THE RUPTURE PROCESS AND AFTERSHOCK DISTRIBUTION OF THE 6 APRIL 1992 M(S) 6.8 EARTHQUAKE, OFFSHORE BRITISH-COLUMBIA

On 6 April 1992, a magnitude 6.8 (M(s)) earthquake occurred in the tri ple-junction region at the northern end of the Cascadia subduction zon e. This was the largest earthquake in at least 75 yr to occur along th e 110-km-long Revere-Dellwood-Wilson (RDW) transform fault and the fir st large earthquake in this region recorded by modern broadband digita l seismic networks. It thus provides an opportunity to examine the rup ture process along a young (<2 Ma) oceanic transform fault and to gain better insight into the tectonics of this triple-junction region. We have investigated the source parameters and the rupture process of thi s earthquake by modeling broadband body waves and long-period surface waves and by accurately locating the mainshock and the first 10 days o f aftershocks using a well-located ''calibration'' event recorded duri ng an ocean-bottom seismometer survey. Analysis of P and SH waveforms reveals that this was a complex rupture sequence consisting of three s trike-slip subevents in 12 sec. The initial rupture occurred 5 to 6 km to the SW of the seafloor trace of the RDW fault at 50.55 degrees N, 130.46 degrees W. The dominant subevent occurred 2 to 3 sec later and 4.3 km beneath the seafloor trace of the RDW fault, and a third subeve nt occurred 5 sec later, 18 km to the NNW, suggesting a northwestward propagating rupture. The aftershock sequence extended along a 60- to 7 0-km-long segment of the RDW fault, with the bulk of the activity conc entrated similar to 30 to 40 km to the NNW of the epicenter, consisten t with this interpretation. The well-constrained mechanism of the init ial rupture (strike/dip/slip 339 degrees/90 degrees/-168 degrees) and of the largest aftershock (165 degrees/80 degrees/170 degrees) are rot ated 15 degrees to 20 degrees clockwise relative to the seafloor trace of the RDW fault but are parallel to the Pacific/North America relati ve plate motion vector. In contrast, the mechanisms of the dominant su bevent (326 degrees/87 degrees/-172 degrees), and the long-period solu tion derived from surface waves aligns with the RDW fault. This sugges ts that small earthquakes (M < 6) in this area occur along faults that are optimally aligned with respect to the regional stress field, wher eas large earthquakes, involving tens of kilometers of rupture, activa te the RDW fault. For the mainshock, we estimate a seismic moment (fro m surface waves) of 1.0 x 10(26) dyne-cm, a stress drop of 60 bars, an d an average slip of 1.2 m. This represents only 21 yr of strain accum ulation, implying that there is either a significant amount of aseismi c slip along the RDW fault or that much of the strain accumulation man ifests itself as deformation within the Dellwood and Winona blocks or along the continental margin.