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 $M_{d}$ 2.5. The mainshock (MS) with $M_{L}$ 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.

Figure 13.13: Seismograms of the mainshock (A) and largest aftershock (B). Z, H1 and H2 are the vertical and orthogonal horizontal components, respectively. The H2 geophone (black) was clipped for these two events, so the gray trace shows the instrument-corrected accelerometer record. Dotted and dashed lines show P- and S-arrivals, respectively. (A) About 1.5 s before the mainshock, a small event occurred. Its traces have been multiplied by 100,000. (B) The large aftershock appears to be a double-event. The P- and S-phases of the first and second event are indicated.
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