Subsections

Mapping the Rupture of the $M_{w}$ 5.4 Alum Rock Earthquake

Margaret Hellweg, Angela Chung, Douglas Dreger, Ahyi Kim and Jack Boatwright (USGS Menlo Park)

Figure 2.49: The Alum Rock earthquake occurred along the Calaveras Fault near San Jose. The ShakeMap (top) shows the epicenter and the levels of shaking experienced. Note the strong shaking (intensity VI) to the southeast of the epicenter (red circle). Both the finite fault line source and plane show rupture to the southeast from the epicenter (bottom).
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Introduction

The largest earthquake since Loma Prieta shook the San Francisco Bay Area at 08:04 pm PDT on October 30, 2007 (03:04 UTC on October 31, 2007, Figure 2.49 (top)). It had $M_{w}$ 5.4. Since the Loma Prieta earthquake, instrumentation in the region has improved, with many more digital stations, including short period, strong motion and broadband sensors. The Shakemap in Figure 2.49 (top) shows the epicenter (star), the level of shaking (colors) and the locations of stations which recorded the event (triangles). The automated finite-source solution (Figure 2.49 (upper part of bottom image)), based on the automated moment tensor, was available 9 minutes after the event. We present here finite fault results from the Alum Rock earthquake.

Moment Tensor Solution

The reviewed moment tensor solution (Figure 2.50) for the Alum Rock earthquake is 98 percent double couple, with strike, dip and rake of 323$^{\circ}$, 87$^{\circ}$and -180$^{\circ}$This motion is consistent with slip along the Calaveras Fault.

Figure 2.50: Moment tensor solution for the Alum Rock earthquake. The automatic solution used stations at short epicentral distances that were clipped or nonlinear. For this reviewed solution, the fits of the synthetics to the data are extremely good.
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Finite Fault Solution

The automated finite-source line solution (Figure 2.49 (upper part of bottom image)) shows that slip was largely located southeast of the epicenter. Although the magnitude of this event is at the lower threshold for finite-source analysis given broadband waveforms and the simplified models used to compute Greens functions, the preliminary line-source as well as felt reports and peak ground motion maps indicate that the rupture had strong southeast directivity. One hour after the event, refined results in the plane source indicated that the fault ruptured southeast from the hypocenter. Two line-source rupture models were produced by the automatic processing. For this event with small $M_{w}$ 5.4, it was not possible to distinguish between the two conjugate line-source models. However, for the line-source parallel with the Calaveras fault (model shown in Figure 2.49 (upper part of bottom image)), most of the slip lies to the southeast of the epicenter. We have prepared a finite-source inversion of the Alum Rock earthquake using the method of Dreger and Kaverina (2000). The model has a single fault plane and constant rake, rupture velocity, and rise time. The data and synthetics were bandpass filtered between 0.01 and 0.3 Hz. The lowpass filtering reduces the importance of high frequency arrivals and was performed because of concerns of the adequacy of the velocity model and Greens functions. Despite this restrictive filtering, southeast rupture is a dominant feature of the slip model (Figure 2.49, lower part of bottom image). Initially, the event ruptures downward and extends about 5 km to the southeast of the eipicenter. This extent of rupture is echoed in the locations of the aftershocks, which occurred exclusively to the southeast of the epicenter, and within 5 km. The overall variance reduction is only 48 percent, and the vertical components, theoretically expected to be small given the focal mechanism, are not fit at all. The details of the slip and the depth of the slip patch are not very well constrained, but are consistent with qualitative waveform analyses and with observed shaking (Figure 2.49 (top)). This model also argues for unilateral southeast rupture.

Perspectives

Future work is needed to more fully document the sensitivity of the finite-source model parameters. The peak slip in the model, assuming a rise time 0.3 seconds and rupture velocity of 80% of the shear wave velocity, is 17 cm. Like the moment tensor analysis, the finite source model produced an $M_{w}$ 5.4.

Acknowledgements

Earthquake monitoring and reporting activities at the BSL are supported by the CISN funding of the California Governor's Office of Emergency Services under contract 6023-5 and the United States Geological Survey project 07HQAG0013.

References

Dreger, D. and A. Kaverina, Seismic remote sensing for the earthquake source process and near-source strong shaking: A case study of the October 16, 1999 Hector Mine earthquake, Geophys. Res. Lett., 27, 1941-1944, 2000.

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