Subsurface structures and their relationship to Northridge earthquake damage in Santa Monica (California)

An experiment designed to examine the relative contributions of focusing and site effects on the seismic wave amplification in the region of intense damage in Santa Monica from the Northridge Earthquake took place in October 1999. We deployed an array of 194 stations in the Santa Monica area. I used amplitudes of seismic waves to estimate near-surface site factors and the effects of deep structural focusing, and P wave first arrivals to invert for the associated deep structure.; The near surface site effects are determined by using normalized spectral amplitudes from direct and coda waves. Coda waves indicated that site factor amplifications are larger south of the Santa Monica fault. Spectral ratios of direct S waves from local earthquakes corrected for site effects show additional amplification, attributable to a deeper structure. The largest amplification values are found from rays travelling a critical path from −10.5° west of north and 46.5° incidence angle.; In an attempt to reproduce the hypothesized focusing from the Northridge earthquake, a ∼4000 lb. shot was detonated at Fort Tejon. Amplification as a result of the Fort Tejon shot was three times south of the Santa Monica fault relative to North. The increased amplification occurs because arrivals from the Fort Tejon shot were close to the critical path (−15.3° incidence, 47.3° azimuth).; To develop a velocity model of the subsurface structure, P wave first arrivals were used in a 3D local earthquake tomography algorithm. Using a plane-layered starting and a model with a priori information, the inversion delineates a basin and a probable location of the Santa Monica fault. I used a 3D ray tracer to show this basin focuses seismic waves from the Northridge Earthquake in the region of concentrated damage and where both aftershocks and shots greatest amplification. I conclude that focusing by a deep basin in Santa Monica could have been responsible for extra amplification of seismic waves during the Northridge Earthquake and so contributed to the anomalous damage.