Summary and Perspectives

Although the earthquakes of the Orinda sequence are small, their study contributes to understanding and defining earthquake hazard in Northern California. They occurred in an otherwise seismically quiet region of the Bay Area, between the Hayward and Calaveras faults. The only previous recent seismicity in the area occurred as a brief sequence in 1977 (Bolt et al, 1977). Thus, this sequence offers the opportunity to learn more about the seismicity and seismic hazard between the two faults. In addition, the many aftershocks provide data for improving the velocity structures between these two major faults, particularly at shallow levels. Cursory comparisons between the Orinda and other nearby sequences with similarly sized mainshocks suggest that the Orinda sequence has produced many more aftershocks. Learning more about the physics at these earthquakes' sources will help understand why this is true.

Figure 2.29: Locations (from P-wave polarization and $t_{s-p}$) of the $M_{L}$ 2.5 foreshock (FS), the mainshock (MS) and the P-wave pulses of the largest aftershock (P1, P2, P3) and a small aftershock exhibiting complexity (Pa, Pb).
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