Separation of Site Effects and Structural Focusing in Santa Monica, California: A Study of High-Frequency Weak Motions from Earthquakes and Blasts Recorded during the Los Angeles Region Seismic Experiment

Shirley Baher, Paul M. Davis, & Gary S. Fuis

Published December 2002, SCEC Contribution #583

Near-surface site effects and the effects of deep structural focusing were estimated in the Santa Monica Mountains and Santa Monica, California from a portable array of 75 seismic stations deployed during the Los Angeles Region Seismic Experiment, Phase II (LARSE II). The objective was to examine further the origin of seismic wave amplification in the region of intense damage in Santa Monica from the Northridge Earthquake. The analysis used normalized spectral amplitudes in the 1-4, 4-8, and 8-12 Hz range in direct and coda waves from local earthquakes in Santa Paula, Northridge, Redlands, and Hector Mine. Coda waves indicated that site factor amplifications are larger south of the Santa Monica Fault relative to the north. Spectral ratios of direct S waves corrected for site effects show additional amplification south of, and adjacent to, the Santa Monica Fault, attributable to focusing by a deeper structure.

Gao et al., (1996) concluded that localized focusing effects contributed to anomalous P and S wave amplification in the Santa Monica damage zone for earthquakes within a specified range of azimuths. In an attempt to reproduce the hypothesized focusing from the Northridge Earthquake, two shots (4000 and 3750 lb.) were detonated, one at Pyramid Lake, a distance of about 69 km to the NNW of central Santa Monica, and the other near Fort Tejon, a distance of 91 km. The azimuth of the shots was chosen to be that expected to give anomalous amplification. At these distances steeply incident seismic energy from Pg/PmP waves are expected to pass through the underground focusing structure and be selectively amplified. After the local site factors are removed the waveforms from the Fort Tejon shot exhibited localized amplification adjacent to and south of the fault, 2-3 times larger than that of the surrounding area. The effect is less for the Pyramid Lake shot which could be due to its higher angle of incidence. The observations lend support to the argument that deep structural focusing is an important factor in determining azimuht-dependent amplification of seismic waves along a basin edge.

Citation
Baher, S., Davis, P. M., & Fuis, G. S. (2002). Separation of Site Effects and Structural Focusing in Santa Monica, California: A Study of High-Frequency Weak Motions from Earthquakes and Blasts Recorded during the Los Angeles Region Seismic Experiment. Bulletin of the Seismological Society of America, 92(8), 3134-3151. doi: 10.1785/0120010168.