The analysis of seismograms from 32 aftershocks recorded by 98 seismic stations installed after the Northridge earthquake in the San Fernando Valley, the Santa Monica Mountains, and Santa Monica, California, indicates that the enhanced damage in Santa Monica is explained in the main by focusing due to a lens structure at a depth of several kilometers beneath the surface and having a finite lateral extent. The diagnosis was made from the observation of late-arriving S phases with large amplitudes, localized in the zones of large damage. The azimuths and angles of incidence of the seismic rays that give rise to the greatest focusing effects correspond to radiation that would have emerged from the lower part of the rupture surface of the mainshock. Thus the focusing and, hence, the large damage in Santa Monica were highly dependent on the location of the Northridge event, and an earthquake of similar size, located as little as one source dimension away, would not be likely to repeat this pattern. We show from coda wave analysis that the influence of surface geology as well as site effects on damage in Santa Monica is significantly smaller than are the focusing effects.
S. S. Gao et al., "Localized Amplification of Seismic Waves and Correlation with Damage Due to the Northridge Earthquake: Evidence for Focusing in Santa Monica," Bulletin of the Seismological Society of America, vol. 86, no. 1 SUPPL. B, pp. S209-S230, Seismological Society of America (SSA), Sep 1996.
Geosciences and Geological and Petroleum Engineering
Keywords and Phrases
amplification; earthquake damage; focussing; Northridge earthquake 1994; spatial distribution; USA, California, San Fernando Valley; USA, California, Santa Monica
San Fernando Valley
Santa Monica Mountains
Santa Monica, California
International Standard Serial Number (ISSN)
Article - Journal
© 1996 Seismological Society of America (SSA), All rights reserved.