Annual Meeting - SCEC.org

Poster Abstracts
possible to observe earthquake processes that cannot be seen in shallow boreholes or at the surface. The SAFOD Main Hole is
a 3.2 km-deep inclined borehole that crosses the active traces of the San Andreas Fault. A 3-component 15 Hz GS-20DM
geophone was deployed within the fault zone at depths between 2350 and 2750 m. Because of the close proximity to
earthquake sources, we routinely observed earthquakes with Mw < 0 with high signal-to-noise ratio. Events were recorded at
three different sampling rates (1000, 2000, and 4000 sps) depending on observation period (2005 - 2011). This study focuses on
earthquakes with S-P time differences shorter than 0.5s, corresponding to hypocentral distance less than 3 km. We determined
source parameters by the Multi-Window Spectral Ratio (MWSR) method (Imanishi and Ellsworth, 2006). The key step in the
method is to stack the ratios calculated from moving windows taken along the record starting with the direct waves. We
confirmed that stacking better suppresses the random noise due to differences in path than single-window methods,
producing a better estimate of the source spectral ratio. For Mw>-1, the calculated stress drops range between 0.1 and 100 MPa.
These observations indicate that there is no breakdown in stress drop scaling for Mw>-1. The stacked spectral ratios show that
corner frequencies of events for Mw