The 1964 Prince William Sound earthquake ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 101, NO. BI, PAGES 523-532, JANUARY

10, 1996

The 1964 Prince William Sound earthquake: Joint inversion of tsunami and geodetic data JeanM. Johnson andKenjiSatake • Departmentof GeologicalSciences,Universityof Michigan,Ann Arbor

Sanford R. Holdahl NationalGeodeticSurvey,CoastandGeodeticSurvey,NationalOceanServices,NOAA, SilverSpring,Maryland Jeanne Sauber Geodynamics Branch,Dabfor TerrestrialPhysics,NASA GoddardSpaceFlightCenter,Greenbelt,Maryland

Abstract. The 1964PrinceWilliam Sound(Alaska)earthquake, Mw=9.2,ruptureda largearea beneaththe continentalmarginof Alaskafrom PrinceWilliam Soundto Kodiak Island.A joint inversionof tsunamiwaveformsandgeodetic data,consisting of verticaldisplacements and horizontalvectors,givesa detailedslipdistribution. Two areasof highslipcorrespond to seismologically determined areasof highmomentrelease:thePrinceWilliam Soundasperitywith averageslipof 18 m andtheKodiakasperitywith averageslipof 10 m. The averageslipon the

faultis 8.6 rn andtheseismic momentis estimated as6.3x1022N m, or over75% of theseismic momentdetermined fromlong-period surfacewaves.

Introduction

The March 28, 1964 Prince William Sound (Alaska) earthquakeruptured an 800-kin-long segmentof the Alaska subductionzone where the Pacific plate is underthrusting beneaththe North American plate. The epicenter,61.04øN, 147.73øW [Sherburneet al., 1969], is locatedin southcentral Alaskaabouthalf way betweenAnchorageandValdez, but the aftershockareaextends300 km eastto CapeYakatagaand 800 km southwest to Kodiak Island (Figure 1). The seismic

moment of theeventis estimated as 8.2x1022 N m,Mw=9.2 [Kanamori, 1977], makingit one of the largestearthquakes ever recorded, secondonly to the 1960 Chile earthquake, Mw=9 .5 .

The tectonicsettingof the PrinceWilliam Soundearthquake is complex. The Pacific plate is subductingin a northnorthwestdirection at about 6 cm/yr [DeMets et al., 1990]. Microearthquake studies [Page et al., 1989; Pulpan and Frohlich, 1985] and reflection and refractionstudies[Brocher et al., 1994] havedelineatedthe structureof the plateinterface and the subductingslab. The plate interface, which in the Kodiak Island area is dipping about 8ø-10ø, becomesvery shallowandbroadin the PrinceWilliam Soundarea,havinga dip of 30-4ø. Furthercomplicating the tectonicsis thepresence of severalaccretedterranes[Joneset al., 1987]. The youngest of theseis the Yakutat terrane,which is in the final stagesof emplacementagainst southern Alaska. Recent modeling of wide angle refraction and reflection data by Brocher et al. [1994] has suggestedthat in PrinceWilliam Soundthe contact between the overlying North American plate and the

subductingYakutat terraneis the Alaskan megathrust,or the plane on which the 1964 earthquakeoccurred.Their work shows the Yakutat terrane as a low-velocity layer overlying

the higher-velocityPacific oceanic crust. Beneath Prince William Soundthis lower-velocitylayer extendsto a depthof approximately 20-25 km, while the Pacific oceanic crust is deeper,at approximately30 kin. The focal mechanismsof the 1964 earthquakeand its aftershocks,which show low-angle thrusting, and the aftershock distribution [Stauder and Bollinger, 1966] are compatible with the interpretationof Brocher eta/. [1994]. Coseismic crustal deformation occurred throughout the source area, causing extensive damage in Alaska. Soon after the earthquake,the vertical and horizontaldisplacements were measuredand compiledby many surveyteams.Vertical uplifts averaged2 m and reacheda maximtun of 11 m on Montague Island [Plafker, 1969]. Maximum vertical subsidencewas approximately2 m. Horizontal displacementsof up to 25 m were observed in Prince William Sound [Parkin, 1969]. The vertical

deformation

of the seafloor

in the Gulf

of Alaska

generateda tsunami which devastatedseveral Alaskan towns, causeddamage in Hawaii (4 m maximum run-up) and on the west coast of North America (13 fatalities, averagemaximum run-up 2 m, maximum run-up 5 m in CrescentCity, CA), and was observedas far away as Australia and Antarctica.It is this tsunami and geodetic data which we use to estimate the slip distributionof the Prince William Soundearthquake. Previous

Seismic Studies

Although the 1964 earthquakeoccurredin the World Wide StandardSeismographNetwork era and thereforewas recorded on high-quality instruments,the enormous size of the event causedmost instrumentsto go off-scale soon after the first P

•Nowat Geological Survey of Japan, Tsukuba. Copyright1996by theAmericanGeophysical Union.

wave

Papernumber95JB02806. 0148-0227/96/95JB-02806505.00

arrival.

This

is true

of instruments

in the teleseismic

distancerange of 30ø-90ø;therefore,there is a lack of body 523

524

JOHNSONET AL.: 1964PRINCEWILLIAM SOUNDEARTHQUAKE 162ø

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Figure1. Aftershocks of the1964PrinceWilliamSoundearthquake locatedbetween March28, 1964and December 31,1965(afterAlgermissen et al., 1969).Hachured areaindicates thePrince WilliamSound asperity asdetermined by RuffandKanamori[ 1983].

wave data that can be usedto studythe momentdistributionof the earthquake.Ruff and Kanarnori [1983] overcame this

Sound,onein the Kodiakarea,anda fourthfor thePattonBay uplift. This inversionis an improvementon the singlefault difficultyby usingP wavesdiffracted by thecore.Theywere model,but it doesnot give any indicationof slip variationsin able to obtain a source time function for the event and estimate the down dip direction.Most recently,Holdahl and Sauber the location of the main moment release. Their results show a

[1994]haveusedthegeodetic datato invertfor a detailedslip large area of momentreleasecoveringthe entire Prince distribution on 68 subfaults. This inversion shows the Prince WilliamSoundarea(Figure1); thisis usuallyreferredto asthe William Soundasperityas a regionwith slip varyingfrom 10 "PrinceWilliam Sound"asperity.Kikuchiand Fukao [1987] to 30 m (Figure 3). Holdahl and Sauber'sresultsalso show a invertedseveralpartiallyclippedP waveformson horizontal components to locateseveralsubevents on the rupturesurface. They also found most of the momentreleaseto have occurred

in the epicentralarea.Recently,Christensen and Beck[1994] have located a secondarea of high momentreleasein the KodiakIslandarea(Figure2). The secondasperitywill herebe called the Kodiak asperity. While thesestudiesdo give a clear indicationof where the

highestmomentrelease,and by implicationthe highestslip,

MARCH 28, 1964 ALASKA EARTHQUAKE

._Z2' 15 ß

10

ß

5

E o

•

0 600

occurred,they give only a minimum estimateof the moment and a lower bound on the averageslip due to the use of

400

200

0

March 28, 1964

diffractedor clippedwaveforms.AlthoughKikuchiand Fukao

[1987] estimatedthe slip distributionin the down dip direction,teleseismicbody wavesusuallyhave poor depth resolution. To morereliablyestimatethemomentandtheslip

200

Km Along Fault Strike Mainshock

Alaska

(Mw=9.2)

60øN

distributionin the down dip direction,we mustturn to other sources of data.

Prince William

Bedng Sea

Sound Asperity

Previous Geodetic Studies 1900

Thereis an enormous baseof geodeticdatafrom the Prince William Sound earthquake.Much of it was collected and describedby Plafker [1969]. This data, which is describedin

the inversionsection,wasusedsoonafterthe earthquake to estimatethe fault parameters and slip by Savageand Hastie [1966]andHastieandSavage[1970].Theyestimated theslip on a singlefaultplaneasapproximately 10m. Theyincluded a small secondaryfault to explain the PattonBay uplift. Miyashitaand Matsu'ura[1978] invertedthe geodeticdatato determinethe slip on four fault planes'two in PrinceWilliam

KodiakAsperity

56 ø

1854

1844

160 ø

Gulf of Alaska

150øW

Figure 2. Asperitydistributiondeterminedby Christensen and Beck [1994]. The upperfigure showsthe along-strike 26 momentdensityin unitsof 10 dyne-cm/km.The lowerfigure shows the map view of the asperitiesdeterminedfrom the upperfigure. The datesof historicearthquakesin the Kodiak segment are listed.

JOHNSONET AL.: 1964PRINCE WILI•AM SOUND EARTHQUAKE 158øW

138øW 66ON

Fairbanks

525

earthquake,the tsunamidata cannotprovide any constrainton estimatesof the slip which occurredon the landwardpart of the fault.

Joint Inversion The limitations discussed previously for each of the inversion methods demonstrate the necessity for a more comprehensiveapproach if we wish to determine the slip

'... •Anch,

distribution on the entire 1964 fault zone. Satake

ß Tide Gauge - Leveling • Triangulation 56ON

Slip in meters 0-5

5-10

10-15 15-20

I

1

20-25

25-30

Figure 3. Slip distributiondeterminedfrom geodeticdata (from HoldaM and Sauber,1994).

regionof high slip eastof Kodiak Island, but this slip is not a resultof the geodeticinversion,as will be explainedbelow. There is a serious limitation to using geodetic data to estimate the slip distribution. Geodetic data gives very good controlon slip occurringon the landwardpart of the fault, but, as is typicalof subduction zoneearthquakes, a greatpartof the slip occurson the oceanicpart of the fault plane. The fact that a large tsunami was generatedshows that significant slip

[1993]

introduceda method for inverting tsunami and geodeticdata simultaneously for the complete down dip slip distribution, usingthe 1944 Tonankaiand 1946 Nankaidoearthquakes as an example. This method is eminently suitable for earthquakes which occur in subduction zones adjacent to continental margins.As long as sufficientgeodeticand tsunamidata exist, the entire slip distribution of the earthquake can be determined. This approach is different than HoldaM and Sauber's [1994] using geodeticdata and a priori information from tsunamis.In the joint inversion, all the data must be satisfied simultaneously. Data

The tsunami generated by the Prince William Sound earthquakewas recordedon analog tide gaugesat points all aroundthe Pacific, at stationswhich are a part of the Seismic Sea Wave Warning System (now Pacific Tsunami Warning System). We chose a wide distribution of these records from Alaska, North America, the Pacific Islands,and Japan(Figure 4). Many have been publishedin a report on the tsunamiby Spaeth and Berkman [1972]. We obtainedthe original tide gauge records and digitized them at 1 min intervals. We removed the tidal component and applied clock corrections where necessary. There are a wide varietyof geodeticdatain the form of point observations taken all over southern Alaska, the islands in

Prince William Sound, and Kodiak. In this study, we used a subsetof the data used by HoldaM and Sauber [1994]. This Geodeticdata is unableto constrainthe estimatesfor this slip. in thestudy ofHoldahl andSauber, thesliponovera thirdof includestwo data types,verticaldisplacementsand horizontal the subfaultscannotbe determinedsolely from the geodetic vectors. The vertical data include (1) displacementsof tide gauges, (2) repeated leveling surveys, and (3) geologic data data. Holdahl and Sauber used slip estimatesprovided by preliminary tsunamimodeling by Johnsonand Satake [1993a] includingchangesin growth limits of coastalmarine species, as a priori information. Without this information, there would beach markers, and bathymetric surveys. The horizontal vectors are computed from preseismic and postseismic have beenno constrainton the slip valuesnear the trench. triangulationsurveys.A completedescriptionof the geodetic occurred beneath the continental

shelf in the Gulf of Alaska.

data and a reference

Previous

Tsunami

Studies

The source area of the 1964 tsunami has been estimated

previouslyby Pararas-Carayannis [1967] andHatori [ 1981]by backwardpropagationof the tsunamitravel times from tide gauge stationswhere the tsunami was recorded.Their estimates

show that the major tsunamiwhich swept the Pacific was generatedmainly from uplift of the continentalshelf in the Gulf of Alaska. No one, however, has previouslyused the tsunamiwaveformsto estimatethe slip distribution.Johnson and Satake[1993a] did a preliminaryinversionof the data to estimatethe offshoreslip, and the resultsshowlarge slip near the trench in the Gulf of Alaska off the Kenai Peninsula and

near Kodiak Island. Althoughfar-field tsunamidata have been used to estimate the slip distribution of other AlaskanAleutianearthquakes which had no landwardextensionof slip [Johnson and $atake, 1993b, 1994], in the case of the 1964

list can be found in HoldaM

and Sauber

[1994]. Fault

Model

The subfault model used in the joint inversion is a simplified versionof the model usedby Holdahl and Sauber [1994]. They used a mosaic of 68 small (-50 by 50 kin) subfaults, 28 spanning the area from Kodiak to the Kenai Peninsulaand 39 coveringPrinceWilliam Soundand the Gulf of Alaska

out to the Alaskan

trench. One additional

subfault is

includedto representthe PattonBay fault on MontagueIsland. We modified this subfault model in several ways. First, we merged sets of four subfaultsinto a single subfault to reduce the computational effort needed to generate the tsunami Green's functions.The presentmodel includeseight subfaults in the Kodiak

area and nine in the Prince William

Sound/Gulf

of Alaska area.We also includedthe PattonBay subfault.The

526

JOHNSONET AL.: 1964PRINCE WILLIAM SOUND EARTHQUAKE

ComputationArea andTide GaugesUsedin Inversion 220'W

200'W

180'W

160'W

60'N

140'W

120'W

'

60'N

,•IJnalaska ,'

NeahBay

ß

PACIFIC

OCEAN

Miyako

nO'N

Fort Rincon Is anta a Joll: ,

20'N

•'• Hilo

ß Wake

ß' Guam I

I

220'W

200'W

'

I

180'W

Nawiliwili Honolulu ] 20'N

I

I

I

160'W

140'W

120'W

Figure 4. Computationarea for 1964 tsunamiand locationsof tide gaugestationsusedin this study.

subfault locationscan be seen in Figure 9, and the fault have more variable mechanisms; therefore, the subfaults in parameters are listedin Table 1. Next, we modifiedthe depths this area have motion aligned with the direction of Pacific of the subfaults. Holdahl and Sauber'spreferred model is plate motion, approximatelyN17*W in PrinceWilliam Sound. consistent with rupture between the North American and Pacific plates in Prince William Sound,hencethe subfaultsare deeperanddippingmore steeplythan the inferredruptureplane suggested by Brocher et al. [1994], described earlier. Therefore, we chosethe depth and dip of the subfaultsto be consistentwith ruptureon the Yakutat terrane-NorthAmerican plate interface.The faults in the Kodiak area (a-d and h-k) are 100 km by 100 km and dip 8*. The faults in the PrinceWilliam Sound area are approximately 100 km by 100 km, but are slightly smallernear the trench and slightly larger along the coast.Subfaultse, f, and g dip 8*, therestdip 3*. The directionof slip for each subfaultis determinedby one of two

methods.

The

aftershocks

in the Kodiak

area have

almost pure dip-slip mechanisms [Stauder and Bollinger, 1966]; therefore,the subfaultsin this area have pure dip-slip motion.

The

aftershocks

Table

1. Fault

Subfault

in the Prince Plane

William

Sound

area

This means that the motion is mainly dip-slip, with a small (rain

where A is the matrix of Green's functions, either a vertical or

for the tsunami data, b is the matrix of observations,and x is

where g is the accelerationof gravity, h is the water height displacedfrom the equilibrium position, d is the water depth, and {• is the flow rate vector. Using digital bathymetryof the Pacific Ocean, we solvethe equationof motion and equationof continuity by a finite difference computationon a staggered grid systemusinga 5'x5' (-10 kin) grid. To ensurean accurate syntheticwaveform, we switch to a l'xl' (