Abstract

Buildings damaged by the Whittier Narrows, California, earthquake of 1 October 1987 were concentrated in several areas, most of which were located 8 to 10 km from the epicenter (corresponding to the critical incidence for SV waves) and were near topographic irregularities, such as the northern part of Whittier, just south of Puente Hills. The purpose of this study is to show the possibility that this anomalous damage pattern is due to the amplification by the topographic irregularity when SV waves are near-critical incidence.

First we examined the accelerograms obtained at the USGS station (7215 Bright Ave.) nearest to downtown Whittier and conclude that the dominantly eastwest motion observed at the station is due to the influence of the building in which accelerograms were recorded. We next demonstrate that strong ground motions recorded by 12 stations in the epicentral area show a polarization direction pattern that is primarily SV-type, which is consistent with a simple thrust fault located at the hypocenter.

Then we calculated the response of a two-dimensional hill with the height 0.3 km and the width 2.4 km to 1) a plane SV wave with a nearly critical angle of incidence, 2) a horizontal line force, 3) a Haskell-type 2D dislocation source, and 4) a Bouchon-type 2D multiple crack source. The results show that the amplification due to the hill relative to the flat surface is more than 1.5 for all the source models. Since this amplification is nearly independent of the source type and spectrum, we conclude that the combined effect of the topographic irregularity and critically incident SV waves might be responsible for the concentration of damage observed during the Whittier Narrows earthquake.

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