ON THE VARIABILITY OF AFTERSHOCK GROUND MOTIONS IN THE SAN-FERNANDO VALLEY

We analyze aftershocks of the 1/17/94 Mw6.7 Northridge earthquake reco rded at a 3-element small-aperture array within the town of Northridge , above the mainshock rupture plane. Many of the M4-5 aftershocks are observed to have a prolonged shaking duration, up to (similar to)8 sec onds, with conspicuous longer period (approximate to 1 s) arrivals in the latter part of the wave train. Recordings of a M4.0 aftershock tha t occurred at 23:49 GMT on 1/17 show the origin of these waves. A slan t-stack cross-correlation method on each of the three components shows that the late arrivals are characterized by low apparent velocities a nd a back-azimuth that is approximately 10 degrees off that of the dir ect arrivals. Based on the inferred apparent velocities and considerat ion of studies in other sedimentary basins, we conclude that these lat er arrivals consist of surface waves generated within the San Fernando Valley. Similar results are obtained for a M3.4 event recorded across the array. The surface waves are not, however, a ubiquitous feature o f the aftershock recordings. We show that other M(similar to)4 events recorded at the same site are characterized by simple displacement pul ses and durations that are typical for their magnitude, suggesting tha t 3-dimensional site response may be difficult to predict in cases whe re the sources are close to a valley or basin and/or the basin structu re is complex.