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 Copyright1996 by the AmericanGeophysical Union. Tectonic Setting 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 ' N. American Plate ,, I i ' a.s. 1 Gorda ', Plate' MF o a.s.2: 46 • •// --•[ ß r-----•------------•-•-:--•-•-•-•••..-.:•::• I •ll r t-'"'"'"•,,,"•••:-'.•.:•:•••.....:•:: • q ',,,. I t-'"'"--'"'"'""""•••':'•••:........:•:..-'_-'.•i %ø%'3,..•o, o1' ::::•:?-.'-::'--3-.::•:......-.:.-.:.-.3:<•i:•:•::'.?.::•:•:• 40øN •e .i::'.'.::i::2:..:"•.:.:i•i•i:•:i:i:i:i:i::.:•:i•:.:-.':.-'.:'.-•i•i•i:i•:i:i: r/ •' "•"'"'"'"'"'""•'"'••'•ot%'•,,,,•// øt I' $"''••l•i•'----'"":'"'"'"'•'-":':•-'•:•••.."..-'..-'..-'i / '• # '"•"'•':-•:••:---\ ¾'"--'"'"'---- "":" I• ............. 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. Motionalong 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