The two largest earthquakes of the sequence were the mainshock on October 19, at 15:32 UTC with 3.5, and an aftershock at 17:50 on October 20, with 3.4 (Figure 13.13). For these two events one of the horizontal components of both the surface and the borehole seismometers was clipped. The black traces in Figure 13.13 show the seismometer records. Fortunately, the clipped component of the borehole seismometer coincides with the single functioning component of the borehole accelerometer. When the instrument response is removed from the accelerometer recording and the trace is integrated, it matches the corresponding component of the velocity sensor (gray traces in Figure 13.13). The offset traces at the beginning of the records in Figure 13.13A show a small event which occurred 1.5 s before the mainshock. These traces, scaled to be 100,000 times those for the mainshock, show some of the range of sizes of the events in this sequence. The large aftershock appears to be a double event (Figure 13.13B) with two clear P and two clear S phases.
Berkeley Seismological Laboratory
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