Event Complexity

Most events in the Orinda sequence, like the MS, have classically simple waveforms, a single P pulse followed by a single S pulse (Figure 2.28A). However, many earthquakes of all sizes exhibit one of two types of complexity. In one type, several events of similar size occur within a very short interval, as a sort of "rapid-fire" burst (Hill et al, 1990, Asch et al, 1997). In the second type of complexity, more than one P-wave pulse arrives before the first S-wave (Figure 2.28 B, C ). Using measurements from waveforms of a few events, we estimate the source location of the P-pulse of each subevent from its polarization (Plesinger et al, 1986, Abercrombie, 1995) and its $t_{s-p}$. As with the other events of the sequence, $t_{s-p}$ values for the subevents lie between 0.58 and 0.7 s, and they are located within the cluster. Figure 2.29A shows as an example the locations of three subevents from the event in Figure 2.28B (P1, P2 and P3) and two subevents from the event in Figure 2.28C (Pa and Pb) in relationship to the mainshock (MS), the $M_{d}$ 2.5 foreshock (FS) and several aftershocks (small dots), as well as to the station BRIB. The distances are given in km. The table (Figure 2.29B) gives the spatial separation of the subevents and the intervals between them. It also gives the ratio of the subevent spacing to the values of $v_{P}$ (4.1 km/s) and $v_{S}$ (2.3 km/s) assumed for the source volume. The intervals between the subevents are consistent with the propagation of rupture at speeds around 0.8 $v_{S}$.

Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
Questions or comments? Send e-mail: www@seismo.berkeley.edu
© 2007, The Regents of the University of California