The February 28, 2001 Nisqually earthquake:GPS Geodesy and quantifying seismic hazard.

Testimony prepared for:Subcommittee on Research, The Committee on ScienceUnited States House of Representatives

Hearing: March 21, 2001. 2:00 PM to 4:00 PM, Rayburn House Office building

Topic: Life in the Subduction Zone: The recent Nisqually Earthquake and Federal Ef-forts to Reduce Earthquake Hazards

M. Meghan Miller, Ph.D.Professor of Geological SciencesCentral Washington UniversityEllensburg, WA 98926-7417

509/963-2825509/963-1109 (fax)meghan@geology.cwu.eduhttp://www.geodesy.cwu.edu/

Tectonic setting of the NisquallyEarthquake:

The February 28, 2001, Mw=6.8Nisqually earthquake broke within thesubducting Juan de Fuca slab, the oce-anic plate that is being thrust under thewestern edge of North America. GPSgeodesy can measure Earth deformationat the millimeter level. The Nisquallyearthquake was the first in the PacificNorthwest to be detected with GPS ge-odesy.

There are three types of earth-quakes that pose seismic risk in the Pa-cific Northwest: those, like the Nis-qually, that break the down goingoceanic plate, those along shallow faultswithin the North America plate, and thepotentially much larger but also less fre-quent ruptures along the subduction zonefault that separates the two plates. Eachof these types has expected characteristicseismic hazard. The deep earthquakes,common during the last century, arestrong and widely felt but on the wholeless damaging at a particular magnitudebecause of their depth. Forecasting sizeand frequency of such earthquakes areperhaps the most difficult using current

methods. The potential size and fre-quency distributions of shallow earth-quakes can be much better known asgeodetic constraints improve, in studiesthat are currently underway and planned.Finally, the subduction zone fault posesspecial hazard, despite its largely off-shore location, because the likely mag-nitude is greater than 8 and may well beclose to 9, exceeding even the 1906 SanFrancisco earthquake and would becomparable in size to the Great 1964Alaska earthquake. Such earthquakesalso generate large tsunamis.

1) How significant were the effects ofthe Nisqually earthquake on the PugetSound region? How are these effectsassessed?

The Nisqually earthquake waswidely felt. Two of the major geologiceffects of the earthquake were wide-spread landslides and liquefaction. TheUSGS led a major scientific effort thatinvolved members of the academic sci-entific community to map and assess thedamage resulting from these processes.Urban development on unconsolidatedmaterials such as landfill are particularlyvulnerable to liquefaction. Develop-

ments on unstable hill slopes are mostsusceptible to landslides.

The co-seismic deformation (thechange in the position of the ground af-ter the earthquake fault has slipped) thataccompanied this earthquake was alsosignificant, and was the first to be ob-served using continuous GPS geodesy inthe Pacific Northwest. These observa-tions give us constraints on the physicsof earthquakes and the parameters, suchas rigidity, that control how the earthresponds to earthquakes. These in turnhave implications for seismic risk as-sessment and planning.

2) To what extent did buildings and landbehave differently than expected in thisearthquake? To what extent shouldcodes, earthquake preparations and theresearch agenda be altered as a result?

The Pacific Northwest is recog-nized as an area where the engineeringcommunity has taken seriously thesometimes abstract determinations of thescientific community, and has systemati-cally worked to strengthen seismic zon-ing in the urban corridor from Seattle,Washington, to Portland, Oregon. Nev-ertheless, as the body of scientific in-formation continues to grow, this col-laboration continues to be important,especially as urban growth places in-creased pressure on remaining hill slopeproperties that may not be suitable fordevelopment from the standpoint ofseismic risk and periods of intense rain-fall.

The co-seismic geodetic defor-mation that we observed in this earth-quake was somewhat smaller than weexpected from seismic parameters. Thispattern has emerged from several recentearthquakes such as large earthquakes inthe California desert during the 1990s.This growing database leads us to reas-

sess our working models for earthquakephysics.

The national research agendaneeds to be strengthened in this area.All the elements from basic research torisk mitigation are the targets of ongoingefforts by the scientific and engineeringcommunities. As new advances in ge-odesy (such as GPS and strain meters)and seismology become available, re-sponsive federal support is needed toimplement state of the art research pro-grams.

3) What is the current depth of our un-derstanding about earthquakes in the Pa-cific Northwest and elsewhere, andwhere should we focus future researchefforts.

The last decade has been a periodof great vitality in Pacific Northwest ac-tive tectonics research. New technologyand new geologic approaches have rap-

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GPS-determined velocity field, Pacific Northwest Geodetic Array (PANGA). This velocity field is plotted relative to North America, as defined by 11 GPS stations in the continental interior and Atlantic passive margin. Two to six years of data from continuously operating GPS stations yield PANGA station velocities; red triangle without velocities show stations with time series less than two years. Error ellipses show two-dimensional 95% confidence regions. Seafloor topography from Smith and Sandwell [1997]. From Miller et al., 2001.

idly advanced our understanding, yet atremendous amount of work remains tobe done in order to quantify the regionalseismic hazard.

Several factors concentrate Pa-cific Northwest seismic hazard in thePuget Sound and Olympic Peninsula re-gion. Seismicity along deep faults suchas the Nisqually earthquake fault, andalong shallow faults in the Puget Low-lands, is regionally concentrated in thisarea. This is a result of arching of theslab underneath western Washington,where it is breaking up in response tothis arching. Earthquakes on the shal-lower faults result from a concentrationof north-south shortening as the Oregon-southern Washington coastal blockdrives into Vancouver Island and is ab-sorbed through earthquake faulting. Fi-nally the locked or seismogenic part ofthe subduction zone fault is very wideunder western Washington, implyinggreater energy release in that area duringinfrequent but great earthquakes.

The northward migration of central Ore-gon results compression across PugetSound. This creates shallow crustalfaults.

Advancing Seismic Risk Assess-ment and Hazards Planning inPuget Sound:

GPS geodesy demonstrates thatthe coastal region from central Oregon tocentral Washington is a coherent block

that is driving into a rigid backstop,Vancouver Island. Approximately 5 mm(about one quarter of an inch) of short-ening occurs each year across the Olym-pic Mountains and Puget Sound. Thisbackground deformation will ultimatelybe released through earthquakes and re-lated processes. The deformation issimilar in magnitude to the annual short-ening across the Los Angeles Basin.The Seattle fault, oriented east-west un-der West Seattle, is the strongest amongseveral candidates for carrying much ofthis deformation and could rupture in anearthquake at least as large as magnitude7.5, or in more numerous smaller events.Because of its proximity to urban corri-dor, such events could rival or exceedthe Northridge earthquake for damageand casualties.

Seattle ought not to be lulled intoa false sense of security, having sus-tained so little damage in the Mw 6.8Nisqually earthquake. The depth of thisearthquake greatly reduced the ampli-tude of shaking experienced in the met-ropolitan region. A shallow event of thatsize, or larger, which is expected on theshallow faults, would not be so kind tothe older buildings and perhaps modernstructures throughout the urban area.

Denser distribution of continuousGPS stations in the Puget Lowlands willcharacterize which faults pose seismichazard. The GPS-determined parame-ters have a direct impact on seismiczoning and building code development:How much strain is accumulating?Where it is accumulating? Which faultsare being loaded with seismic deforma-tion? How wide are the down-dip rup-ture patches on shallow faults? Thesefindings will constrain the likely size,location, and frequency of earthquakeson active faults in Puget Sound. Be-cause the Seattle, Tacoma Narrows,

Southern Whidbey Island and othersimilar faults are shallow (likely seismicsources at 10 - 15 km depth), they posemuch graver seismic risk than the faultruptured in the Nisqually earthquake.

Mitigation strategies, communitypreparedness, and response planning de-pend on the accuracy of fault parameterdeterminations. GPS geodesy is a newtechnology that is rapidly advancing ourability to set scientific constraints onthese parameters.

GPS geodesy and the Nisquallyearthquake:

Central Washington Universityreceived NSF support to densify thePANGA continuous GPS network in thePuget Lowlands in order to address criti-cal questions regarding the Nisquallyearthquake and future Pacific Northwestearthquakes. Partners in this effort in-clude the Southern California Earth-quake Center, the U.S. Geological Sur-

vey, and UNAVCO (the UniversityNAVSTAR Consortium).

1 . How is the budget of north-southshortening (5 mm) that accumulateseach year between coastal centralWashington and Vancouver Islandrelieved? The budget is comparablein size to that across the Los AngelesBasin. Should we expect a flurry ofearthquakes on the Seattle, TacomaNarrows or Whidbey Island fault,like those in the last 15 years in LosAngeles which followed a long pe-riod of dormancy?

2. Is the series of deep earthquakes re-corded in 1939, 1946 and 1949 atypical sequence? Are we enteringanother period of such events, begin-ning with the 1999 Grays Harbor andthe 2001 Nisqually earthquakes, tobe followed by some near-term fu-ture event(s)?

3 . Do earthquakes relieve all of thestrain in the Puget Lowlands or doother processes play a role?

4. Can these deep earthquakes and trig-ger earthquakes on shallow faults, assuggested by recent patterns ofearthquakes in southern Alaska andEl Salvador? The December 1999Kodiak earthquake is similar to theNisqually earthquake in that bothruptured the slab although the styleof faulting is different . The January2001 El Salvador event is probablymore similar to the Seattle event, butwith about 3 times the energy re-lease. That earthquake was also fol-lowed by a complex aftershock se-quence that included faulting in theupper plate. It is clear from theKodiak and El Salvador examplesthat these slab events can be fol-lowed by aftershocks or triggeredevents in the upper plate. This may

be explained by static stress changes,or by patterns of deformation that aretemporarily perturbed after such alarge earthquake.

Nisqually earthquake response de-ployment:

We have undertaken an immedi-ate deployment of seven continuous GPSstations in the Puget Lowlands in orderto quantify the distribution of slip on ac-tive faults in the metropolitan region.We currently have four teams in the fieldfinding suitable sites for installation, ne-gotiating permits, and making the in-stallations. These seven stations willadd to the more widely spaced existingnetwork of 40 stations, which spans theregion from northern California to theinternational border, and from the coastas far east as Idaho and Nevada.

PANGA (the Pacific NorthwestGeodetic Array)

The Pacific Northwest GeodeticArray (PANGA) is an international con-sortium of institutions committed to us-ing continuous GPS geodesy to furtherunderstanding of Earth deformation inthe Pacific Northwest, including seismichazard assessment. Central WashingtonUniversity coordinates PANGA. Insti-tutions that participate in fundedPANGA projects include CentralWashington University, the GeologicalSurvey of Canada, the U.S. GeologicalSurvey, University of Washington, Uni-versity of Oregon, Oregon State Univer-sity, and University of Alaska. Manyother academic institutions and govern-ment agencies participate in annualPANGA Investigator CommunityMeetings. Data analysis is performed atthe PANGA Data Analysis Facility in

the Geodesy Laboratory at CentralWashington University, an NSF facility.

The core projects that have es-tablished PANGA and supported thePANGA Data Analysis Facility at Cen-tral Washington University since 1997have been funded by the National Sci-ence Foundation, through the Earth Sci-ences Research programs. Supplemen-tary funding has come from the U.S.Geological Survey – National Earth-quake Hazards Research Program (Ex-ternal Research), the National Aeronau-tics and Space Administration, andprivate gifts from Sun Microsystems.

Translating Scientific Results intoBenefits to Society:

An essential element of long-term earthquake hazard mitigation is theestablishment and refinement of hazardmaps. The data from these GPS stationswill be used as input to probabilisticseismic hazard analysis to refine futureversions of these hazard maps. By betterdefining areas most susceptible to strongshaking, future land use planning cantake this into account, thereby improvingseismic risk mitigation in this rapidlydeveloping urban and suburban region.Furthermore, data from the PANGAnetwork will rapidly provide earthquakeinformation that will be useful to emer-gency responders, helping them to targetand prioritize their response efforts.

Precise location data provided byGPS is of much broader utility thanPANGA’s scientific goals. This networkof GPS base stations will be widely usedby federal, state, county, city and privateparties for routine surveying applica-tions. These uses support damage repa-rations, landslide and liquefaction map-ping, road repair, life line and damagemapping and reparation, GIS applica-tions for urban and seismic planning in-

CGPS installations in progressCGPS installations in progress

Nisqually earthquake

CGPS on geodetic monumentCGPS on geodetic monument

CGPS on spindly tower or roof topCGPS on spindly tower or roof top

Pack Experimental Forest

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Issaquah Alps

Tumwater

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Gold MountainWeatherval

Nisqually earthquake response continuous GPS site installations. The stations are constrained Nisqually earthquake response continuous GPS site installations. The stations are constrained to be on bedrock from the standpoint of monument design. Bedrock map from M. A. Jones, to be on bedrock from the standpoint of monument design. Bedrock map from M. A. Jones, USGS Professional Paper 1424-C. (map updated 3/18/01 8:15 a.m.)USGS Professional Paper 1424-C. (map updated 3/18/01 8:15 a.m.)

cluding HAZUS, road reparations andhill slope stability planning, and othergeotechnical needs directly related tomitigation.

The Future: EarthscopeThe existing PANGA network,

as well as its in-progress enhancementfollowing the Nisqually earthquake,forms a critical first step towards under-standing the processes that cause earth-quakes in the Pacific Northwest. Thenascent GPS technology promises enor-mous results if implemented morebroadly. Each time the scientific com-munity has attempted to learn somethingabout how the Earth deforms using GPSas a tool, we discover far more than weset out to measure and we must refor-mulate the questions in light of resultsthat exceed our expectations. The Earthis a complex and intriguing laboratory,sometimes messy, but always rewarding.GPS is an unprecedented tool for char-acterizing the Earth’s dynamics.

The scientific community ispoised to expand these observations in amanner that will support a systematicaccounting of seismic hazard in manyvulnerable states: through the Earthscopeinitiative. This initiative from the scien-tific community has NSF's NationalScience Board approval and is awaitingcongressional support. It is a multi-agency collaboration between the NSF,USGS, NASA, and DOE.

PANGA is one of a few continu-ous GPS networks in the western UnitedStates. The scientific community hasrecognized the potential synergy ofcombining GPS observations throughoutthe country, with concentrated “clusters”of geodetic instrumentation in areas thatare known to experience many earth-quakes. This planned Earthscope net-

work, termed the Plate Boundary Obser-vatory or PBO, has three levels ofdeployment: (1) a sparse network ofGPS stations that covers the stable con-tinental interior and provides key con-straint for understanding the deformingregions, (2) a backbone of GPS stationsthrough all the areas from the RockyMountain westward at 200 km spacingto provide even, systematic characteri-zation of the active mountain buildingregions, and (3) clusters that focus on

Earthscope PBO backbone network

areas where deformation is caused byeither earthquake faulting or volcanicactivity occur. The Pacific Northwesthas been targeted for such clusters thatwill constrain earthquake faulting andvolcano deformation.

The PBO is one of several com-ponents of Earthscope. It provides aperspective that is unique among theelements of Earthscope, setting seismol-ogical and other observations within thecontext of how the Earth deformsthrough time. Together the elements ofEarthscope provide a state of the artcharacterization of the actively deform-ing continent on which we live.

Cascadia GPS Cluster proposed forEarthscope.

International Aspects and GlobalUniqueness of Earthscope

Earthscope has two internationalcomponents: (1) partnerships with Can-ada and Mexico and (2) similar initia-tives in Japan and Taiwan. The scien-tific community is committed to

developing international partnershipswith Canada and Mexico that wouldcomplete the picture of the deformingcontinent beyond our national bounda-ries.

Japan has implemented a similarproject of 1000 GPS sites within theirsmall country. The U.S. has providedscientific expertise to this project. In thewake of the 1999 Chi-Chi earthquake,Taiwan is planning to implement a PBOmodeled after the U. S. plan, and isseeking the experience and expertise ofour scientific community to support thiseffort.

The U.S. scientific community ispoised to implement the Earthscope ini-tiative that would provide urgentlyneeded observations on a global scale.The investigator community is verystrong, and practiced in interdisciplinarystudies. Furthermore, we dwell on avaried active plate boundary that in-cludes microcosms of each of Earth’smajor tectonic environments: subduc-tion zones of two important types inCascadia and Alaska, a the textbook ex-ample of a transform boundary in theSan Andreas fault, and rifting by exten-sion in the Great Basin of Nevada, Utah,and adjacent states. No similar projectsboast this wealth of tectonic environ-ments.

SummaryBecause of dramatic growth in

our understanding of seismicity in thePacific Northwest over the last decade ormore, scientists were not surprised bythe February 28, 2001, Mw = 6.8 Nis-qually earthquake. Continued integra-tion of scientific results into urban plan-ning and risk mitigation requiresenhanced support for the new technolo-gies that can help scientists map thelikely locations, size and frequency of

future earthquakes on shallower faults,which pose much more serious risk tolife and property.

Recent technological advancesinclude the use of GPS to study how theplanet Earth’s tectonic plates deform inreal time. With NSF, NASA, USGS,and Sun Microsystems support, the Pa-cific Northwest Geodetic Array(PANGA) has piloted applications ofthis technology in the Pacific Northwest.PANGA has responded to the Nisquallyearthquake by initiating installation ofseven new stations in the Puget Low-lands region. This has been undertakenwith NSF support and in partnershipwith the geodetic investigator commu-nity including the Southern CaliforniaEarthquake Center, UNAVCO, and theU.S. Geological Survey.

The scientific community ispoised to dramatically extend ourknowledge base by implementing thesetechnologies at an unprecedented scalein the planned projects that make up

Earthscope, which has been approved bythe National Science Board. ThroughEarthscope, the scientific communitywill provide meaningful constraints forurban planning and emergency responsemeasures, in addition to advancing ourbasic research in the areas of earthquakephysics, the physics of deforming volca-noes, and the forces that drive plate tec-tonics and mountain building within thecontinents.

Recommendation:Federal funding of earthquake

sciences needs to be strengthened tosupport the growth of technology such asGPS geodesy. This includes Earthscope,which has been approved by the Na-tional Science Board, funding to earth-quake response agencies, and basic re-search initiatives implemented throughthe research programs of the NSF.