On the variability of aftershock ground motions in the San Fernando Valley

We analyze aftershocks of the 1/17/94 Mw6.7 Northridge earthquake recorded 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 ~8 seconds, with conspicuous longer period (≈1 s) arrivals in the latter part of the wave train. Recordings of a M4.0 aftershock that occurred at 23:49 GMT on 1/17 show the origin of these waves. A slant-stack cross-correlation method on each of the three components shows that the late arrivals are characterized by low apparent velocities and a back-azimuth that is approximately 10 degrees off that of the direct arrivals. Based on the inferred apparent velocities and consideration of studies in other sedimentary basins, we conclude that these later 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 of the aftershock recordings. We show that other M~4 events recorded at the same site are characterized by simple displacement pulses and durations that are typical for their magnitude, suggesting that 3-dimensional site response may be difficult to predict in cases where the sources are close to a valley or basin and/or the basin structure is complex