Does the faulting in large and small earthquakes involve different physical processes? This question remains open because of the pervasive difficulty in geophysics of performing controlled experiments. To find the answer, it is necessary, for example, to separate source from path and site effects in seismograms. A sequence of small earthquakes which occurred near Orinda, California, offers an opportunity to explore this question. This sequence occurred under Berkeley Seismological Laboratory's station BRIB (37.92 N, 122.15 W). At the surface are a broadband seismometer and an accelerometer. In addition to the surface installation, there is a borehole at the station equipped with a 3-component geophone and a 3-component accelerometer at a depth of 119 m. The sequence began on October 19, 2003, at 14:35:27 UTC, with an earthquake with 2.5. The mainshock (MS) with 3.5 followed about an hour later. In the next week and over the course of the next 3 months there were more than 4000 aftershocks ranging in magnitude from -2.5 to 3.4.
Berkeley Seismological Laboratory
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