The 1992 Cape Mendocino Earthquake Broadband

Home , 1980 Eureka earthquake, Cape Mendocino, Gorda Ridge, Mendocino Fracture Zone

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B7, PAGES 16,043-16,058,JULY 10, 1996

The 1992 Cape Mendocino earthquake: Broadband determination of source parameters

Michael T. Hagerty and SusanY. Schwartz Instituteof Tectonicsand W.M. Keck SeismologicalLaboratory, University of California, SantaCruz

Abstract. The April 25, 1992, Cape Mendocinoearthquake (Ms 7.1) hasrenewed speculationabout the natureof subductionalong the Cascadiasubduction zone and the associatedseismic hazard. This eventmay representthe first large (M > 6) thrustevent alongthe entireCascadia subduction zone in historictimes (last 200 years). We analyze long-periodsurface waves and broadband body wavesin orderto estimatethe mainshock sourceparameters. We alsoexamine broadband body wavesfrom the nearby1991 Honeydewearthquake (M 6) in orderto assessthe contributionsof bothrupture complexityand unmodeled source and receiverstructure in the CapeMendocino waveforms. From both body and surfacewave inversions,as well as forward modelingof body waves,we estimatea bestdouble couple mechanism for the Cape Mendocino earthquake(strike=330_10 ø,dip=12___2 ø,rake=75___15 ø,seismic moment=l.93x1019 N m, and Mw=6.8). This mechanismcontains a significantcomponent of slip in the estimated directionof Gorda-NorthAmerica plate convergence. Although this earthquakehad suitablegeometry for relievingstrain accumulated by Gorda-NorthAmerica plate convergence,we cannotresolve whether it occurredon the interplatemegathrust or on a fault within the overridingaccretionary prism. We find evidencefor southwest(offshore) rupturetoward an azimuthof 240ø . In addition,we find evidencefor early aftershocksin both the teleseismicallyrecorded body wavesand in locally recordedstrong motions. We model one aftershockdelayed -13 s from the mainshockwith a mechanismthat is differentfrom the mainshockmechanism but is consistentwith the north-southtrending, horizontalcompression found offshore within the Gordaplate. We postulatethat this aftershockand two additionallarge, strike-slip aftershocks that ruptured the Gordaplate within 24 hoursof the mainshockwere causedby the transferof stressaccumulated across the Cascadiasubduction zone and accretionaryprism far offshore,to the Gordaplate, where it reducedthe normalstress across NW-SE orientedfaults, triggering failure. The complexityof fault interactionsnear the Mendocinotriple junction needs to be understood beforepotential seismic hazards of the southernCascadia subduction zone can be quantified.

Introduction earthquake(Mw 6.0) was the largestrecorded onshore event The April 25, 1992, Cape Mendocino earthquakewas in the vicinity of the Mendocinotriple junction [McPherson located near the town of Petrolia, along the northern and Dengler, 1992]. The smallersize and similar thrust California coast. It was followed the next day by two large mechanismof the Honeydew earthquakemake it a good aftershocks(Ms 6.6 and 6.7) located 25 km offshore of event for testing the suitability of the synthetic Greens Petrolia, within the Gorda plate (Figure 1). The mainshock functionsused to model the Cape Mendocino earthquake. elevateda 25-km sectionof the coastnear Cape Mendocino Both the Cape Mendocino and the Honeydewearthquakes by as much as 1.4 m [ Carver et al., 1994] and generateda were followed within 24 hoursby large, strike-slipevents small tsunami that was recordedby tide gaugesalong the locatedoffshore within the Gordaplate (Figure 1 and Table California and Oregon coasts[Oppenheimer et al., 1993]. 1). As the Cape Mendocino earthquake is the only The earthquake was recorded by a local array of strong instrumentallyrecorded large thrust event to occur along the entire length of the Cascadia subduction zone, it motion instruments,by regional networksof short-period instruments,and by globally distributedbroadband digital representsunique evidence in supportof seismicsubduction instruments. The Harvard centroid moment tensor (CMT) of the Gorda plate beneathwestern North America. In this solution[Dziewonski et al., 1993] for the Cape Mendocino paper we analyzeteleseismic recordings of both the 1992 mainshock indicates thrust motion in the direction of Cape Mendocino and the 1991 Honeydewearthquakes in order to derive information needed to improve inferred Gorda-NorthAmerica plate convergence.Prior to understandingof the southernCascadia subduction zone. the Cape Mendocino earthquake,the 1991 Honeydew

Papernumber 96JB00528. The Cape Mendocino earthquake occurred in the 0148-0227/96/96JB-00528509.00 vicinity of the Mendocinotriple junction, a broadlydefined

16,043 16,044 HAGERTYAND SCHWARTZ:THE 1992CAPE MENDOCINO EARTHQUAKE

41 øN

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' a.s. 1 Gorda ', Plate'

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125øW 123.5øW Figure 1. Map of the Mendocino triple junction showing locations and lower hemispherefocal mechanismsof earthquakesdiscussed in this paper (Cape Mendocinoand Honeydewmechanisms are determinedin this paper; aftershockmechanisms (a.s.1 and a.s.2) are from Dziewonskiet al. [1993]). Inset map showsthe geometryof the tectonicplates near the triple junction. Trianglesrepresent local strong motion stations (CM,Cape Mendocino; P,Petrolia); open circles denote aftershockswith magnitudegreater than 3.5 that occurredbefore the first large aftershock(a.s. 1). SAF,SanAndreas fault; MF,Mendocino fault; CSZ,Cascadia subduction zone.

zone located at the southern end of the Cascadia concentratedalong the Mendocinofault, the Gordaridge, subduction zone and the northern terminus of the San and the Blancofracture zone [ Tobinand Sykes,1968]. The Andreas fault, where the Gorda, North America, and Gorda plate is deforminginternally under predominantly Pacificplates meet (Figure 1). This regionof the northern north-southcompression [Silver, 1969]. This compressive California coastis one of the mostseismically active areas stressis due to both the southwardcomponent of motion of the continental United States. The majority of alongthe Blancofracture zone, which causesthe Juande seismicity is located offshore, diffusely distributed Fuca plate to the northto convergeupon the Gordaplate, throughout the Gorda plate [Smith et al., 1993] and and to the continuednorthwesterly migration of the triple

Table 1. SummaryInformation for EarthquakesDiscussed in Text

Event Date Time, Latitude, Longitude, Depth, UF deg deg km

CapeMendocino Apr.25, 1992 1806:04 40.33 -124.23 10.5 Aftershock1 Apr. 26, 1992 0741:40 40.43 -124.57 19.3 Aftershock2 Apr. 26, 1992 1118:26 40.39 - 124.57 21.7 Honeydew Aug.17, 1991 1929:40 40.29 -124.24 8.7 Honeydewaftershock Aug.17, 1991 2217:15 42.18 -125.64 11.0

Locationsare from D. Oppenheimer(personal communication, 1992). HAGERTY AND SCHWARTZ: THE 1992CAPE MENDOCINO EARTHQUAKE 16,045 junction.Motion along the Mendocinofracture zone is zone to depthsof 200-300 km and the frequentshallow predominantlyright-lateral, with somedip-slip movement thrust events characteristic of other subduction zones. near shore[Smith et al., 1993; Schwartz,1995]. Silver Riddiough[1984], from an analysisof seafloormagnetic [1969] suggestedpossible underthrusting of the Gorda anomalies,concludes that the Gorda-NorthAmerican plate platebeneath the Pacific plate, but recent offshore seismic velocitieshave been slowing for the past 12 Myr and that profiles[Smith et al., 1993;McPherson, 1989] as well as since3.5 to 2.5 Ma, the southernportion of the Gordaplate analysis of offshore seismicity, indicate that the hasceased to subductand is movingparallel to the trench. Mendocinofracture zone dips steeply(-80 ø) northward Ando and Balazs [1979] interpret crustal deformation which wouldpreclude any underthrustingof the Gorda acrosswestern Washington and the absenceof large thrust platebeneath the Pacific plate, except possibly for a small eventsduring the past 150 yearsas indicativeof continuous region near the coastwhere the Gorda plate appears aseismic subductionof the Juan de Fuca plate, while thickenedin crosssection [Smith et al., 1993]. The older, Spence [1987] arguesfor a diminished slab pull force morerigid Pacificplate appears to confineseismicity to a incapableof drivingsubduction. Nevertheless, a greatdeal narrow band near the Mendocino fracture zone, while the of evidence is accumulatingthat suggeststhe Cascadia younger,hotter, more buoyantGorda plate deforms subduction zone (CSZ) is seismically active with internally in responseto the north-southcompressive characteristic subduction event recurrence intervals of 200- stress. Silver [1971b], from an analysis of ocean 400 years. The Gorda ridge is seismicallyactive [Tobin bathymetry,magnetic anomalies and acousticreflection and Sykes, 1968] and has a well-defined, actively uplifted studies,suggested that the Gordaplate accommodates this median valley indicative of slow spreading[Atwater and north-southshortening by failing alongNE-SW oriented Mudie, 1968]. Silver [1969,1971a] found evidence of 2-3 left-lateral faults that formed originally as ridge-parallel cm/yr of late Cenozoic underthrustingof the continental extensionalfaults near the spreadingGorda ridge and were margin in offshore reflection profiles. More recently, laterreactivated. Failure along NE striking,near vertically Clarke [1992] identified several offshore fold-and-thrust dipping,left-lateral faults is furtherevidenced by studiesof belts forming west verging,imbricate fans that mergeinto the 1980 Eureka earthquake [Lay et al., 1982] and singlethrust faults that extend4-5.5 km abovethe Gorda- associatedaftershock trends [ Eaton, 1981]. North America plate interface and deform offshore From the diffuse natureof the seismicityit is clear that Holocene sediments. Onshore paleoseismicity studies the Mendocinotriple junctiondeviates substantially from along the Pacific northwest coast further corroborate the classic triple junction model where three rigid, continued subduction. Clarke and Carver [1992] estimate nondeformingplates meet at a single point. This is earthquakerecurrence intervals of 150-550 yearsfrom late somewhatunsettling since many of the fundamentalideas Holocene deformation of onshore sediments of the that form the basisfor plate tectonicswere developedwith accretionary prism, believed to be episodic slip on the the Mendocino triple junction in mind. Even the megathrustrather than localized displacementsbased on traditionally depicted offshore location of the triple the recognition of coeval deformations along large junction has becomesuspect. Clarke [1992] from an stretchesof the northern California and Oregon coasts. analysisof offshoreacoustic reflection data and onshore Nelsonetal. [1995],using high-resolution 14Cdating of geologicmapping, estimates the presentlocation of the buried trees and herbaciousplants, find evidencethat the triple junction to be onshore, approximately 25 km most recent prehistoricevent occurred in the early 1700s southeastof CapeMendocino in a broadregion defined by and ruptured mostof the lengthof the CSZ. A ruptureof the Petrolia, Cooskie, and Mattole shear zones near the this extent could produceearthquakes of magnitude8.4 or northflank of the King Range[see Clarke, 1992,figure 2]. greater [Heaton and Kanamori, 1984, 1985; Clarke and This impliesthat the offshoresegment of the SanAndreas Carver, 1992] and presents a serious threat to coastal from Point Arena to Punta Gorda has been or is being Pacific northwest. abandonedin favor of faults farthereast, an idea supported The Cape Mendocino earthquake is important in by the lack of seismicitysince 1906 along the 100-km-long assessingthis threat sinceit may be the first large event to offshore segmentof the San Andreas fault and by the occur on the megathrustin historictimes (last 200 years). partitioningof Pacific-NorthAmerican plate motion near As the earthquakedid not producea surfacerupture and Punta Gorda over a 35 to 80-km-wide zone of faults was not followed by aftershockswith similar mechanisms located well east of the San Andreas [Freymueller and that define a ruptureplane, the only meansof determining Segall, 1994]. This is compatiblewith models of the the fault geometry is by analyzing the seismicradiation Pacific-North American plate boundary north of San pattern. Franciscoin which the crustalboundary, located beneath the SanAndreas, is beingabandoned in favor of the deeper, Event Analysis lithosphericplate boundarybeneath the Haywardfault to the east[Furlong, 1984]. We separatelyanalyze long-period surface waves (150- The natureof subductionalong the Cascadiasubduction 300 s) and broadbandbody waves(2.5 Hz to 50 s). Useful zone is fervently contested. Many researchersquestion sourceparameters (strike, dip, rake, moment,source time whether subduction there occurs seismically or function) can potentiallybe extractedfrom either data set. aseismicallyand even whether subductionis occurringat However, given their different frequency contentsand all. Althoughthere is little doubtthat subductionoccurred varyingdegrees of sensitivityto the sourceparameters, it is in the past, there is a paucity of evidence that supports not surprisingthat these data sets often yield different subductioncontinuing today. The Cascadiasubduction estimatesof the same sourceparameters. In addition, the zone lacks the seismically well-defined Wadatti-Benioff sourceparameters are extractedfrom the data setsusing 16,046 HAGERTY AND SCHWARTZ: THE 1992CAPE MENDOCINO EARTHQUAKE different techniques,with inherentlydifferent resolving Table 2. Stationsand PhasesUsed in Body and Surface capabilities.Nevertheless, it is hopedthat after accounting Wave Analyses for the varying sensitivitiesand assumptionsof each method,robust estimates of the sourceparameters will be Stn. Net. Dist., Azim., Surface Body obtained. We will seethat by comparingsource parameters deg deg Waves Waves obtainedindependently from the differentdata setsand methods,a priori informationcan be introducedinto the AAK IDA 95.69 346.23 R1,R2 independentanalyses that will improveresolution of the BDF IDA 89.94 110.61 P, SH BJI GSN 82.92 317.73 R1,G1,G2 P source parameters. Each data set offers a different CAN GEO 109.63 240.09 R1,R2 frequencywindow throughwhich to view the earthquake CTAO GSN 102.36 54.19 R 1,R2 rupture. We begin our analysiswith the long-period CHTO GSN 108.75 316.77 G1,G2 surface waves which are more sensitive to the overall or ERM GSN 66.18 305.54 R1,R2,G1 P, SH ESK IDA 72.32 30.94 P, SH averageproperties than to the finer detailsof the rupture. GDH GSN 46.18 27.79 R2 Later we analyzebody wavesto refine our estimateof the GUMO GSN 82.02 280.96 R1,R2 P, SH fault geometryand reconstructthe temporalhistory of the HIA GSN 74.49 322.35 R1,R2,G1,G2 P rupture. HRV GSN 39.03 69.01 P, SH INU GEO 73.96 302.79 G1,G2 P, SH KEv GSN 68.22 10.39 R1 P, SH Surface Wave Analysis KIP GSN 34.30 246.84 P Data. The surface wave data set consists of three- KMI GSN 101.58 317.44 R1,R2,G1,G2 KONO GSN 73.35 22.48 R1,R2,G2 P, SH component,long-period digital data recorded by stationsof LSA GSN 102.97 328.87 R1,R2,G1 three global seismicnetworks: the Global Seismograph LZH GSN 92.05 322.91 G1,G2 Network (GSN), the International Deployment of MAJO GSN 72.47 303.16 R1,G1,G2 P, SH Accelerometers (IDA) and GEOSCOPE. Fundamental MDJ GSN 72.60 314.04 R1,R2,G1,G2 P, SH NNA IDA 68.08 129.08 R1,R2,G1 P, SH mode Rayleigh wave arrivals (R1, R2, and R3) were OBN IDA 83.52 10.96 P, SH isolated on the vertical componentsusing appropriate RPN IDA 68.53 165.75 P, SH groupvelocity windows. The horizontalcomponents were SCP GSN 34.91 73.79 P rotated and the Love wave arrivals (G1, G2, and G3) were SNZO GSN 98.36 221.79 R1,R2 SSB GEO 83.19 33.67 R1,R2,G1 P isolatedon the transversecomponents. The isolatedphases SUR IDA 150.89 85.33 R1,R2 (R1-R3, G1-G3) were subsequentlylow-pass filtered and TOL GSN 83.34 42.04 R1,R2 P, SH examined. Phases with low signal to noise, unreliable UNM GEO 30.00 126.88 R1,R2 P instrumentresponse, or anomalousfocusing effects were WMQ GSN 91.42 337.49 R1,R2,G1,G2 P ZOBO GSN 76.70 125.00 G2,G3 discarded. Unfortunately, many of the Love wave recordingsfor the Cape Mendocinoearthquake had a low signalto noiseratio andcould not be used,greatly reducing IDA, InternationalDeployment of Accelerometers;GEO, GEOSCOPE;GSN, Global SeismographNetwork. the numberof phasesavailable. The 38 Rayleighwave and 26 Love wave phasesmeeting the abovecriteria are listed in Table 2. Spectral moment tensor inversion. We analyzelong- period (150-300 s) Rayleigh and Love waves using a errorsof the step-oneinversion can be usedto assessthe spectral moment tensor inversion method, developedby suitability of the sourceduration and the phase velocity Kanamori and Given [1981] and modified to a two-step model. By stepping over increasing values of source procedurethat separatesthe sourcefiniteness effects from duration, we can choose an optimal duration for each the centroid depth and moment tensor determinations periodand wave type (R or G) basedon the step-oneerrors. [Romanowicz and Guillement, 1984]. The simultaneous In the step-twoinversion we invert different subsetsof the inversion of Love and Rayleigh wave spectrais further datafor the momenttensor components. The surfacewave describedby Zhang and Kanamori [1988]. excitationcoefficients are functionsof depth,suggesting a As in all wave propagationproblems, it is impossibleto meansto locate a best centroiddepth by minimizingthe exactly separatethe effects of the sourcefrom thoseof the step-twoleast squaresinversion error. Unfortunately,the propagationpath on the recordedsignal. In our analysiswe vanishing near the Earth's surface of the excitation must correct the observed surface wave spectra for coefficientsthat weight the M x•and My z momenttensor propagationand source finitenesseffects using several componentsin the inversion prevents a stable depth Earth models (phase velocity, group velocity, anelastic estimationfor shallow thrust events. In the following attenuationand sourceexcitation) before inverting for the analysiswe fix the sourcedepth at 15 km to stabilizethe source parameters (moment tensor and source duration). inversion.Since the resulting moment tensor changes very The purpose of the two-step inversion is to separatethe little for depths between 10 and 20 km, this should not dependence of the estimated source duration on the affectthe inversionresults adversely. assumed velocity model from the dependence of the Surface wave results. Figure 2 is a plot of the estimateddepth and momenttensor on the assumedsource weighted,step-one least squaresinversion errors at each excitation model. We perform the inversion at seven period for sourcedurations ranging from 1 to 100 s for discreteperiods (155, 175, 200, 225, 256, 275, and 289 s), three different global phase velocity models: PREM chosento correspondto the normalmode periods at which [Dziewonskiand Anderson, 1981], M84C [ Woodhouseand several Earth models are tabulated. The least squares Dziewonski,1984], and MPA [Wong, 1989]. PREM is a HAGERTY AND SCHWARTZ: THE 1992 CAPE MENDOCINO EARTHQUAKE 16,047

0.25 T M84C • MPA 155 175

0.20 200 0.15 225

256 0.10 275 0.05 289

0.00 I I i I 0 20 40 60 0 20 40 60 0 20 40 60 Duration (sec) Figure 2. Weighted step-oneleast squares errors versus source duration. Plot showsduration estimates for Earth models PREM, M84C, and MPA used to correct for the propagationof Rayleigh and Love wavesat eachof sevenperiods (155, 175,200, 225,256, 275, and289 s) prior to the step-oneinversion. spherically symmetric model, while M84C and MPA was chosen because it resulted in a minimum residual in incorporatelateral heterogeneityup to angulardegrees 8 the step-twoinversion. From Table 3 we seethat the main and 12, respectively. From Figure 2 we see that the differencebetween the inversionsat differentperiods are optimal sourceduration dependson the phasevelocity the calculatedvalues of the Mxz andMvz momenttensor modelused. The scatterin optimalsource duration for the components.This is dueto theinstability in estimatesof differentperiods could reflect a frequencydependence of thesetwo componentsfor shallowevents. In Figure3 we the centroidtime, howeverit morelikely reflectsdeviations showthe corrected Rayleigh and Love amplitude and phase of the real Earthfrom the phasevelocity models, which are spectraat 256 s alongwith the predictionsfor the moment more severe at shorterperiods. Since MPA consistently tensorobtained by invertingall sevenperiods of Rayleigh yields the least step-oneinversion error, we use it from and Love wave spectrasimultaneously. The Rayleigh here on in our analysis. By averagingover the optimal wave spectralfits are fairly good,but thereis considerable sourceduration times for eachperiod we obtainan average scatterin the Love wave spectra.Some of the scatterin the source duration of-20 s. Rayleigh and Love wave amplitudescan be attributedto The moment tensor inversionswere performed using errorsin the sphericallysymmetric, global attenuation (Q) Rayleighand Love wave spectratogether at singleperiods, model used. Published data on attenuation of surface at all periods simultaneously,and using Rayleigh spectra waves and normal modes differ by more than 5% in the alone at all periodssimultaneously. The inversionresults 150 to 300 s period range [e.g., Zhang and Kanamori, are summarizedin Table 3 along with the resulting best 1988]. Our useof greatcircle paths and neglect of possible double couples. Prior to inversion the spectra were focussingeffects contributeadditional amplitude errors. correctedusing phasevelocity model MPA [Wong, 1989] Fortunately,it is the phasespectra that are key to resolving and anelastic attenuation model K70 [Kanamori, 1970]. the fault orientation in the spectral moment tensor The averageoceanic model (RA) [Regan and Anderson, inversion;amplitude errors primarily affectthe estimatesof 1984] was used to compute the source excitation seismic moment. Since we are most interested in the coefficients. This particular combinationof Earth models doublecouple representation of the source,it is instructive

Table 3. SurfaceWave MomentTensor Solutions for the 1992 CapeMendocino Earthquake

R+G

T= 155 T= 175 T=200 T=225 T=256 T=275 T=289 All Periods All Periods

M•o, 0.31_+0.040.26_+0.04 0.34_+0.03 0.38_+0.03 0.45_+0.02 0.44_+0.02 0.42_+0.01 0.39_+0.03 0.46_+0.03 Myy- Mxx -0.74_+0.07-0.63_+0.06 -0.66_+0.05 -0.68_+0.05 -0.69_+0.03 -0.67_+0.02 -0.67_+0.02 -0.67_+0.05 -0.44_+0.01 Myy+ Mxx -1.84_+0.04-1.75_-+0.04 -1.7•.03 -1.71+0.24 -1.78-•-0.02-1.70-M).01 -1.58_+0.01 -1.71_+0.03 -1.78_-+0.55 Mvz 1.73_-_4-0.550.53_-+0.61 1.82_-+0.60 2.37_-+0.66 3.50-M).76 3.20-M).62 2.60•.50 1.97_+0.72 0.83_-+0.50 Mxz -3.13_+0.55-0.83_+0.61 -1.81_+0.60 -0.63_+0.66 -0.64_+0.77 -3.08+.063 -4.79_+0.50-1.81_+0.72 -0.54_+0.50

Strike, deg 337.10 345.79 341.24 333.24 328.79 339.18 338.21 340.29 327.87 Dip, deg 13.34 31.37 16.07 16.22 12.74 10.11 8.43 15.64 29.11 Rake,d• 123.51 110.28 111.89 80.08 71.31 111.66 128.00 109.44 90.53 M o, x10 N m 3.94 1.85 3.00 2.91 3.91 4.71 5.65 3.10 1.90

Moment tensorconvention is after Kanamori and Given [ 1981]. 16,048 HAGERTY AND SCHWARTZ: THE 1992 CAPE MENDOCINO EARTHQUAKE

Rayleigh Waves Love Waves 0.5

ß ß

e ß

0.3 e e ß •e

0.2 .

0.1

0.0 • • • • •

ß ß ß ß ß .. I ß ß ß ß ß ß ß

I I I I I ß I I I 60 120 180 240 300 360 0 60 120 180 240 300 360 Azimuth(deg) Azimuth(deg) Figure3. Fitsto Rayleighand Love wave amplitude and phase spectra at 256 s period.Predictions (solidlines) are for the step-twoinversion for moxnenttensor of Rayleighand Love wave data at all sevenperiods simultaneously.

to examine Table 3 for consistenciesin the solutions to We perform a linearized inversionof the teleseismicP determinewhich doublecouple parameters are well- andSH waveformsto determinethe momenttensor and the resolvedby the inversions.The strikeis -330-340 ø and the sourcetime function. Following Langston et al. [1982], dip is -8-14ø;however the momentis poorlyresolved we writethe P andSH displacementsas

(1.85-5.65x1019N m) and trades off with the dip. Also, the nr 5 rakeis notwell-constrained, and we cannotconfidently pick between a rake of 70 ø and a rake of 120ø . uP'Sn(t,r,z)=EEM•s•.[T(t-1=1 J=l v•.)* Hf'Sn(t,r,z)] (1) Examinationof the amplitudeand phase fits for different doublecouple models spanning the range in rakereveals whereMj arethe five momenttensor elements, s•: are the that the main differencein the two solutionsis the fit to the sourcetime function amplitudes, T(t-x•:) is an elementof Lovewave phase spectra in a narrowrange of azimuthsand thesource time function, Hj arethe media step function thatthe Rayleigh waves cannot distinguish between the two responses(Greens functions) for the individual moment valuesof rake. This is problematicsince there is much tensorelements, and the asteriskdenotes convolution. We scatterin the Love wavespectra, due in partto the low parameterize the source time function as a series of signal-to-noiseratio for theLove waves for thisevent, and overlappingtrapezoids (subevents) asdescribed byNdb •lek we would rather not choosea solutionbased on a few Love [1984]. Each trapezoidcan be consideredthe result of wavephases. We will seein thenext section that the body convolvingtogether two boxcars,one representing the waveshelp to resolvethis dilemma. subeventrise time Ax r andthe other the subevent rupture duration Az: Body Wave Analysis T(t-'c•)= B/xz(t)*B/xzr(t),,•k=AX(k_l), k=l,n'• (2) Data and method. In this sectionwe analyze teleseismicP and SH bodywaves digitally recorded by In thisway, horizontal directivity due to unilateral rupture broadbandstations of the GSN, IDA and GEOSCOPE canbe incorporatedby varyingthe subeventrupture networks. In order to avoid the effectsof the Earth'score durationfor stations atdifferent azimuths according to andupper mantle discontinuitites on the waveforms,we restrictour distance range to 300-90ø. P wavearrival times A'c'= A'c[1- VrpCOS( Or_O) ] (3) pickedon the vertical components are used to window60 s whereAz is therupture duration of an elementof thefault of waveformdata. The instrument response isdeconvolved plane in the absenceof directivity,Vr is the horizontal fromthe traces,and the waveformsare integratedto rupturevelocity, p is theray parameter, Or is therupture displacement.After rotating the horizontal components, azimuth, and 0 is thestation azimuth [e.g., Ben-Menahem, theabove procedure is appliedto the SH arrivalsrecorded 1961;Ndb•lek, 1984]. We wishto relatethe differences on thetransverse components. The resulting waveforms between the synthetic and data waveforms toperturbations arelow-pass filtered and decimated to five samplesper tothe model parameters. Following Langston etal. [1982], second.Table 2 liststhe station parameters. wewrite the linearized system of equations: HAGERTY AND SCHWARTZ: THE 1992 CAPE MENDOCINO EARTHQUAKE 16,049

Table 4. Body Wave Moment Tensor InversionResults and PublishedSolutions for the 1992 Cape MendocinoEarthquake

Source Mxx M.•y Mx•, Mr M): Strike, Dip, Rake, Mo deg deg deg N m

Model 1 -0.33 -0.60 0.18 -0.39 1.74 331 13 76 1.9x1019 HarvardCMT * -0.79 -1.42 0.45 -0.62 6.39 331 9 68 6.7x1019 NEIC * -0.40 -2.19 -1.47 -1.01 4.11 51 25 142 5.1x10•9 Oppenheimer* 350 13 106 4.5xl019 PDE first motion õ 352 13 90

*CMT, Centroidmoment tensor; from Dziewonski etal. [ 1993]. *NEIC,National Earthquake Information Center; from Sipkin [1994]. •+FromOppenheimer etal. [ 1993]. õPDE,Preliminary Determination of Epicenters.

Ac= AAp (4) correspondingbest double couple mechanism(q•=331 ø, 15=13ø, X,=76ø) is similar to the Harvard centroid moment where Ac is a vector of differences between the observed tensor (CMT) solution ((•=331 ø, 15=9ø , )•=68ø) and synthetic waveforms, A is a matrix of partial [Dziewonski,aet al., 1993] and contains a significant derivatives of the syntheticwaveforms with respectto the componentof thrust motion in the direction of inferred model parameters, and A p is a vector of model Gorda-NorthAmerica convergence.However the resulting perturbationsfor which we wish to solve. We invert (4) for bodywave moment, 1.93x1019 N m, is muchsmaller than the model perturbations using a singular-value the Harvard CMT moment (Table 4). This discrepancy decompositionmethod [Wiggins, 1972]. The body waves will be addressedlater in this paper. The resultingfits to for this event display a great deal of complexity which the waveforms are shown in Figure 4 along with the proveddifficult to model with a simplepoint source. In the correspondingsource time function. The waveformfits are following sectionwe investigatethree different modelsfor generally good but note the unmodeledenergy late in the the Cape Mendocino earthquake:a single point source,a waveforms, particularly at azimuths to the northwest for point source with horizontal directivity, and two point which the direct P wave is nodal. There are several sources combined. possible explanations for this misfit energy, including Body wave modeling results: Single point source. In source or receiver structure that is unmodeled by our our first model (model 1) we neglectpossible directivity simplisticlayer-over-a-half-space Greens functions, and a effectsand parameterizethe sourceas a point sourcewith a complex sourcetime function of extendedduration. We total ruptureduration of 9 s. A ruptureduration of 9 s was can rule out anomalously large propagation phases determined from preliminary body wave inversions and originating far from the sourceregion (e.g., PcP) since was also found by Velasco et al. [1994] using empirical thereis no moveoutof the late phasewith distancefrom the Greens functions. For this model the subeventrupture source. In the following sectionswe will rule out possible durations and rise times were set to 1 s so that the source extendedduration source complexity, and by modelingthe time function is describedby nine overlappingtriangles. 1991 Honeydew earthquakewe will rule out unmodeled The synthetic Greens functionswere computedfor a 25- receiver structure. km-thick layer with a compressionalvelocity of 6 kin/s, The resultingbody wave mechanismis slightlyrotated underlainby a half-spacewith a compressionalvelocity of from our surface wave mechanisms. We would like to 8 km/s. The results of the moment tensor inversion are havesome estimate of how well the bodywaves resolve the summarizedin Table 4 along with the correspondingbest double coupleparameters. We could attemptto estimate doublecouple mechanism. By performingthe inversionat confidencebounds on the doublecouple parameters from severaldepths we estimatethe sourcedepth to be 8-18 km. the formal inversion errors associated with the moment We cannotestimate the depth to better precisiondue to the tensor components, but the results would be difficult to strongtrade-off between the point-sourceGreens functions interpretand probablyoverly optimistic. Instead,in order and the sourcetime function. This degradesour ability to to delimit the range of parameters((•,15,•) that will determinethe sourcedepth from the relative timing of the adequatelyfit the waveforms, we forward model the data depth phases (pP,sP for P) as errors incurred from using a range of double couple models. The forward computing Greens functions at the wrong depth are problem can be written as Grn=d, where G is a matrix of absorbedinto the resulting source time function in the lagged point source Greens functions for a particular inversion.In the analysesto follow we fix the sourcedepth doublecouple model, rn is a vectorof sourcetime function at i0.6 km, the optimal depth determinedby analysisof amplitudeswe wish to solve for, and d is the data vector. short-period regional data (D.Oppenheimer, personal By invertingfor rn, usingstandard least squares techniques, communication,1992). We tried severaldifferent starting we are simultaneouslydeconvolving a best sourcetime modelsto examine the influenceof the startingmodel on function from all the waveforms, d. To determine the the outcome of the inversion and all runs converged to parameterresolution we performa grid searchover values similar results within two to four iterations. The of strike, dip, and rake and invert for the source time 16,050 HAGERTY AND SCHWARTZ: THE 1992 CAPE MENDOCINO EARTHQUAKE

wMo•

MDJ •

KIP

SH: wao -J \\,,¾•',.•

MDJ ERM,•. ESK INU

$(t)

0.2 ,''••• NNA

0.1 RPN

ß i ' I i i l I I 0 2 4 6 8 10 sec 0 60 sec

Figure 4. Plot of P and SH waveformsfor the 1992 CapeMendocino earthquake. Solid lines are data; dashedlines are syntheticsfor model 1. Also shownis the correspondingsource time function.

function, s(t). The results of the grid search can be we impose positivity constraintson the solutionusing the evaluatedeither by the least quaresinversion error or by methodof nonnegativeleast squares [Lawson and Hanson, the weighted root-mean-squareresidual (wrms) between 1974]. To reduce bias due to the late energy in the the data and synthetics In order to avoid unrealistic waveformsthat is unmodeledby our Greensfunctions, we negativeexcursions of the resultingsource time functions, windowthis energyout prior to computings (t) and the HAGERTY AND SCHWARTZ: THE 1992 CAPE MENDOCINO EARTHQUAKE 16,051

(a) (b) Strike=34o o Strike=30 o • 0.54 '- Dip=13 o E 0.52 Rake=130 o ,_• 0.520.54 - ß -u 0.50 -o 0.50 ß ß .• ß _c 0.48 .r: 0.48 .? • 0.46 ß ß ß ß ß ß ß • 0.46 ß 0.44 • • • • I • • 0.44 • • ! 8 10 12 14 16 18 20 50 60 70 80 9o lOO

Strike= 3500 Strike= 330 o • 0.54- • 0.54 Dip=13 ø E 052- Rake=9øø E0 52 -u 0.50 ß -o 0.50 ß 2 _c 0.48 .r: 0.48 .? • 0.46 ß . ß • 0.46 0.44 • • I T ' ? ' • • I 0.44 I • I 6 8 10 12 14 16 18 20 50 60 70 8O lOO

Strike =3300 Strike = 32o o 0.54 0.54 • 0.52 Rake=75 o • 0.52 Dip= 13o g 0.50 ß ß g 0.50 ß _c 0.48 ._• 0.48 ß ß ß ._•2• ß • 0.46 ß . . . ß •: 0.46 . 0.44 • I I • ' • ' I • I 0.44 I 6 8 10 12 14 16 18 20 SO 60 70 80 9O lOO Dip, 6(ø) Roke, X (0)

310

32O

330

340

350 360

4O 5O 6O 7O 8O 90 100 110 120 130 140 Rake (deg) Figure5. Slicesthrough the three-dimensional errorvolume formed by solving the forward problem for a rangeof strike,dip and rake for theCape Mendocino earthquake. (a) Weightedrms error (wrms) versusdip for fixed strike and rake, (b) wrms versus rake for fixed strike and dip, and (c) wrms contours versusstrike and rake for fixed dip (13ø).

wrmserrors. Figures5a-5c show different slices through This canbe seenin Figure5c wherewe plot the wrmserror the three-dimensionalerror volumeformed by solvingthe versusstrike and rake for a fixed dip of 13ø. The trade-off forwardproblem for a rangeof strike,dip, and rake. Figure between strike and rake is nearly perfect and presentsus 5a showsthat for fixed valuesof strike and rake, the dip is with a problem:how do we choosea beststrike and rake? well-resolvedand its value changesslowly within the We must introducesome form of a priori informationto acceptablerange of strike and rake. Figure 5b narrowthe range of possiblevalues of strikeand rake. The demonstratesthat for a fixed dip of 13ø, the rake is well- surfacewave analysis constrained the striketo lie between resolved. However,unlike the dip, the resultantvalue of 330ø and 340ø. From Figure5c we seethat by restricting rakeis stronglydependent on the assumedvalue of strike. the striketo be in this range,the rake is constrainedto lie 16,052 HAGERTYAND SCHWARTZ:THE 1992CAPE MENDOCINO EARTHQUAKE

Table 5. Body Wave Moment TensorInversion Results and PublishedMechanisms for the 1991 HoneydewEarthquake

M•c M•, Mxv M• M>z Strike, Dip, Rake, Mo Source " deg deg deg N m

Thisstudy -0.35 -0.92 -0.33 0.13 0.57 28 32 97 1.3x1018 HarvardCMT* -0.47 -0.72 0.46 0.08 1.55 311 22 51 1.9x1018 NEIC ñ 0.38 -2.43 -1.19 -1.02 1.91 48 38 141 3.3x10 ]8 PDE first motion 348 22 90

From Dziewonski et al. [1992]. FromSipkin [ 1993].

between 70 ø and 80ø . Thus, combiningthe well-resolved waveforms is not due to unmodeled source or receiver parts of the surfaceand body wave analysesallows us to structure. arrive at a solution that is better constrained than either Unilateral rupture model. In our secondmodel of the individual solution. This is perhapsexpected, since the Cape Mendocino event (model 2), we model the sourceas verticalradiation pattern of teleseismicbody waves is most a horizontally propagating, unilateral rupture. The sensitiveto the dip, whereasthe horizontalradiation pattern moment tensorwas held fixed to the point-sourcemoment of surface waves is •nost sensitive to the strike and rake. tensor (model 1). The resulting effects of horizontal As mentioned earlier, the effects of the source trade off directivityon the waveformsare includedby adjustingthe with those of Earth structure on the recorded waveforms. subeventrupture duration at differentazimuths according The trade-offs become more severe as the source duration to equation(3). We triedseveral different values of rupture increases. Furthermore,for large events(M>7) our failure velocityv r and azimuth q)rand found bestresults for v r = to include the effects of finite rupture extent in our point- 2.5 km/s and q)r = 240ø, indicating updip (southwest) source Greens functions introduces additional errors. In rupture. Ammon et al. [1993] and Velasco et al. [1994] orderto testthe adequacyof our Greensfunctions to model usedthe Honeydewevent as an empiricalGreens function Earth structure for this region, we analyze P and S H to infer that the Cape Mendocinoearthquake ruptured waveformsfrom the nearby 1991 Honeydewevent (M w 6) towards an azimuth of 230ø and 262+26ø , respectively, which is better approximatedas a point sourcethan the consistentwith our findings.Modeling of locallyrecorded larger Cape Mendocinoevent. By usingsome of the same stationsas for the Cape Mendocino event, we can assess the role of unmodeled source and receiver structure in the Honeydew Grid Search Cape Mendocinowaveforms. Investigation of umodeled receiver structure: The 0.50 1991 Honeydew earthquake. The results of our body 0.45 wave inversionfor the Honeydew event are summarizedin 0.40 II Table 5, alongwith otherpublished solutions for this event. 0.35 The resultsof performinga double couplegrid searchare 0.30 110+20 ø displayedin Figure 6. By defining a maximum allowable 0.25 • • • • • 40 80 120 160 wrms of 0.33, we can estimatebounds on the doublecouple Rake (deg) solution:q)=30+20 ø,/5=35+5 ø,Z=110+25 ø. Thisrange of solutionsencompasses the U.S. GeologicalSurvey (USGS) 0.55 0.50 moment tensor solution but is quite different from the 0.45 Harvard CMT solutionand the Preliminary Determination 0.40 of Epicenters(PDE) first-motion solution(Table 5). This 0.35 is in contrast to Velasco et al. [1994], who found that the 0.30 /5:35+5 ø CMT Honeydew mechanism (311ø/22ø/51ø),with its 0.25 • • • • I I similarity in strike and rake to the CMT Cape Mendocino o 10 20 30 40 5O 6O mechanism (331ø/9ø/68ø ) was compatible with their Dip (dcg) empirical Greensfunction results. However, on the basis 0.55 of the grid search we infer that the Honeydew strike is ,,, 0.50 I rotatedwith respectto the Cape Mendocinostrike and the • 0.45 fault planesare more steeplydipping. This could account • 0.40 ß-• 0.35 for the difficulty Velasco et al. [1994] encounteredin •: o.3o (•: 30-Z-_10 ø obtainingrelative sourcetime functionsfor stationsto the 0.25 I northwest. Figure 7 is a plot of the data along with 1 O0 200 3OO syntheticwaveforms generated for our bestdouble couple Strike (deg) mechanism(Figure 1) determinedfrom the grid search. Figure 6. Slices through the three-dimensionalerror The fits to the waveformsare generallygood and suggest volumeformed by solvingthe forwardproblem for a range that the unmodeled energy in the Cape Mendocino of strike,dip, andrake for the 1991Honeydew earthquake. HAGERTY AND SCHWARTZ: THE 1992CAPE MENDOCINO EARTHQUAKE 16,053 KEV WMQ HIA• • / • "•« •' KONO

TOL

ERM

/ HRV MA•O

KIP.½ - • NNA

0.2

0.1

0 1 2 3 4 5 6 7 sec Figure7. Plotof P andSH waveformsfor the 1991 Honeydew earthquake. Solid lines are data; dashed linesare syntheticsfor the grid searchdouble-couple mechanism (30ø/35ø/110ø). Also shownis the correspondingsource time function. strongmotions [ Oppenheimer et al., 1993]also favors an Multiple point sourcemodel. In orderto seewhy a updip(westward) rupture propagation. multiplepoint sourcemodel may be appropriatefor the Althoughthe inclusion of westwarddirectivity improves CapeMendocino earthquake, it is instructiveto examine the fit to theearly part of the waveforms,the late arriving the sourcetime function that results when a single point energyremains poorly modeled. We triedseveral different sourcehaving the mechanismof model 1 is permittedto approachesto modelthis energy, including a finite-fault rupturefor an extendedduration (27 S). Figure8 shows modelin whichthe fault strikeand dip werefixed but the sourcetime functionsthat resultfrom solvingthe forward amountand direction of slipwere allowed to varyover the problemusing singular-value decomposition (svd) and fault plane. Althoughfits to the early portionof the nonnegativeleast squares (nnls) methods. The negative waveformswere improved over the pointsource models, excursionof the svd solutionafter 9 s and the reappearance fits to the later portion of the waveformswere not of positivemoment release in both the svd and nnls significantlyimproved. In ouropinion the large number of solutionsafter 19 s suggestthat during the intervalfrom 9 additionalfree parametersintroduced in the finite-fault to 19 s, energywas releasedfrom a secondevent with a modelingis notjustified by thesmall improvement in fit. mechanism different from the first. Further evidence for 16,054 HAGERTYAND SCHWARTZ:THE 1992CAPE MENDOCINO EARTHQUAKE s(t) " mechanismfor the secondevent for a delay time of 14 s are 0.2 nnls displayedin Figure 10. Note the improvedfits to the later parts of the waveforms for P waves at azimuths to the northwest and for SH waves at azimuths to the northeast. 0.1 The improved fit to the waveforms over both extended duration and finite-fault models argues for a change in mechanism from the first event. Although the exact relationshipbetween the timing of pulsesin the locally s(t) recordedstrong motions and in the teleseismicallyrecorded 0.2 svd displacementsis not well-understood,it is interestingthat the bestfitting delay time (14 s) is comparableto the onset 0.1 of the secondarypulse seen in the strongmotion recordings (13 s). Owing to the lack of a clear onset of the second event, there are simply too many parametertrade-offs to confidently constrainthe mechanismof the secondevent .... I , , , , I .... • • • I , , , , I , , without a priori information. In our analysiswe used 0 5 10 15 20 25 Greensfunctions computed at the depthof the first event Seconds (10.5 km); a changein the depth of the secondevent will introduce additional errors. Figure 8. Extended duration sourcetime functionsfor the Cape Mendocino earthquakethat result from solvingthe While we do not feel the details of the second forwardproblem using nonnegative least squares (nnls) and mechanism are well-constrained, the majority of singular-valuedecomposition (svd) methods. mechanisms which gave a good fit to the data were characterized by a horizontal, north-south oriented compressionalaxis. This suggestsan offshorelocation for the second event, in the region of the triple junction multiple ruptures comes from local strong motion dominatedby horizontal north-southcompression. In a recordings. Figure 9 shows three componentsof separatestudy of the mechanismsof aftershocksoccurring accelerationrecorded at stationsCape Mendocino and several days after the Cape Mendocino earthquake, Petrolia operatedby the California Strong Motion Schwartz [1995] found that the majority of aftershocks InstrumentProgram (CSMIP). Both stationsare located locatedoffshore and could be groupedinto threedifferent within 10 km of the epicenter(Figure 1). Discretepulses of energysome 13 and 33 s after the arrival of the primary shearwave are inferred to be secondaryshear waves from their enhancedamplitudes on the horizontalcomponents. Cape Mendocino Vertical Apparentlythese aftershocks occurred too soonafter the mainshockto retriggerlocal short-periodinstruments of the Northern California Seismic Network (NCSN) as the first .....•.. aftershocklisted in the NCSN earthquakecatalog occurred East some 14 min after the mainshock. Althoughthe secondary pulsesare visible at several of the local strongmotion stations,their timing is not sufficientlyclear to locate their origin with respectto the mainshock.There is no obvious Noah moveout of the later pulses with distance from the mainshock epicenter, however, which would preclude significant offset of the secondary event from the . . . i . . . i . • . . mainshock,particularly in the north-southdirection for which the station geometry provides better moveout Petrolia Vertical resolution. In our final model of the Cape Mendocinoevent (model 3), we parameterizethe sourceas two discreteruptures of 9 and 10 s duration. We fix the parametersof the first source East to thoseof model 2 and invert the data for the parameters of the second source: moment tensor, source time function and delay time with respectto the onsetof the first source. Not surprisinglythere are great trade-offsin all of these parameters. Our approachis to fix the onsettime of the secondsource and invert for perturbationsto the moment tensorand sourcetime function. We performedinversions using delay times from 1 to 20 s. Several different delay Seconds times yielded similar fits to the waveformsbut resultedin very different mechanismsfor the secondevent. A delay Figure 9. Raw, three-componentaccelerations recorded at time of 14 s gave the bestfit to the waveforms. The fit to local strongmotion stationsCape Mendocinoand Petrolla. the waveforms and the correspondingdouble couple Arrowsdenote secondary arrivals discussed in thetext. HAGERTY AND SCHWARTZ: THE 1992 CAPE MENDOCINO EARTHQUAKE • 16,055

SH:

KEV

OBN

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ESK ..-••,•,•.•TOL

*••f••.•"•BDF s(t)

0.2 NNA

o.1

0 2 4 6 8 10 12 14 16 18 20 sec 60 sec

Figure 10. Plot of P and SH waveformsfor the Cape Mendocinoearthquake Solid lines are data; dashedlines are syntheticsfor model 3. Also shownare the sourcetime functionsand focal mechanisms for the two point sourcesof model 3. 16,056 HAGERTY AND SCHWARTZ:THE 1992CAPE MENDOCINO EARTHQUAKE depth intervals. Although each depth interval was and Yu's [!986] proposedHumboldt plate separatingthe characterizedby a different averagesource mechanism, all GDZ and the North American plate, as clearly indicativeof of the mechanisms shared a north-south, horizontal Garda-Humbaldtplate convergence.However, we hesitate compressionalaxis. The mechanismfor our best delay to introduce an intervening plate to account for the time (14 s) is a rotated thrustmechanism similar to those discrepancybetween our resulting slip direction and the found for several aftershocks in the 15-22 km depth inferred direction of Garda-Narth America plate interval of Schwartz's [1995] study. It is importantto note, convergence but rather note that the tectonics of the however,that while a rupturefront propagating at 2.5 km/s Mendocino triple junction are extremely complicated,so towards an azimuth of 230 ø for 9 s would extend into the that earthquakeslip vectorsmay not be wholly indicative zone of offshore aftershockslocated by Schwartz[1995], of plate motions. The Cape Mendocinoearthquake served our findingsare not consistentwith a continuousrupture of to relieve strainaccumulated as a resultof convergenceof the mainshockand aftershockfault planes,but with two, the Gardaand North America plates. Determiningwhether discrete ruptures separated by several seconds of this event actually ruptured the interplate megathrust, quiescence. however, is not possible from our analysis alone but Table 4 summarizesthe resultsof our body wave model dependson independentinformation about the locationand as well as published solutionsfor the Cape Mendocino depth of the subductedGarda plate near the hypacenter. earthquake. Note the large discrepanciesamong the Jachens and Griscom [1983] interpret isostaticresidual different solutions, particularly in the estimatesof the gravity and aeromagnetic[Jachens and Griscom, 1994] seismic moment. The moment of the second event, anomaliestrending S60øE to delineatethe southernedge of 0.49x1019N m, is about25% of the mainshockmoment the Garda plate. Their depth profile for the top of the and correspondsto an event of magnitude6.4. With the subductedGarda plate dips 9øE and locates the plate inclusionof the secondevent, the total body wave moment interface at around 10 km beneath Cape Mendocino. for model 3 is about 2.5x1019 N m. This is still less than However, Verdonck and Zandt [1994] interpret three- half the size of the Harvard CMT moment, though it is dimensional local earthquake tamagraphy results to comparableto a momentof 2.79x10•9 N rn for thisevent indicate velocitiestypical of the Garda crust at depthsof estimatedfrom geodeticdata [Oppenheimeret al., 1993]. 15-20 km beneath Cape Mendocino. They image the The differences in seismic moment estimates from our subductedGarda slab dipping 8øE, flexing downward6- body wave inversionsand from the Harvard CMT can be 12øSEand with a southernedge trending S75øE. Southof attributed to several possiblecauses. We use broadband the southernedge of the Garda plate they infer a thickened recorded body waves with peak energies around 20 s North America crust (up to 30 km thick) which they period, while the Harvard CMT algorithmsimultaneously attributeto the combinedeffects of regionalcompressive invertslong-period body waves(T>45 s) and surfacewaves stressesand partial filling of the slab window that results (T> 135 s). Given the larger amplitudeof the surfacewaves from passageof the triple junction. Verdonckand Zandt at theseperiods, they can be expectedto stronglyinfluence [1994] suggestthat the residual gravity anomaly which the Harvard CMT solution. Hence the differences in Jachens and Gricscom [1983] associatewith the southern seismic moment could reflect the different sensitivities of edge of the Garda plate may instead represent the surfaceand body waves to the earthquakerupture. From southwestedge of the zone of thickenedNorth America our spectralanalysis of the surfacewaves, we found that crust southof the Garda plate. A deeperlocation of the the seismicmoments estimated at different periodswere Garda plate beneathCape Mendocinois also favoredby highly variable and tradedoff with the dip. Furthermore, Smith et al. [1993], who suggestthat a doubleplane of this trade-off is dependenton the Earth model used to microseismicity dipping 11-12ø eastward defines the compute the source excitation coefficients and on the subductedGarda plate. In contrast,models of heat flow assumedcentraid location and depth [see,e.g., Zhang and near Cape Mendocino [Lachenbruch and Sass, 1980] are Lay, 1990]. Although a larger seismic moment is not consistentwith a thin (10 km) North Americanplate near compatiblewith the body wavesfor the 10.5 km depthand the coast. Given the uncertaintyin the three-dimensional 13ø dip usedto modelthem, it might be accommodatedby plate geometrynear Cape Mendocino,it is not possibleto a deepersource depth and a shallowerdip. The variations say whetherthe Cape Mendocinoearthquake ruptured the in solutionsfor this event (Table 4) and for the Honeydew interplatemegathrust or a low-angle thrustfault within the earthquake (Table 5) highlight the difficulties in overriding Cascadia accretianary prism. Based on the determiningaccurate source parameters for earthquakesin shallower depth (8.7 km) and the more steeplydipping thisregion. fault planes of our solution for the 1991 Honeydew earthquake,it is unlikely that this event rupturedthe plate Discussion boundaryor evenon the samefault as the CapeMendocino earthquake. The Cape Mendocinomainshock mechanism The relative motion between the Juan de Fuca and North is consistent with the horizontal, NE-SW oriented American plates, computedfrom the NUVEL-1 model compressionalaxis and resulting NW striking tectonic [DeMets et al., 1990] at the epicentrallocation is 3.4 cm/yr fabric of the northerncoast ranges [Jennings et al., 1977]; in the directionN59øE. Our inferred slip directionfor the however it is also consistent with the direction of Garda- mainshock,N74øE, containsa significantcomponent of North America plate convergenceand the expectedslip motion parallel to this direction. Tanioka et al. [1995] directionon the interplatemegathrust. obtained a similar slip vector for the Cape Mendocino Our resultssuggest that the CapeMendocino earthquake earthquakewhich they regard,in light of Wilson's[1986] triggeredaftershocks that occurredonly secondsafter the model of a Garda deformation zone (GDZ) and Prescott mainshock but had different mechanisms than the HAGERTY AND SCHWARTZ: THE 1992 CAPE MENDOCINO EARTHQUAKE 16,057 mainshock. In addition, both the Cape Mendocino and References Honeydew events triggered large offshore aftershocks within 24 hoursof their occurrence.Evidently, the Gorda Aremort,C. J., A. A. Velasco,and T. Lay, Rapid estimationof rupturedirectivity; application to the 1992 Landers(M•=7.4) plate acts as an efficient guide to transfer stress and Cape Mendocino (M•=7.2), California earthquakes, accumulatedacross the subductionzone and overriding Geophys.Res. Lett. , 20, 97-100, 1993. accretionaryprism far offshorewhere it resultsin internal Artrio, M., and E. I. Balazs, Geodetic evidence for aseismic subructionof the Juande Fuca plate, J. Geophys.Res., 84, deformation of the Gorda plate. Slip on shallow thrust 3023-3028, 1979. events onshore, resulting from the horizontal, NE-SW Atwater, T. M., and J. D. Muddie, Block faultingon the Gorda directed compressionacross the accretionaryprism, may rise, Science, 159,729-731, 1968. act to lower the normal stress across faults offshore in the Ben-Menahem, A., Radiation of seismic surface waves from finite Gorda plate, causing them to fail in response to the moving source,Bull. Seismol.Soc. Am., 51, 401-435, 1961. prevailing N-S compressionoffshore. In an analogous Carver,G. A., A. S. Jayko,D. W. Valentine,and W. H. Li, Coastal uplift associatedwith the 1992 CapeMendocino earthquake, manner,earthquakes offshore in the Gorda plate and along northernCalifornia, Geology,22, 195-198, 1994. the Mendocino fracture zone can alter stress levels at the Clarke, S. H., Jr., Geologyof the Eel River Basinand adjacent subduction zone. For instance, the recent 1994 Mendocino region: Implicationsfor late Cenozoictectonics of the southern Fault earthquake (Mw 6.9) ruptured towards Cape Cascadiasubduction zone and Mendocinotriple junction, AAPG Bull., 76, 199-224, 1992. Mendocino and may have loaded the subduction zone, Clarke, S. H., Jr., and G. A. Carver, Late Holocene tectonicsand potentiallyhastening the next largethrust event [Dengler et paleoseismicity,southern Cascadia subduction zone, Science, al., 1995]. The nearnessin time of offshore aftershocksto 255, 188-192, 1992. the Cape Mendocino earthquakehighlights the rapidity DeMets,C., R. G. Gordon,D. F. Argusand S. Stein,Current plate with which stress can be transferred between the onshore motions,Geophys. J. Int., 101,425-478, 1990. Dengler, L. A., et al., The September1, 1994 Mendocinofault and offshoreregimes. earthquake,Calif Geol., 48, 43-53, 1995. Dziewonski,A.M., and D. L. Anderson,Preliminary reference Earth model, Phys.Earth Planet. Inter., 25, 297-356, 1981. Summary Dziewonski,A.M., G. Ekstrom,and M.P. Salganik,Centroid- momenttensor solutions for July-September,1991, Phys.Earth In this paperwe have analyzedboth the 1991 Honeydew Planet. Inter., 72, 1-11, 1992. and 1992 Cape Mendocinoearthquakes with a view toward Dziewonski,A.M., G. Ekstrom,and M.P. Salganik,Centroid- understandingthe nature of subductionalong the Cascadia moment tensor solutionsfor April-June, 1992, Phys. Earth subduction zone. Using both body and surface wave Planet. Inter., 77, 151-163, 1993. inversions and forward modeling, we estimate a best Eaton,J.P., Distributionof aftershocksof the November8, 1980, Eurekaearthquake, Earthquake Notes, 52, 44-45, 1981. double couple mechanismfor the 1992 Cape Mendocino Freymueller,J. T., and P. Segall, Distributionof deformation earthquake of q)=330+10 ø, 15=12+2ø, 3• =75+15ø . acrossthe Pacific-NorthAmerica plate boundary,northern Determining whether this event representsrupture on the California (abstract),Eos Trans. AGU, 75(44), Fall Meet. Gorda-North America interplate megathrust or on a Suppl., 163, 1994. Furlong, K.P., Lithosphericbehavior with triple junction horizontal detachment surface within the overriding migration:An examplebased on theMendocino triple junction, Cascadia accretionary prism is not possible without Phys.Earth Planet. Inter., 36, 213-223, 1984. improved resolution of the complex three-dimensional Heaton,T. H., andH. Kanamori,Seismic potential associated with geometryof the Mendocinotriple junction. Both the Cape subductionin the northwesternUnited States,Bull. Seismol. Soc.Am., 75, 933-942, 1984. Mendocinoand Honeydewevents were followed by large eventsoffshore, suggesting that althoughthe GordaPlate is Heaton,T. H., and H. Kanamori,A reply to H. Acharya's "Commentson 'Seismicpotential associated with subductionin deformable, it is also an efficient stressguide and is thenorthwestern United States' ", Bull.Seismol. Soc. Am., 75, capableof generatinglarge earthquakes.The observation 891-892, 1985. of early aftershocksto the Cape Mendocinoearthquake Jachens,R. C. andA. Griscom,Three-dimensional geometry of theGorda plate beneath northern California, J. Geophys.Res., with differingmechanisms underscores the complexity of 88, 9375-9392, 1983. faultinteractions at theMendocino triple junction that must Jachens,R. C., andA. Griscom,Structure of theMendocino triple be understood before the nature of subduction of the junctionbased on newaeromagnetic data (abstract), Eos Trans. Cascadiasubduction zone and the potentialseismic hazard AGU, 75(44),Fall Meet.Suppl., 474, 1994. can be assessed. Jennings,W. W., R. G. Strand,and T. H. Rogers,Geologic map of California,scale, 1:750,000, Calif. Div. of Minesand Geol., Sacramento, 1977. 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