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An application of seismic tomography to basin focusing of seismic waves and Northridge earthquake damage

Shirley Baher, & Paul M. Davis

Published February 2003, SCEC Contribution #684

Damage during the 1994 Northridge earthquake was anomalously concentrated in Santa Monica, California. Aftershock studies suggest that the basin substructure caused localized amplification of seismic waves by focusing the energy. However, the geometry and velocities of the geological structure are uncertain, relying on extrapolation from widely separated boreholes. We present results of a tomographic study which includes inverting over 30,000 travel times from a seismic array experiment on a 194 station array and a 10-km-long Vibroseis survey. We find that the damage zone is located above a subbasin several kilometers deep where low-velocity sediments of the northeastern Los Angeles basin are separated from higher-velocity rocks of the Santa Monica Mountains by the steeply dipping Santa Monica-Potrero Canyon fault zone. Three-dimensional ray tracing through this structure shows that focusing of rays from the Northridge earthquake epicenter occurs in the region where damage was greatest and where aftershock amplitudes are magnified. We also find that near-surface P velocities, while low in the Santa Monica basin, do not correlate with the zone of damage. In a related study [ Baher et al., 2002 ], site effects determined from coda waves are better correlated with damage, suggesting that compliant site effects were also important in localizing the damage distribution. We conclude that the anomalous damage was caused by a combination of site and basin focusing effects. Such geometries may be quite common in earthquake zones worldwide where similar results have been observed.

Baher, S., & Davis, P. M. (2003). An application of seismic tomography to basin focusing of seismic waves and Northridge earthquake damage. Journal of Geophysical Research, 108(B2), 2122. doi: 10.1029/2001JB001610.