Abstract

A finite difference technique by which an earthquake wave field recorded at the Earth's surface could be extrapolated backward in time to produce an image of the source was presented by McMechan (1982). The resulting image is dynamic and reveals the temporal and spatial configuration of the acoustic equivalent of the source. The method was successfully tested on synthetic data, but no real earthquake data satisfying the prerequisites for processing were available in 1982. The data must be recorded close to the source and must be spatially dense. In January of 1983, a unique data set was recorded by the U.S. Geological Survey within Long Valley Caldera in eastern California. Three events were chosen from the aftershock sequence. Preprocessing of the data for each event includes construction of a true amplitude section, filtering, and extrapolation to produce unaliased, equally spaced observations. Extrapolation of these data through a previously determined velocity structure produces coherent images in which both the source location and radiation pattern are visible. The images are also consistent with previously determined focal mechanisms. The results demonstrate the feasibility of imaging real earthquake sources.

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