Possible Causes of Enhanced Damage during Northridge Earthquake

During the 1994 Northridge earthquake, the Sherman Oaks area in the southern boundary of the San Fernando valley, California, experienced much greater damage than neighboring regions at similar distances from the epicenter. In order to investigate the causes of the concentrated damage we use aftershock data recorded by about 30 three-component seismic stations that we installed in the area after the earthquake. Analysis of spatial variation of peak amplitudes, coda and S-wave amplification factors, liquefication maps, strong-motion data, and available geological information suggests that focusing from lens-like structures and site effects are the two most-likely causes for the concentrated damage. The strongest evidence for focusing is the systematic azimuthal variation of amplification factors in the aftershock data. We show using finite difference synthetic seismograms that the azimuthal variation can be modeled using simple focusing structures. Other possible candidates for the damage will also be discussed.