Northridge earthquake damage caused by geologic focusing of seismic waves

Despite being Located 21 kilometers from the epicenter of the 1994 Northrid ge earthquake (magnitude 6.7), the city of Santa Monica experienced anomalo usly concentrated damage with Mercalli intensity IX, an intensity as Large as that experienced in the vicinity of the epicenter. Seismic records from aftershocks suggest that the damage resulted from the focusing of seismic w aves by several underground acoustic Lenses at depths of about 3 kilometers , formed by the faults that bound the northwestern edge of the Los Angeles basin. The amplification was greatest for high-frequency waves and was Less powerful at lower frequencies, which is consistent with focusing theory an d finite-difference simulations.