RUPTURE DIRECTIVITY AND SLIP DISTRIBUTION FOR THE M(S )6.8 EARTHQUAKEOF 6 APRIL 1992, OFFSHORE BRITISH-COLUMBIA - AN APPLICATION OF THE EMPIRICAL GREENS-FUNCTION METHOD USING SURFACE-WAVES

On 6 April 1992 at 13:55 UT, an earthquake of M(S) = 6.8 occurred alon g the Revere-Dellwood-Wilson (RDW) fault, in the triple-junction regio n at the northern end of the Cascadia subduction zone. This was the fi rst significant event in this region recorded by modem digital broadba nd seismic networks, thus providing the first opportunity to examine t he rupture process of a major earthquake along this young oceanic tran sform fault. In this article, an empirical Green's Function technique is applied to regional and teleseismic surface waves to estimate the r upture directivity, the extent of rupture, and the slip distribution a long the Revere-Dellwood-Wilson fault associated with this earthquake. The 20-sec low-pass-filtered relative source time functions (RSTF's) are single pulses with an azimuthal variation in the pulse width. This suggests that the rupture propagated to the NW (315 degrees +/- 20 de grees), along the Revere-Dellwood-Wilson fault (striking 326 degrees). Higher frequency RSTF's reveal two discrete subevents. The clear azim uthal variation in the time separation of these subevents requires tha t relative to the first subevent, the second is located 13 to 20 km in the direction 345 degrees +/- 20 degrees. Using the RSTF at HRV (perp endicular to the rupture direction), a total rupture length of 35 km i s estimated, with the bulk of the slip concentrated in a 20-km-long se gment of the RDW fault to the northwest of the epicenter. Two peaks ar e observed in the estimated slip distribution, with maximum Values of 1.8 and 1.1 m, respectively. The rupture model derived from this analy sis is similar to that obtained from the analysis of body waves and is consistent with the results of aftershock studies. The latter indicat e a paucity of aftershock activity (and low moment release) in the 20- km-long segment of the RDW fault to the NW of the epicenter. A distinc t peak in aftershock activity 30 to 40 km to the NW of the epicenter L ikely represents the termination of rupture. The good agreement betwee n the results of this study, the rupture model estimated from body-wav e analysis, and the aftershock distribution bode well for the applicat ion of the empirical Green's function method using surface waves. It s uggests that this method could be applied to large, historic earthquak es in this region, for which regional and teleseismic surface waves ar e often the most reliable data set.