The Orinda Earthquake Sequence: Complexity in Small Earthquakes

Margaret Hellweg

Introduction

How different are large and small earthquakes? In large earthquakes, such as the 2004 Sumatra event, we expect a long lasting rupture process with several episodes of energy release. We usually imagine that earthquakes with magnitudes smaller than 4 are point sources in space and time. The events in sequence of earthquakes which occurred near Orinda, California offers an opportunity to explore this question. On October 19 a $M_{L}$ 3.5 earthquake (MS) occurred almost directly below Berkeley Seismological Laboratory's station BRIB (37.92 N, 122.15 W). At the surface are a broadband seismometer and an accelerometer. In addition, there is a borehole at the station, with a 3-component geophone and a 3-component accelerometer sampled at 500 sps at a depth of 119 m. The sequence began on 19 Oct 2003 at 14:35:27 UTC, about an hour before the MS with a Md 2.5 foreshock, and included more than 4000 aftershocks ranging in magnitude from - 2.5 to 3.4 over the course of the next 3 months.

Event Complexity

The record of the mainshock (15:32:52 UTC on October 19 with $M_{L}$ 3.53 (Figure 14.1A) is quite simple, as we expect from a point source. However, the slightly smaller aftershock the next day (October 20, 17:50 UTC, $M_{L}$ 3.4, Figure 14.1B) exhibits a number of P-wave pulses before the arrival of the S-waves. Even smaller events exhibit similar complexity (Figure 14.1C), with several P pulses of similar amplitude arriving before the first S-wave. For each P pulse, there is a corresponding S pulse. Using time-domain polarization analysis to find the azimuth and incidence angles (Plesinger et al., 1986, Abercrombie, 1995), the direction to the source of the waves can be determined. The distance to the source can be derived from the delay between the P- and S-waves from the same source. I performed this analysis for a selection complex events to see if the P-waves came from different locations. Figure 14.2 shows the locations of three P-wave sources for the $M_{L}$ 3.4 aftershock of October 20 and two for a $M_{L}$ 0.5. The mainshock location and the locations of the foreshock and a number of other small, simple aftershocks are also shown. Each P-wave pulse from each these events clearly stems from a different location.

Perspectives

The source of the complexity observed in these two events, as well as in other small events of the Orinda sequence, is as yet undetermined. There are three possible causes for the complexity observed here. The simplest explanation is that several earthquakes of similar size occur with in a very short time of each other, thus that the events are independednt. It is also possible that the rupture process of the event is uneven, and P-wave pulses are generated as the rupture propagates through the medium, releasing energy in separate episodes. Finally, the later events may be triggered in the stressed medium by the passing waves of the first event. Further investigations of the timing of the later arrivals may provide insights. However, an important remaining unknown which can influence the interpretation is the velocity structure in the source region.

References

Abercrombie, R.E., Earthquake locations using single-station deep borehole recordings: Implications for microseismicity on the San Andreas fault in southern California. J. Geophys. Res., 100, 24003-24014, 1995.

Plesinger, A., M. Hellweg and D. Seidl, Interactive high-resolution polarization analysis of broad-band seismograms. J. Geophysics, 59, 129-139, 1986

Figure 14.1: Waveforms for the mainshock (A), largest aftershock (B) and a $M_{L}$ 0.5 afterhock of the Orinda sequence. Note that there are three P-wave pulses in the large aftershock, and at least two in the small one. It may be that these are because several events happen within a short time, or this may be due to rupture propagation.
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Figure 14.2: Locations of the mainshock (large, solid circle), and a number of small, simple aftershocks (small, solid circles). P-wave locations for the largest aftershock are indicated by the black, open circles (P1, P2, P3). For the small aftershock in Figure 14.1C they are marked by gray, open circles.
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