TECTONICS, VOL. 14, NO. 5, PAGES, 1095-1103,OCTOBER 1995 Seismotectonicsof the April 25, 1992, Petrolla earthquake 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 earthquakes 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 aftershocks(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 the first 10 s for themainshock, 0.7 x 1019Nm in the first 8 s for explain the spaceproblem betweenthe Blanco FractureZone in the first aftershock,and 0.9 x 1019Nm in the first 2 s for the the north and the Mendocino Fault 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 andHumboldt 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 Seismology(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 ........