The NCSN has reliably located earthquakes to less than M 1 level since 1984, and most sites consist of short-period seismometers. During the period 1984-2004 ending before the Parkfield M6 mainshock, 30 M 1.3 - 3.0 NCSN-derived repeating sequences were identified with a total event number of 178 (Nadeau and McEvilly, 2004). With the higher level of detection of micro-earthquakes, the borehole High Resolution Seismic Network (HRSN) has revealed a larger number of repeating earthquakes ranging in magnitude from -0.4 to +1.7. Recording of the HRSN deep borehole sensors began in early 1987, but the original data acquisition system failed in 1998. In August 2001, the HRSN network was upgraded and three new borehole stations were installed to improve resolution of the structure, kinematics and monitoring capabilities in the SAFOD drill-path and target zone. During the period 1987-1998, 187 HRSN repeating sequences were identified with a total event number of 1123. With more sequences that repeat more frequently due to their smaller event size, the HRSN catalog significantly increases the amount of repeating data available for analysis and provides a better opportunity to examine recurrence properties.
Berkeley Seismological Laboratory
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