Introduction

Are small earthquakes like large earthquakes? Does their faulting involve the same physics? How small do aftershocks get, and where do they occur? Do they exhibit similar complexity to that observed in large events? These questions are difficult to answer, because small earthquakes and their tiny aftershocks are rare in our recordings.

Figure 2.28: Example waveforms of (A) the mainshock, which is simple, and (B) the largest and (C) a small aftershock, which are complex. This is most easily seen in the S-waves (gray shading), which have more than one large arrival om B amd C.
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A sequence of small earthquakes near Orinda, California, occurred almost under the BSL's station BRIB (37.92 N, 122.15 W). At the surface, this station has a broadband seismometer and an accelerometer. As it is also a borehole station of the Northern Hayward Fault network, it has a geophone and an accelerometer, each with 3-components, at a depth of 119 m. The borehole instruments are sampled at 500 sps. The sequence began on October 19, 2003, at 14:35:27 UTC, with an earthquake with $M_{d}$ 2.5. The mainshock (MS, $M_{L}$ 3.5, Figure 2.28A) followed about an hour later. Over the course of the next 3 months, there were more than 4000 aftershocks, with magnitudes ranging from -2.5 to 3.4.

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