults have not yet been carried out. Many of the faults are likely segmented, although the generally poor preservation of evidence for most recent events and relatively great elapsed time since most recent events make studies of segmentation difficult.

Quaternary deformation other than faulting is present in southwestern Utah. Anticlines, monoclines, tilted stream terraces, and domal uplifts occur in several areas, usually associated with faults (figure 3). The largest such structures are the Elsinore "fault" (actually a monocline which may overlie a buried fault) along the west edge of the Sevier Valley near Richfield (Anderson and Barnhard, 1987), and the Cedar City-Parowan monocline, which connects the Hurricane and Paragonah faults and may also overlie a buried fault (Anderson and Christenson, 1989). Smaller, faulted anticlinal folds occur: (1) near Beaver in Quaternary alluvium (Machette and others, 1984; Machette, 1985), (2) east of Panguitch in Quaternary alluvial fans and late Tertiary Sevier River Formation and basalt, on the hanging wall of the Sevier fault (Anderson and Christenson, 1989), and (3) in the North Hills south of Cedar City in 1 million-year-old basalt on the hanging wall of the Hurricane fault (Anderson and Christenson, 1989). Warping of the Bonneville shoreline east of the axis of the Cove Creek Dome (shown as Pleistocene-suspected Quaternary in figure 3) west of Cove Fort may indicate possible Holocene-latest Pleistocene uplift in the area (Oviatt, 1991). A series of stream terraces showing increasing tilt with age (Callaghan and Parker, 1961) along the Sevier River near Elsinore indicates uplift of 0.1-1.0 mm/yr (0.004-0.04 in/yr) (Anderson and Barnhard, 1987). Modern stream-channel-pattern anomalies in the Sevier River, perhaps indicating recent uplift, are found south of the Drum Mountains faults (Anderson and Barnhard, 1987), near Elsinore (Anderson and Barnhard, 1987) and east of Panguitch (D.W. Jorgensen, Colorado State University, written communication to R. E. Anderson, 1988).

The seismogenic potential of faults, folds, and other structures in southwestern Utah is not well known. Faults in the Sevier/Black Rock Desert area, such as the Clear Lake, Pavant, Beaver Ridge, and Drum Mountains faults, are thought to merge at shallow depths (generally less than 5 km [3 mi]) with underlying low-angle detachments, or to be surface projections of these detachments (Allmendinger and others, 1983; Anderson and others, 1983; Crone and Harding, 1984). Other faults to the west such as the House Range and Snake Valley faults also intersect low-angle detachments at shallow depths (Allmendinger and others, 1983; Smith and Bruhn, 1984). The Mineral Mountains, San Francisco Mountains, and Beaver Basin faults intersect


39° N.

38° N

114° W. ARIZONA 113° W

u 10 20 30 40 Miles I r -H H—,—h H 0 10 20 30 40 50 60 Kilometers

Scale

N 1! 2C W.

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EXPLANATION

Faults Holocene-latest

Pleistocene Pleistocene-

suspected Quaternary

BB - Beaver Basin BR - Beaver Ridge CL - Clear Lake CM - Cricket Mountains DM - Drum Mountains H - Hurricane HR - House Range LV - Little Valley MM - Mineral Mountains PR - Paragonah PV - Pavant PW - Parowan S - Sevier SF - San Francisco Mts SN - Snake Range SV - Scipio Valley T - Tabernacle W - Washington

Anticlines-Uplifts Pleistocene-

/ suspected Quaternary

CC - Cove Creek JF - Joseph Flats NH -North Hills P - Panguitch

Monoclines Holocene-latest

Pleistocene Pleistocene-

CP E

suspected Quaternary

- Cedar City-Parowan - Elsinore

Figure 3. Quaternary faults and folds of southwestern Utah (modified from Hecker, in preparation).

f


Ground Shaking

Figure 4. Drum Mountains fault scarp, northeast of Delta (photo by Suzanne Hecker, 1991).

subhorizontal detachments at greater depths of about 10 km (6 mi) (Smith and Bruhn, 1984). Surface rupture on these faults may occur in response to movement on the underlying detachments. Currently, we do not known whether movement on these underlying, regionally extensive detachments can generate moderate to large earthquakes. Little is known of the subsurface geometry of other major faults in southwestern Utah, but for purposes of seismic-hazard evaluation at this time, all are considered to be potentially seismogenic.

Major folds such as the Elsinore monocline and the Cedar City-Parowan monocline may be surface expressions of deep faults capable of generating large earthquakes. Other folds such as those cut by numerous axial faults in the Panguitch, Beaver, and Cedar City (North Hills) areas may have formed in association with movement on major range-front faults and may themselves not be seismogenic. The origins of other structures such as the Cove Creek Dome and Joseph Flats uplift remain unknown, as does their potential to generate moderate to large earthquakes.

EARTHQUAKE HAZARDS

Moderate to large earthquakes, generally magnitude 5 and greater, can cause a variety of hazards. Foremost among these is strong ground shaking, which is the most widespread and damaging of the earthquake hazards. Other hazards include surface faulting, tectonic subsidence, liquefaction, slope failure, and flooding.

Historically, earthquakes with MMIs of VI and greater (generally magnitude 5.0 and greater) have occurred in the ISB in southwestern Utah, with the largest centered in the Richfield-Elsinore area (1901, 1921) and north of St. George in the Pine Valley Mountains (1902) (see Seismicity above). The threshold at which architectural damage to poorly constructed buildings occurs is MMI VI. The largest earthquakes expected in the region (magnitude 7.0 + ) may have MMIs several intensity units higher than those experienced in the largest historical earthquakes.

National ground-shaking maps at a scale of 1:7,500,000 by Algermissen and others (1990) provide a basis for assessing earthquake ground-shaking potential in southwestern Utah. In their study, generalized seismic-source zones were defined based on historical seismicity and patterns of Quaternary faulting, and source parameters from each zone were used in a probabilistic analysis to calculate peak horizontal ground accelerations and velocities with a 10 percent chance of being exceeded in 50 and 250 years (figure 5). In general, damaging ground motions are those with peak horizontal ground accelerations greater than 0.1 g. For the 50-year exposure time, accelerations of 0.1 g may be exceeded over most of the populated area of southwestern Utah, including Richfield, Cedar City, and St. George (figure 5). For the 250-year exposure time, such damaging levels of ground shaking may occur throughout southwestern Utah, except parts of western Beaver and Millard Counties. Except for a small area in seismic zone 3 of the Uniform Building Code (UBC; International Conference of Building Officials, 1991) in eastern Millard County north of Fillmore, all of southwestern Utah is in UBC seismic zone 2B, an area of moderate earthquake risk. The seismic zonation map in the UBC is based on levels of peak horizonal ground accelerations for the 50-year exposure time, which for zone 2B ranges from 0.1 to 0.2 g.

The historical earthquake record, particularly near Richfield, indicates that accelerations exceeding 0.2 g have likely occurred in historical time in the area and that perhaps seismic zone 3 should extend farther into southwestern Utah. The geologic record indicates that large earthquakes (magnitude 7 + ) producing much stronger ground shaking may also occur, although less frequently than in northern Utah. To accurately assess the adequacy of southwestern Utah's UBC seismic-zone designations, a detailed probabilistic analysis of ground shaking, incorporating paleoseismic information for faults, is needed.

Figure 5 and the UBC seismic zonation map consider ground shaking on bedrock, and do not reflect the effect of local site conditions on ground shaking. Deep alluvial basins and soft soils have been shown in northern Utah along the Wasatch Front and elsewhere to amplify peak velocities for certain lower-frequency ground motions most damaging to


B

Figure 5 . Probabil ist ic e a r t h q u a k e accelerat ion m a p s for U t a h s h o w i n g accelerat ions (numbers indicate hor izontal accelerat ions expressed as percent of gravi ty) w i t h a 1 0 percent chance of being e x c e e d e d in: (A) 5 0 years , and (B) 2 5 0 years (A lgermissen and o thers , 1 9 9 0 ) .

tall buildings (Hays and King, 1984). The same effects are likely in southwestern Utah, and such amplifications may occur near the centers of deep alluvial basins, particularly in the Basin and Range province west of Interstate Highway 15. However, few cities are in such areas, and few tall buildings are present, reducing the impact of amplified low-frequency ground motions on existing structures. Likewise, soft soils such as thick beds of saturated, soft clay are not widespread in populated areas of southwestern Utah as they are in northern Utah, except perhaps in the Sevier Desert area near Delta.

Surface Fault Rupture

The potential for surface faulting exists along all of the faults shown in figure 3. Such faulting may be characterized by vertical surface displacements up to several meters, with cracking, tilting, and graben formation in a zone of deformation along the fault up to several hundred meters wide. Tectonic subsidence, or permanent lowering of the downdropped side of the fault accompanying surface rupture, may occur over large areas and affect entire basin floors. In the ISB, earthquakes of at least magnitude 6.0 to 6.5 are thought to be required before the fault will rupture to the surface (Machette and others, 1991; Smith and Arabasz,

1991). However, it is conceivable that rupture on some faults, such as those which merge with shallow detachments in the northern part of the area may not be accompanied by such large earthquakes.

In order to assess the relative likelihood of surface fault rupture, it is necessary to look at the rupture history of a fault, both in terms of most recent events and long-term slip rates. Because data are lacking for most faults, only a preliminary estimate of relative likelihood is possible at this time. Figure 6 is modified from Christenson (in preparation), and shows the relative likelihood of rupture on faults in southwestern Utah grouped into two classes: (1) moderate to high, and (2) moderate to low. The overlap in classes reflects the ambiguity and incomplete nature of the data.

The greatest potential (moderate to high) is thought to be on faults with relatively high long-term slip rates, or on faults with evidence for events during Holocene or latest Pleistocene time. The Sevier, Hurricane (figure 7), and Paragonah faults are included in this category because of their relatively high long-term slip rates, which suggest recurrence intervals of 10,000 years or less. Although Holocene movement has not been demonstrated on these faults, it has not been precluded either. All faults in southwestern Utah with evidence for a single Holocene-latest Pleistocene event


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EXPLANATION

Moderate to high

J Moderate to low

H - Hurricane PR - Paragonah S - Sevier

113° W.

0 10 20 30 40 Miles 1 i—S H—i—h H 0 10 20 30 40 50 60 Kilometers

Scale

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Figure 6 . Sur face- faul t - rupture-hazard map of sou thweste rn Utah (modif ied f r o m Chr istenson, in preparat ion) .


Figure 7 . V i e w to nor th f r o m atop the Hurr icane fau l t scarp east of Hurr icane (photo by Gary E. Chr is tenson, 1 9 8 8 ) .

are also included in the moderate to high category. However, all lack evidence for recurrent movement in the last 30,000 years, and because of the apparent long average recurrence, these faults may have a relatively lesser surface-fault-rupture hazard than the Hurricane, Sevier, and Paragonah faults. No faults in southwestern Utah are considered to have as high a surface-fault-rupture hazard as the Wasatch fault in northern Utah (Anderson and Christenson, 1989; Christenson, in preparation; Hecker, in preparation). Those faults with low long-term slip rates, recurrence intervals generally exceeding many tens of thousands of years, and most recent events probably more than 10,000 years ago, and in some cases only suspected in all of Quaternary time, are considered to have a moderate to low potential.

These assessments of surface-fault-rupture hazards refer to the relative probability of rupture based on a qualitative analysis of past fault history, which is generally poorly known. Because recurrence intervals are not uniform and events may cluster on certain faults, in certain geographic areas, or in certain time intervals (Wallace, 1987; Machette and others, 1991), rupture could occur on any of these faults at any time.

Liquefaction

Conditions favorable for liquefaction occur chiefly along streams and in some central basins of southwestern Utah where shallow ground water and sandy soils are found. Sand boils resulting from liquefaction were reported along the

Sevier River near Richfield after the 1901 magnitude 6 l/i Richfield earthquake (Williams and Tapper, 1953). Anderson and others (1990) have evaluated liquefaction potential in the Sevier Valley and found a low to moderate potential (5-50 percent chance in 100 years) along the Sevier River in Sevier County. Liquefaction potential has generally not been evaluated for the rest of southwestern Utah. However, Mabey and Youd (1989) have completed a statewide map of the Liquefaction Severity Index (LSI), which indicates the amount of lateral displacement (in inches) expected in areas most susceptible to liquefaction-induced ground failure (gently sloping, saturated, Holocene flood-plain deposits). They show all areas with LSIs of 5 or greater, indicating 5 inches (13 cm) or more lateral displacement, with a 10 percent chance of being exceeded in 10, 50, 250, and 1,000 years. The 1,000-year exposure time represents the maximum extent of possible liquefaction. Figure 8 shows areas in southwestern Utah where liquefaction-induced ground failure of 5 inches (13 cm) or more (LSI of 5 or greater) may occur in the 1,000-year exposure time, based on the LSI evaluation of Mabey and Youd (1989) and the map of shallow ground water (less than 10 m [30-ft] deep) by Hecker and others (1988). For this evaluation, susceptible soils are assumed to be present in all areas of shallow ground water. For the 50-year exposure time, no areas of LSI greater than 5 occur in the southwestern Utah region except in the northeast corner along the Sevier River in Juab County (figure 8). For the 250-year exposure time, all areas in figure 8 are included except those around Sevier Lake and in extreme western Millard County.


39° N.

38° N.

37" N

EXPLANATION

/ Area with a 10 percent chance in 1000 years for liquefaction-induced ground failure of 5 inches (13 cm) or more, indicating the likely maximum extent of possible liquefaction.

114s W. A R I Z O N A 113° W. N

112°W.

0 10 20 30 40 Miles

n I -H H 1 S H 0 10 20 30 40 50 60 Kilometers

Scale

F igure 8 . L i q u e f a c t i o n h a z a r d m a p of s o u t h w e s t e r n U t a h ( c o m p i l e d f r o m H e c k e r a n d o t h e r s , 1 9 8 8 ; M a b e y a n d Y o u d , 1 9 8 9 ) .


As shown in figure 8, liquefaction is not a widespread hazard in southwestern Utah, and the likelihood of significant liquefaction-induced ground failure is generally low. The areas of greatest hazard are along streams in flood-plain areas such as along the Sevier and Virgin Rivers where soil and ground-water conditions are favorable and soil in stream banks is not confined laterally and may slide into the stream. Broad areas of possible liquefaction-induced ground failure are found in basin floors of western Utah such as the Escalante and Sevier Deserts, Sevier Lake Basin, Beaver River, and Snake Valley.

Other Hazards

Other earthquake hazards include chiefly slope failures and flooding. Rock falls are most common, and may occur in earthquakes of magnitude 4.0 and greater (Keefer, 1984). Because of the many steep cliffs and canyons in southwestern Utah, rock falls will likely be numerous in a moderate to large earthquake. Clouds of dust from rock falls in the Pine Valley Mountains were reported in the 1902 Pine Valley earthquake (Williams and Tapper, 1953). Rock falls will be most destructive to roads, aqueducts, or other structures in canyons and at mountain fronts.

Larger, deeper-seated landslides commonly occur in and around the epicentral areas of magnitude 5 and larger earthquakes (Keefer, 1984). The landslide hazard in southwestern Utah is discussed in Harty (this volume), and in general earthquake-induced landslides will likely occur in the same areas of existing landslides and landslide-prone geologic units. Areas of particularly high landslide susceptibility during earthquakes are near springs where soils are saturated and slopes are steep. Snow avalanches triggered by earthquakes in winter pose another potential hazard.

Flooding during earthquakes may occur as a result of: (1) dam failure, (2) diversion or destruction of canals, aqueducts, water lines, or streams, (3) increased ground-water discharge, (4) seiches in lakes and reservoirs, and (5) tectonic subsidence in areas of lakes, reservoirs, or shallow ground water. Several significant dams and many smaller structures are present in the area. The flooding hazard from dam failures is outlined in Harty and Christenson (1988). The many irrigation canals and aqueducts pose potential flood hazards to any areas downslope from a break. Large, permanent displacements of conveyance structures may occur where such structures cross faults and landslides, but damage may occur nearly anywhere as a result of ground shaking. Culinary water supplies may likewise be disrupted and cause local flooding if conveyance structures are damaged. The hazard from increased ground-water discharge is unknown but may cause local flooding near springs and streams.

Seiches may flood shorelines of lakes and reservoirs. The general lack of extensive development around lakes and permanent impoundments in southwestern Utah reduces the potential for damage in shoreline areas from this hazard.

However, seiches may overtop and damage dams, potentially causing a dam failure with flooding downstream. Areas of shallow ground water (generally less than 10 feet [3 m]; see Hecker and others, 1988) or impounded water on the downdropped side of faults may be subject to local flooding accompanying tectonic subsidence. Such areas are not widespread and generally are not found in urban areas in southwestern Utah.

CONCLUSIONS AND RECOMMENDATIONS

The area of greatest earthquake hazard in southwestern Utah is within the southern Intermountain seismic belt (ISB), a zone of shallow, diffuse seismicity which trends from Richfield through Cedar City and St. George. The ISB encompasses most of southwestern Utah except the northwestern corner west of Delta and Milford. The larger historical earthquakes in the area, predominantly near Richfield and Elsinore, have reached magnitude 6 to 6l/i. Active late Quaternary deformation, including surface rupture on faults (probably accompanied by large-magnitude earthquakes), is apparent from geologic studies in southwestern Utah. Single Holocene and latest Quaternary displacements are evident on many faults in southwestern Utah, but there is little evidence for recurrent Holocene/latest Quaternary displacement. Faults thus appear less active than many in northern Utah. However, data are incomplete and ambiguous on many faults.

Hazards accompanying moderate to large earthquakes include ground shaking, surface fault rupture, tectonic subsidence, liquefaction, slope failure, and flooding. The most frequent and strongest ground shaking is expected within the ISB in the eastern and southern parts of the study area. Potentially seismogenic faults capable of rupturing the surface, accompanied by tectonic subsidence and strong ground shaking, are found throughout the area, although the seismogenic potential of some shallow faults in the northern part of the area is not clear. The highest liquefaction potential is along stream-valley bottoms within the ISB, particularly along the Sevier and Virgin Rivers. Slope failures in canyons and mountain slopes, and flooding along streams, lake shorelines, and downslope from damaged water-conveyance structures, will likely accompany moderate to large earthquakes.

Earthquakes pose a risk to life and property. As such, they must be considered in land-use planning, particularly for siting critical facilities. Ground shaking is presently handled through building codes, and buildings must conform at least to the minimum standards set in the UBC for the appropriate seismic zone. Other hazards such as surface faulting, liquefaction, and slope failures are best handled in land-use ordinances which require detailed site evaluations to identify hazards and recommend hazard-reduction measures.


Decisions regarding appropriate land use can then be based on the results of these evaluations. Earthquakes are a hazard that cannot be avoided, but wise land-use practices and strict adherence to building codes can greatly reduce losses.

ACKNOWLEDGEMENTS

This paper was reviewed by Walter J. Arabasz of the University of Utah, and Kinun M. Harty and William R. Lund of the Utah Geological Survey. We thank them for their helpful comments.

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