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DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF SANTA BARBARA, SANTA BARBARA, CALIFORNIA 93106-9630
INSTITUTE FOR CRUSTAL STUDIES UNIVERSITY OF CALIFORNIA, SANTA BARBARA, SANTA BARBARA, CALIFORNIA 93106-1100
WARREN & SELBERT, INC., 222 E. CARRILLO STREET, SUITE 310 SANTA BARBARA, CALIFORNIA 93101
U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD MENLO PARK, CALIFORNIA 93101
Abstract
The Garner Valley downhole array (GVDA) in southern California (33°41.60'N, 116°40.20'W) is a five-element array of three-component, dual-gain force balance accelerometers that are capable of measuring accelerations from 3 x 10–6 to 2.0 g over a frequency range from 0.0 to 100 Hz. The accelerometers are placed at depths of 0, 6, 15, 22, and 220 m. The lithostratigraphy is 19 m of soil overlying a 24-m layer of weathered granite below which is granite (tonalite). The array, only 7 km from the San Jacinto fault, is located at the northern end of the Anza seismic gap on the San Jacinto fault, where an M 6.5 or greater earthquake can be expected. It is only 35 km from the Indio segment of the San Andreas fault, which last ruptured with an M > 8 earthquake around 1700 A.D. From its installation in July 1989 through July 1991, GVDA has recorded 280 earthquakes with magnitudes ranging from 1.2 to 4.7 and epicentral distances ranging from from 0 to 110 km. The maximum acceleration recorded was 89 cm/sec2 from an ML 4.2 earthquake 15.8 km (hypocentral distance) from GVDA. The average amplification of the apparent seismic moment between 220-m depth and the surface is about 13 for over five orders of magnitude in the size of the recorded earthquakes. The spectral ratio 0/220 of horizontal acceleration amplitude spectrum shows a mean amplification of about 10 for the frequency range of 2.0 to 30 Hz for 17 events. Resonance peaks exist at about 1.7, 3.0, and 12.0 Hz where the spectral ratio 0/220 is nearly 40. Analysis of the acceleration spectra of two specific earthquakes that have nearly the same hypocenter but with different magnitudes, M 4.2 and 2.5, shows that the weathered granite zone, beneath the soil, strongly attenuates the spectral amplitudes for frequencies greater than 40 Hz. The impedance of the soil relative to the weathered granite amplifies all frequencies by about a factor of 3, leading to spectral levels that are greater at the surface than at 22-m depth, near the top of the weathered granite. The acceleration spectra at each depth show resonant peaks at the same frequencies. The spectra at each depth are well correlated for both earthquakes. The acceleration spectrum at 220 m is nearly constant for frequencies less than 60 Hz, which implies that there is little or no attenuation.
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