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Test of a simple empirical Green's function method on moderate-sized earthquakes

Institut de Physique du Globe de Strasbourg Université Louis Pasteur-CNRS, 5, rue René Descartes, 67084, Strasbourg Cedex, France

Abstract

In regions of low to moderate seismicity, accelerometric data may be scarce or nonexistant. This makes it difficult to use empirical or semi-empirical methods for estimating the strong ground motion of a potentially damaging earthquake. When a few small events are nevertheless well recorded locally, we propose to use a simple empirical Green's function method to obtain rough strong-motion estimates for a hypothetical magnitude 5 to 7 earthquake. The method is equivalent to applying a source-dependent linear filter to the small-event records. At low frequencies, tests with synthetics computed for a circular fault model show that the method may be valid at distances of a few fault lengths from the source, but it fails at predicting the near-field terms at shorter distances. Spatial aliasing problems, due to the sampling of the large-event fault surface with a finite series of subfaults, put another limit at high frequencies. Applying the method to an earthquake sequence in central California, we also show that uncertainties in the small-event source parameters may cause important errors in the strong-motion prediction. Despite these quite severe limitations, we consider that rough estimations of strong ground motion are possible by making a series of hypotheses on the source process. We show, for example, using a small earthquake recorded at the surface and in a 500-m-deep borehole, how the SH strong-motion acceleration spectra could be estimated at depth, next to a mine, and at the surface, for a hypothetical M 6.5 earthquake in the sediment-filled valley of the upper Rhine Grabben.

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