On October 19 and 20, 2003, two earthquakes with 3.5 and 3.4 occurred ENE of Orinda, CA. Fortuitously, their hypocenters were located almost directly below Berkeley Seismological Laboratory's station at Russell Reservation Field Station, BRIB (37.92 N, 122.15 W). This station is equipped at the surface with a Guralp CMG-3T in a 35 m posthole installation and a FBA-23 accelerometer. In addition, the station has a 3-component Oyo HS-1 geophone and a 3-component Wilcoxon 731A accelerometer in a borehole at a depth of 119 m.
Since it is a short-period instrument, the Oyo HS-1 geophone in the borehole at BRIB usually only records local events. In the days, leading up to October 19, 2003, there were several small earthquakes, but they belonged to a sequence in Danville/Alamo, CA, further to the south and east. The Orinda sequence began on October 19, 2003 at 14:35 UTC (07:35 PDT) with an earthquake with 2.5. This event was followed by more than 15 smaller events, the largest of which, at 15:12 UTC with 1.67, was also located. Just under one hour later, a larger earthquake occurred, the 3.5 mainshock of the sequence.
The two largest earthquakes, the mainshock at 15:32 UTC on October 19 with 3.5, and the aftershock at 17:50 on October 20 with 3.4, were clipped on one of the horizontal components of both the surface and the borehole seismometers. Fortunately, the clipped component of the borehole seismometer coincides with the single functioning component of the borehole accelerometer.
Of the many events which occurred during the first 24 hours of the sequence, only 14 appear in the catalog of the Northern California Seismic Network (NCSN) with locations and magnitudes. While the catalog reports depths of approximately 10 km for the two largest quakes, the S-P times at the station BRIB for all the events in the sequence range from 0.58 to 0.7 s. They must therefore be located less than 6 km below the station. The borehole instrument recorded more than 4000 fore- and aftershocks in the first week of the sequence. At the beginning of December aftershocks continued at a rate of 6 or more per day, with a 2.9 aftershock recorded January 1, 2004.
Standard magnitudes cannot be determined for most of the earthquakes in the sequence: the events are too small to be recorded at other stations and, strictly speaking, the local magnitude scale is not defined for events as close as 6 km. To determine the magnitude threshold for the events, I calibrated a ``manual magnitude'' scale using the 14 events from the catalog. Following the definition of local magnitude (Richter, 1935), I measured the peak-to-peak amplitudes in the instrument-corrected velocity seismograms of the two
This sequence provides a well-recorded multitude of tightly clustered, small events ranging over 5 magnitude units in size. It thus offers an excellent opportunity to investigate various aspects of event scaling and aftershock statistics.
Richter, C.F., An instrumental earthquake magnitude scale, Bull. Seismol. Soc. Am., 25, 1-32, 1935.
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