THE 1992 CAPE-MENDOCINO EARTHQUAKE - BROAD-BAND DETERMINATION OF SOURCE PARAMETERS

The April 25, 1992, Cape Mendocino earthquake (M(S) 7.1) has renewed s peculation about the nature of subduction along the Cascadia subductio n zone and the associated seismic hazard. This event may represent the first large (M > 6) thrust event along the entire Cascadia subduction zone in historic times (last 200 years), We analyze long-period surfa ce waves and broadband body waves in order to estimate the mainshock s ource parameters. We also examine broadband body waves from the nearby 1991 Honeydew earthquake (M 6) in order to assess the contributions o f both rupture complexity and unmodeled source and receiver structure in the Cape Mendocino waveforms. From both body and surface wave inver sions, as well as forward modeling of body waves, we estimate a best d ouble couple mechanism for the Cape Mendocino earthquake (strike=330 /- 10 degrees, dip=12 +/- 2 degrees, rake=75 +/- 15 degrees, seismic m oment=1.93 x 10(19) N m, and M(w)=6.8). This mechanism contains a sign ificant component of slip in the estimated direction of Gorda-North Am erica plate convergence. Although this earthquake had suitable geometr y for relieving strain accumulated by Gorda-North America plate conver gence, we cannot resolve whether it occurred on the interplate megathr ust or on a fault within the overriding accretionary prism. We find ev idence for southwest (offshore) rupture toward an azimuth of 240 degre es. In addition, we find evidence for early aftershocks in both the te leseismically recorded body waves and in locally recorded strong motio ns. We model one aftershock delayed similar to 13 s from the mainshock with a mechanism that is different from the mainshock mechanism but i s consistent with the north-south trending, horizontal compression fou nd offshore within the Gorda plate. We postulate that this aftershock and two additional large, strike-slip aftershocks that ruptured the Go rda plate within 24 hours of the mainshock were caused by the transfer of stress accumulated across the Cascadia subduction zone and accreti onary prism far offshore, to the Gorda plate, where it reduced the nor mal stress across NW-SE oriented faults, triggering failure. The compl exity of fault interactions near the Mendocino triple junction needs t o be understood before potential seismic hazards of the southern Casca dia subduction zone can be quantified.