Introduction

On the Parkfield segment of the San Andreas Fault, repeating earthquake seismicity is observed with highly similar waveforms, suggesting that the events occur on the same patch of fault repeatedly (Nadeau et al., 1995). Surrounding these repeating clusters are areas inferred to creep. A shallow cluster of such repeating events is the drilling target of the NSF EarthScope San Andreas Fault Observatory at Depth (SAFOD) experiment. Imanishi et al. (2004) studied waveforms and spectra for one of the SAFOD target events using data obtained from the SAFOD Pilot Hole, and determined a Mw 2.1 and a depth of 2.1 km. From their corner frequency measurements they find a static stress drop of 8.9MPa. On the other hand, Nadeau and Johnson (1998) proposed an asperity-loading model to infer that slip in each event was on the order of 6.6cm, and a stress drop of 240MPa. If a rigidity of 12 GPa, more appropriate for the shallow depth of the event is used, a stress drop of 100MPa is obtained with their method. The difference between 8.9 and 100 MPa bears directly on the nature of the faulting mechanics, whether frictional sliding or rock fracture processes are operating in these events. In this study we reconcile the difference in the reported stress drop estimates using a finite-source inverse method to determine the rupture area, slip distribution, spatially variable stress drop, and rupture velocity of the small repeating earthquakes.

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