Effects of Local Geologic Conditions on Strong-Ground Motions in the Vicinity of Coalinga, California

ABSTRACT

An array of six portable digital event recorders (GEOS) recorded eighteen aftershocks (M 1-4.3) during a three-day deployment period to investigate the effects of local site conditions on earthquake-generated ground motions in the community of Coalinga. Large frequency-dependent amplitude variations due to local site conditions were observed over distances less than 1.5 km. The sites underlain by alluvium, including two sites in the community of Coalinga show evidence of site resonances with inferred amplifications of horizontal motion exceeding a factor of 10 in some cases. In general, the wide dynamic range broad-bandwidth signals recorded on the array show considerable amplification of the higher frequency components of shaking (5-15 Hz), suggesting that the alluvial deposits underlying the community of Coalinga could have been a contributory factor in the damage to old brick structures observed during the May 2, 1983 Coalinga earthquake.

First posted February 23, 2010