, VOL. 14, NO. 5, PAGES, 1095-1103,OCTOBER 1995

Seismotectonicsof the April 25, 1992, Petrolla and the Mendocino triple junction region

Yuichiro Tanioka, Kenji Satake,and Larry Ruff Departmentof GeologicalSciences, University of Michigan, Ann Arbor

Abstract. The April 25, 1992, Petrolia earthquake(Ms 7.1) 124ø34.47'W.The parametersof the secondaftershock (AF2) are occurredat the southerntip of the Cascadiasubduction zone. origin time 11:18:25.8 (GMT); location, 40ø23.40'N, This is thelargest thrust earthquake ever recorded instrumentally 124ø34.30'W.These occurred near Cape Mendocino, in the Cascadiasubduction zone. The earthquakewas followed where the Pacific, North American, and Gorda plates meet by two large strike-slip (both Ms 6.6). Moment (Figure 1). The Gorda plate is the southernpart of the Juan de releaseof eachof theearthquakes is as follows: 4.0 x 1019Nm in Fuca plate, south of the Blanco Fracture Zone. In order to thefirst 10 s for themainshock, 0.7 x 1019Nm in thefirst 8 s for explain the spaceproblem betweenthe Blanco FractureZone in the first ,and 0.9 x 1019Nm in the first 2 s for the the north and the Mendocino Zone in the south, Wilson second aftershock. These indicate that the mainshock and each of [1986] claimed that the Gorda plate is not a rigid plate and the aftershocksmay have different tectonicbackgrounds. The deforms internally. He called it the Gorda deformation zone bestdepth estimates of the mainshockand the two aftershocksare (GDZ). Seismicitystudies by Smithand Knapp [1980] andSmith 14 km, 18 km, and 24 km, respectively.The slip directionof the eta/. [ 1993] alsosupported this hypothesis. mainshockis betweenN75øE andN80øE. This slip directionis The potentialfor a largeinterplate earthquake in the Cascadia not consistent with either the relative motion of the North subductionzone (CSZ) where the Juan de Fuca plate or GDZ Americanand Juan de Fucaplates (N60øE) or betweenthe North subductsbeneath the North American plate has been debated. Americanplate and the Gordadeformation zone (N40øE).It has The CSZ possessesmany featuresof typical subduction,zones been suggestedthat the North American-Pacificplate motion is includingan active volcanicarc and accretionaryprism. On the accommodatedby right-lateralslip on both the San Andreasand basisof comparativesubductology [Uyeda, 1982], Heaton and Maacama-RodgersCreek-Hayward fault systems;the intervening Kanamori [1984] argued that CSZ is capable of great block is the Humboldtplate. If we modify the relativemotion of earthquakes.Geological studies seem to offer strongevidence for the southernmost Gorda deformation zone to conform with the greatearthquakes in CSZ [Adams, 1990; Atwater, 1992; Atwater seismicitytrends and allow the Humboldt-Pacificplate motion to et al., 1995]. On the otherhand, it hasbeen essentially aseismic be about half the total North American-Pacific motion, then the through recorded history; there has not been a single verified Gorda deformation zone-Humboldt relative motion matches the underthrustingearthquake. directionof the Peta'oliaslip vector.Also, the mixture of focal The tectonicsof the CapeMendocino region is complex.First, mechanisms in the two distinct aftershock clusters can be the locationof the San Andreasfault (SAF) in this region is not explained by motion between the Gorda deformation zone and well known becausethere is little seismicactivity (Figure 1) and Pacificplate and the Humboldtand North Americanplates. The the fault is coveredby thick sedimentsalong the coast.Second, Gorda deformationzone is subductingbeneath the Humboldt on the eastern side of SAF, several active faults exist and are plate in the Cape Mendocinoarea, and the Petroliaearthquake parallel to the SAF [Herd and Helley, 1977; Hill et al., 1990]. ruptured the entire subduction segment between the Gorda These are called the Garberville-Maacama-Rodgers Creek- deformationzone and Humboldt plate. Hayward fault zone (GMRH). Seismicity on the GMRH fault zone is much more active than the S^F in this region. The continuationof this GMRH fault systemextends into the Cape Introduction Mendocinoregion. Herd [1978] suggestedthat thesefaults form the easternboundary of a Humboldt plate which is boundedon An earthquakewith Ms 7.1 occurrednear Cape Mendocino on the west by S^F and on the north by GDZ. Prescott and Yu April 25, 1992. The origin time and location given by the [1986], Griscom and Jachens [1989], and Smith et al. [1993] NationalEarthquake Information Center (NEIC) are 18:06:05.25 discussedvarious aspectsof the eastwardstepping of the S^F. (Greenwich mean time (GMT)), 40ø19.94'N, 124ø13.70'W. The Third, the region north of the Cape Mendocino region is the mainshockepicenter is near the town of Petrolia, Californiai It accretionaryprism of the CSZ, with rising anticlinesthat are wasfollowed by two large(both Ms 6.6) aftershocksthe next day underlainedby active thrusts and high angle reverse faults (April 26). The NEIC parametersof the first aftershock(AF1) [Kelseyand Carver, 1988; Clark, 1992]. The orientationof these are origin time 7:41:40.0 (GMT); location, 40ø26.12'N, anticlinesand faultsis E-W to SE-NW near the Cape Mendocino region. In thispaper, we reportour analysisof teleseismicsurface and Copyright1995 by the AlnericanGeophysical Union. body waves to determine the source parametersof the 1992 earthquakesequence. We will then show that the mainshockhas Paper number95TC01975. the characteristicsof a subductionearthquake between the GDZ 0278 -7407/95/95TC-01975 $10.00 and the Humboldt plate. The tectonicsof the Cape Mendocino

1095 1096 TANIOKAET AL.: SEISMOTECTONICSOF1992 PETROLIA EARTHQUAKE

50

41

45 NA

40

z 40 ß' 120 Petrolia Eq. oi.. I Southern edge of ß I. I GoFJGSlab I I Pacific I ?, Ocean I I

Focal Depths

+ <15.0 km 39 • >15.0 km Point Arena AF + ß 4,

ß 50 km x"• GVF

125 124 123 longitude, w Figure1. Seismicityof northernCalifornia for 1980-1991time period after Castillo and Ellsworth [ 1993]. Stars arethe epicenters of the 1992Petrolia main shock and two largeaftershocks, AF1 andAF2. The dashedline is thesurface projection of thesouthern edge of theGorda slab after Jachens and Griscom [ 1983]. Inset map shows theentire Cascadia subduction zone with plate boundaries. Abbreviations are BSF, Bartlett Springs fault; CSZ, Cascadiasubduction zone; GDZ, Gorda deformation zone; GF, Garberville fault; GVF, Green Valley fault; HBF, Healdsburgfault; JDF, Juande Fucaplate; LMF, Lake Mountainfault; MF, Maacamafault; MFZ, Mendocino faultzone; NA, NorthAmerican plate; PAC, Pacific plate; and SAF, San Andreas fault. region,as we mentionedabove, can be well explainedby theslip periodsurface waves recorded at 15 stationsof GlobalDigital directionand aftershock distribution of thePetrolia earthquake. SeismographNetworks (GDSN), IncorporatedResearch Institute for (IRIS), and GEOSCOPE networks.The station parameters are shown in Table 1. The inversion was canied out Mechanism of the Mainshock in thetime domain on thefiltered records (period range from 50 s to 300 s). The depth is fixed at 15 km. The doublecouple We performed centroid moment tensor (CMT) inversion solutions of the moment tensor are shown in Table 2 as well as in [Dziewonskiet al., 1981'Fukushirna et al., 1989]using very long Figure 2. The mechanismis a thrusttype, with a shallowdip TANIOKA ET AL.: SEISMOTECTONICS OF 1992 PETROLlA EARTHQUAKE 1097

Table 1. Station Parmneters Station Azimuth, Distance, Mainshock Aftersl•ck 1 Aftersl•ck 2 Name deg deg CMT Body CMT Body CMT Body Wave Wave Wave ALE 9.1 46.7 P ANMO 105.4 14.9 x x x BDF 110.7 89.8 BJI 317.8 83.0 CHU 301.5 71.9 COL 338.7 28.2 x x COR 7.1 4.3 x x ERM 305.6 66.3 P s P GSC 128.7 7.6 x x x GUMO 281.1 82.2 x P HKY 301.7 72.8 P HRV 69.0 38.9 x P s x P x P s ISA 134.7 6.4 x x x KIP 247.1 34.4 P s KIV 9.6 95.3 P KMI 317.6 101.7 x P KONO 22.6 73.3 x P x P x LZH 323.0 92.2 x P NNA 129.2 67.9 P MAJO 303.3 72.6 x P x P x P OBN 10.8 82.6 P P PAS 140.8 7.7 x x x PPT 207.7 62.3 P RAR 215.5 69.7 x SBC 148.1 6.8 x x x SCP 148.1 34.8 P P s SNZO 221.9 98.4 P SSB 33.8 83.1 P TKO 302.0 72.4 P TOL 42.1 83.3 x P ZOBO 125.1 76.6 P Note that azimuthand distanceare for the mainshock.The symbolX representsthe station which we usedfor CMT inversion.The symbolsPans S representthe stationswhich we usedfor P wave andS wave. CMT is the centroid moment tensor.

angle of 11ø and a strike of 342ø. The seismicmoment from the dip angle.On the otherhand, body wave analysisoffers a better CMT inversionis 4.9 x 1019Nm. controlon the dip anglebut lesscontrol on the stxike.From the In the case of a shallow thrust event, surface wave analysis CMT inversionwe fixed the strikeof the fault planeat 342ø and constrainsthe strike of the focal planesbut poorly determinesthe performedthe body wave inversionto find the dip anglesand

Table 2. Source Parameters of Mainshock and Aftershocks Mainshock Aftershock 1 Aftershock 2 CMT Strike 342 ø / 144 ø 32 ø / 296 ø 36 ø / 127 ø Dip 11o / 79ø 62ø / 79ø 80ø / 85ø Rake 107 ø /87 ø 13 ø / 152 ø -6 ø/-169 ø Mo 4.9x 1019Nm 0.9x 1019Nm 1.3x 1019Nm Mw 7.1 6.6 6.7 Non-DC 12 % 5 % 1% Centroid time 12.3 s 6.0 s 4.4 s Body wave Depth 14 km 18km 24 km or deeper Strike 342ø(fixed)/170ø 212ø /116 ø (fixed) 216ø/307 ø (fixed) Dip 20ø / 70ø 74ø / 70ø 70ø / 87ø Rake 83 ø /93 ø 21 ø / 163 ø -3 ø/-160 ø Mo 4.0x 1019Nm 0.7 x 1019Nm 0.9x 1019Nm Duration 10 s 8 s 1.6 s Localstress drop 50 bar 20bar 2,800bar CMT is centroid moment tensor 1098 TANIOKA ET AL.: SEISMOTECTONICS OF 1992 PETROLlA EARTHQUAKE

Mainshock a) Initial deth estimate using CMT solution O ...... I ...... I ...... 0.82 0.8 ...... •...... •-...... '.-'...... E•10 bestdepti 0.78 ...... •...... •...... ,•.,,....._...... ?...... ?...... <14km ...... 0.76 ...... 2o

//-' ,i" ! • -- 'dip: 80' 25 0.74 l..•--.Z..i.-.-.?•...-i...... i...... -- -di'"'=75 ø :30 ,,: ---dip: 70ø 0.6 0.7 0.8 0.9 correlation coefficient 0.720.7 - ...... - I I I I 140 ø 145 ø 150 ø 155 ø 160 ø 165 ø 170 ø 175 ø 180 ø b) depth estimate using the final solution Strike of the auxiliary plane i ii ill ill ii iii ii iii iii ii iii i I

•v lo bestdeptL••

25 _

CMTsolution The final solution 0.6 0.7 0.8 0.9 correlation coefficient Figure 2. (top) Graph showsthe correlationcoefficient between observed and synthetic seismogramsas a function of various Figure3. The correlationcoefficient between the observedand strikesof auxiliary plane for four different dip angles.(bottom) syntheticseismograms for the mainshock are plotted for point Focal mechanism solutions for the centroid moment tensor sourcedepths: (a) initialdepth estimate using CMT solution,(b) (CMT) inversionand the final model are shown. depthestimate using the final solution. source time function. We used teleseismic P waves from 21 functionfor the bestdouble couple mechanism, shown in Figure stations and S waves from 2 stations of the GDSN, IRIS, and 4, hasone dominantpulse of 10 s duration,similar to the duration GEOSCOPE networks. The procedure of the body wave of 9 s estimatedby Velascoeta/. [ 1994]. inversion is as follows. First, we inverted the body waves to estimate a reasonabledepth using the double couplemechanism obtained from the CMT inversion [e.g., Tichelaar and Ruff, Mechanism of the Two Largest Aftershocks 1991]. Figure 3a showsthe variation of the correlationbetween the observedand computed waveforms as a functionof the depth. We performed the CMT inversion for the two large The bestdepth estimate is 14 km. Next, we fixed the depthat 14 aftershocksusing the very long periodsurface waves recorded at km and varied the dip angleof the auxiliary plane from 65ø to 10 stations.The stationparameters are shownin Table 1. The 80ø by a 5 ø interval and also varied the strike of the auxiliary depth is fixed at 10 km for both events. The double couple planefrom 140ø to 180ø by a 5 ø interval.We performedthe body solutions of the moment tensor are shown in Table 1 as well as wave inversion for each double couple mechanism.Figure 2 Figure 5. The mechanismfor both aftershocksis strike-sliptype, shows the variation of the correlation between the observed and with strikes of 32 ø and 296 ø for the first aftershock and strikes of computedwaveforms as a function of the strike of the auxiliary 36 ø and 127 ø for the second aftershock. The seismic moment plane for four different dip angles. The set of strike and dip from the CMT inversionis 0.9 x 10]9 Nm for the first aftershock angles which producedthe best correlationrepresents the best and 1.3 x 10]9 Nm for the secondaftershock. double couplemechanism. As can be seenin Figure 2, the best In the case of strike-slip events, similar to the shallow thrust mechanismhas a strike of 165ø-170ø and a dip angleof 70ø for events,the dip anglesof focal planesare betterconstrained from the auxiliary plane. This indicatesthat the slip directionfor the body wave analysisthan from surfacewave analysis,while the fault plane is betweenN75øE and N80øE. Finally, we confirmthe strikesfor both planesare better constrainedfrom surfacewave best depth by reinvertingthe seismogramswith the best double analysisthan from body wave analysis.We thereforefixed the couple mechanism(strike, 170ø; dip,70ø; rake,93ø). Figure 3b strikeof bothplanes to thosedetermined from the CMT inversion showsthat the bestdepth estimate is still 14 km. The sourcetime andperformed the bodywave inversionfor both aftershocks.We TANIOKA ET AL.: SEISMOTECTONICS OF 1992 PETROLIA EARTHQUAKE 1099

Source time functions of the NE-SW trending plane from NW60øto NW90 ø and lO SE60øtoSE90 ø. We performedthe body wave inversionfor each FA Mainshock doublecouple mechanism and for the depthrange from 10 to 30 km. Figure 5 showsthe variationof the correlationbetween the observedand computedwaveforms as a function of dip angle. The dip anglewhich producedthe bestcorrelation represents the best doublecouple mechanism. The bestdouble couple solution o , for both aftershocksis shown in Figure 5 as well as Table 2. • Aftershock1 Figure 6 showsthe variationof the correlationbetween obsex•,ed and syntheticseismograms as a function of the depth with the best double couple mechanisms.The best depth for the first aftershockis 18 km. The best depth for the secondaftershock is p 0 24 km or deeperand not well resolved.The sourcetime functions • 10 for the best double couple mechanismsare shown in Figure 4. • Aftershock 2 E The source time function for the first aftershock has one Eo 5 MO=0.9 X 1019 Nm dominantpulse of 8 s duration.The sourcetime functionfor the secondaftershock has one dominantpulse of 1.6 s durationmuch I shorter than the first aftershock. These estimates are smaller than the durationsof 14 to 15 s estimatedby Velascoet al. [1994].

o lO 20 30 40 50 time, sec Duration and Stress Drop Figure 4. Sourcethne functionsand the final focal mechanisxn Figure4 showsthe momentrate as a functionof time or the solutions for the mainshock, the first aftershock, and the second source time function deconvolved from observedseismograms. aftershock. The seismic moment released in the first dominant The durationof the first pulse for the mainshockand the first pulseis alsoshown. aftershockis 10 s and 8 s, respectively. The seismic moment releaseduring this first pulse is calculated as4.0 x 1019and 0.7 x used teleseismic P waves from nine stations for the first 1019Nm, respectively. These are both slightly smaller than those aftershock and from six stations for the second aftershock. The obtainedfrom the CMT inversion(4.9 x 1019and 0.9 x 1019Nm, stationparameters are shownin Table 1. We varied the dip angle respectively).If the discrepancyin momentestimates between

Aftershock 1 I .... i .... i .... i .... i .... i .... final solution CMT

0.8

.o 0.6

• 0.4 o O.2 --bodywave

70 80 90 80 70 60 dip angle Aftershock 2 I .... i .... i .... i .... i '' ' final solution CMT

._o: 0.6 Ibody ? 0.4 u0.2 ...... ,.... 7o dip angle Figure 5. Left graphsshow the correlationcoefficient between the observed and synthetic seismograms for the two largeaftershocks as a functionof dip angle.Two focalmechanism solutions for eachaftershock are shown on the right: the final solutionand the CMT solution. 1100 TANIOKA ET AL.' SEISMOTECTONICS OF 1992 PETROLIA EARTHQUAKE

aftershock 1 aftershock 2 N58øE. This is not consistentwith the slip direction of the 0 ...... • ...... • ...... Petroliaearthquake (Figure 7a). Wilson[1986] suggestedthat the oceanic lithosphere between the Blanco Fracture Zone and E10 Mendocino Fault Zone (MFZ) is not a rigid plate and defor•ns •15 internally as it subducts along the Cascadia subduction zone ß 20 (Figure 8). He called it the Gorda deformationzone. Basedon his 25 results,the motion of the GDZ near the Cape Mendocinoregion

30 relative to the Pacific plate is estimatedas about5.8 cm/yr in the 0.85 0.9 0.95 1 0.5 0.6 0.7 0.8 0.9 1 east-west direction. The relative motion between North American correlation coefficient correlation coefficient plate and GDZ (shown as GD1 in the Figure 7a) then becomes Figure 6. The correlationcoefficient between the observedand 5.4 cm/yr in the directionof N40øE. This cannot explainthe slip syntheticseismograms for the two large aftershocksis plotted for directionof the Petroliaearthquake either (Figure 7a). variouspoint sourcedepths.

a) CMT and body wave inversionsis real, these numbersindicate PAC GD1 that about 80 % of the moment is released in the first 10 s. The duration of the second aftershock is less than 2 s, much shorterthan the othertwo. The seismicmoment of the first pulse iscalculated as 0.9 x 1019Nm, which is also slightly smaller than F that obtainedfrom CMT inversion(1.3 x 1019N•n). It is •11111•pIvector noteworthythat 70 % of the seismicmoment is releasedin the NA 5 cm/yr first 2 s. The local stressdrop Ac• can be computedfrom the seismic moment M 0 and a half durationof the first pulse x as b) PAC GD1

16 GD 2 where v is rupturevelocity. The aboveequation is modifiedfrom the stressdrop derivationfor a circular fault given by Kanamori and Anderson[1975]. This formula gives the stressdrop of the Slip vector NA first subevent,which could be different from the staticstress drop (static stressdrop is the averagestress drop over the entire fault c) area). Assuminga rupturevelocity v = 3 km/s, we get local stress PAC GD1 dropsof 50 bars, 20 bars, and 2.8 Kbars for the mainshockand two aftershocks.While the absolutevalue of stressdrop is model- ...••••,•••GD2 dependentand small error in the esti•nationof durationeasily producesmore than a factor of 2 difference of estimated stress drops, the stressdrop of 2.8 Kbar for the second aftershockis very large. This means that the second aftershock had an unusuallyhigh rupturevelocity, an unusuallyhigh stressdrop, or NA both.Michael [1992] statesthat the secondaftershock produced Figure 7. Velocity vectors (thin arrows) show the relative an unusually large amount of energy in the frequencyrange of motion of various plates around the Cape Mendocino triple 0.3-3 Hz. This may correspondto the short durationand high junction.The thick arrowsshow the slip directionof the Petrolia stressdrop of the secondaftershock. earthquake(N80øE). (a) The relative motion of Pacific (PAC), North American (NA), Juan de Fuca (JDF), and Gorda deformation zone (GD1), which is based on NUVEL1 model Tectonic Interpretation of the Petrolia [DeMetset al., 1990] and Wilson[1986], is shown.(b) Velocity Earthquake triangleshows the relative plate motion of PAC, NA, and Gorda deformationzone (GD2) when we assumethat the slip direction The slip directionof the bestdouble couple mechanism for the of the Petroliaearthquake represents the relativemotion between Petrolia earthquake is between N75øE and N80øE. This is NA and Gorda deformation zone. The relative motion of the consistentwith the resultsof the geodeticanalysis reported by Gorda deformation zone to PAC is corrected to E7øS (GD2), Oppenheimeret a/.[1993]. If this earthquakeis a subduction insteadof E-W (GD1). (c) Velocity triangle showsthe relative event betweenthe North American and Juan de Fuca plate, the plate motion of PAC, Humboldt plate (HB), and GD2 by slip direction of the earthquake must be consistent with the assuming that the slip direction of the Petrolla earthquake relative motion between these plates. The relative motion representsthe relative plate motion betweenHB and GD2. The between the North American and Juan de Fuca plates at the dashedarrows represent the en'or in the relative velocity of ItB epicenterof the Petrolia earthquakecalculated froin NUVEL 1 andGD2 basedon the relative velocitybetween NA and HB, 2.5 model [DeMets et al., 1990] is 3.4 cm/yr in the direction of _+0.6 cm/yr [Prescottand Yu, 1986]. TANIOKA ET AL.: SEISMOTECTONICS OF 1992 PETROLIA EARTHQUAKE 1101

41 ø 1 26øw 5O km

mainshock JDF CSZ z

o o

M iiiiiii!ii;gDZ: ...... HB ......

...... PAC

..

ß 40

0

40øN 50 km PAC I I Honeydew 8/17/91 125 ø 124 ø Figure 8. A schematic illustration of the tectonics of the southern part of the Cascadia subductionzone (CSZ) with our Longitude, W final mechanismof the Petroliaearthquake and the HarvardCMT Figure 9. Aftershockdistxibution of the Petxoliaearthquake from mechanism of the Honeydew earthquake which occurred on Oppenheimeret al. [1993] with the bestmechanism of the main August 17, 1991. In the Gorda deformation zone (GDZ), shock.The starrepresents the epicenterof the xnainshock. schematicstrain symbolsshow direction and relative magnitude of extension(outward arrows) and compression(inwm'd arrow::) from Wilson[1986]. Abbreviationsare JDF, Juande Fuca Prescottand Yu [1986] suggestedthat the North American- HB, Humboldt plate; NA, North Alnerican plate; and P,•g, Pacific plate motion is accommodatedby right-lateral slip on Pacific plate. both the San Andreasfault (SAF) and Maacama-RodgersCreek- Hayward fault system(MRH). The interveningplate is called the Humboldt plate. They also claimed that the slip rate of the MRH The aftershockdistribution of the Petxoliaearthquake (Figure fault systemis 2.5 + 0.6 cm/yr, which is almosthalf of the total 9) showstwo linear clusters,one in the southand the other in the North American-Pacificplate motion. If the slip directionof the north. The mechanismsof about 50% of earthquakesin the Petrolia earthquake represents the direction of the GDZ- southerncluster are E-W strike slip (D. Oppenheimer,personal Humboldt plate motion, the relative velocity between GDZ and communication,1992). If this linear clusterrepresents the MFZ, Pacific plate becomes5.9-7.4 cm/yr (Figure 7c). The subducted the strike of the MFZ should be E7øS in the Cape Mendocino areacalculated from this velocity is 21-24x 103km 2, whichis region, instead of E-W. By combining this orientation with an consistentwith theresult of Wilson[1986], 21.3 x 103km 2. Our assumptionthat the slip direction of the Petrolia earthquake conclusionfrom this analysis is that the Petrolia earthquake representsthe relative motion betweenGDZ andNorth American representssubduction of the Gorda deformationzone underneath plate, the relative motion between GDZ and Pacific plate the Humboldtplate. The relative motionis shownin Figure7c. becomes12-15 cm/yr (shown as GD2 in Figure 7b), which is As we mentionedabove, the aftershockdistribution (Figure 9) quite fast. Wilson [1986] showed that the area generatedin the shows the two clusters in the north and south. We already GDZ since1.77 Ma is 21.3 x 103k m2 basedon magnetic mentionedthat the southerncluster includes the MFZ type (E-W anomaly isochron data. We assumethat the relative velocity strike slip) events.In addition,the northerncluster also includes betweenGDZ and Pacific plate is linearly decreasingtoward the the same E-W strike-slipevents. This E-W motion betweenthe north near CSZ, and eventually it becomesthe relative velocity Humboldt and North American plates can be explained as betweenJuan de Fuca and Pacific platesat the borderof Ju:mde follows (see Figure 8 and 10). Near the Cape Mendocinoregion, Fuca plate and GDZ. Then the subductedarea since 1.77 Ma, accretionary prism exists north of the Humboldt plate and which correspondsto the generatedarea, can be calculated.The deforms in a almost N-S direction, producing the geological subductedarea for the GD2 model becomes33-39 x 103 km 2. feature called the Eel River syncline [Clark, 1992]. This N-S This is too large comparedwith the resultof Wilson[1986], 21 x convergenceof the accretionaryprism near Cape Mendocinoand 103km 2. Therefore the GD2 model, shown in Figure7b, is not the directionof the North American-Humboldtplate motion can acceptable. producethe E-W strike-slipmotion betweenHumboldt plate and 1102 TANIOKA ET AL.: SEISMOTECTONICS OF 1992 PETROLIA EARTHQUAKE

Figure10. A schematicillustration of theplate inotion near the Cape Mendocino region. The two sides show the sequenceof blockmotions, the right side shows the plate configuration sometime after an "increment" of motion. ThePacific plate (PAC) is heldfixed, and motions of theGorda deformation zone (GDZ), NorthAmerican plate (NA), andHumboldt plate (HB) arerepresented by largearrows, relative to PAC.

the accretionary prism of the CSZ. To illustrate these The aftershockdistribution of the Petroliaearthquake (Figure interactions,a simplifiedschematic view of the plate movements 9) shows that the Petrolia earthquake ruptured the entire is show in Figure 10. The plate configurationshown in the left subductionsegment between the Gorda deformationzone and the frame would become as shown in the right frame when the Humboldt plate. The Petrolia earthquake should be the Pacific plate is fixed. characteristicevent in this segmentand may representthe largest size of a subductionearthquake possible in this segment.Note that it requires48 yearsto accumulatethe 2.7 m of displacement Discussion [Oppenheimeret al., 1993] at our predictedrate of 5.6 c•n/yr, althoughno large earthquakehas beenreported previously in this The slip directionof the Petrolia earthquake,N75øE-N80øE, region. North of this segment, the Gorda deformation zone or indicatesthat the earthquakeis a subductionevent betweenthe Juande Fuca plate subductsbeneath the North Panericanplate. Gorda deformation zone and the Humboldt plate. There was The characteristicearthquake in this portion of the Cascadia another thrust event, the August 17, 1991, (Ms 6.2) Honeydew subduction zone would be much larger than the Petrolia earthquake.The epicenterof this earthquakeis very closeto that earthquake. of the Petrolia earthquake(Figure 8). The slip directionof the In addition,there is anotherfault systemon the easternside of Honeydew earthquake from the Harvard CMT solution the Garberville fault. This fault systemincludes the Groganfault [Dziewonski et al., 1993] is N80øE, the same as that of the and the Lake Mountain fault zone. If this fault system Petrolia earthquake.This confirms that the Gorda deformation accommodatesthe North American-Pacificplate motionwith the zone is subducting beneath the Humboldt plate in the Cape San Andreas fault and GMRH, another small plate may exist Mendocino region (Figure 9). Seismic structure study by betweenGrogan fault and Itumboldt plate (G.A. Carver,personal Verdonckand Zandt [1994] indicatesthat the top of the Gorda communication, 1993). Therefore, although the northern slabis deeper(-20 km) thanthe depthof the •nainshock(our best boundaryof this small plate is not clearly defined, theremay be depthis 14 km); thusthe earthquakerepresents underthrusting of another subductionsegment between this small plate and the the trench-fill sedimentcomplex on the top of the subduction Gorda deformation zone. Rupture of this segmentmay cause interface. Smith et al. [1993] show that the existenceof a double anotherlarge earthquake. seismic zone in this region is due to a new thrust fault that subductsa fragmentof continentalcrust eastward. Regardlessof Acknowledgments. We thank J. Johnsonfor helpful commentsand the details of above models, the Petrolia earthquakedoes not Stewart S•nith and David Verdonck for critical reviews. This work is representGorda-North America convergence;instead, it is due to supportedby the NationalScience Foundation (EAR-940553) and USGS Gorda-Humboldtconvergence. (1434-92-G-21787).

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