The Orinda Earthquake Sequence

Margaret Hellweg

Introduction

On October 19 and 20, 2003, two earthquakes with $M_{L}$ 3.5 and $M_{L}$ 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.

The Seismicity

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 $M_{d}$ 2.5. This event was followed by more than 15 smaller events, the largest of which, at 15:12 UTC with $M_{d}$ 1.67, was also located. Just under one hour later, a larger earthquake occurred, the $M_{L}$ 3.5 mainshock of the sequence.

The two largest earthquakes, the mainshock at 15:32 UTC on October 19 with $M_{L}$ 3.5, and the aftershock at 17:50 on October 20 with $M_{L}$ 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.

Figure 16.1: Three-component recording of the mainshock on the borehole instruments. H1 and H2 are the two orthogonal horizontal components. For this figure, the H2 recording has been replaced by the instrument-corrected and integrated record from the borehole accelerometer. The offset traces in the first 3 s of the figure have been scaled by a factor of 100,000 to show the ``pre-event noise'', in this case a very small foreshock.

When the instrument response is removed from the accelerometer recording and the trace is integrated, it matches the corresponding component of the velocity sensor. Figure 16.1 shows the three component recording of the mainshock, with the vertical trace at the top and the two orthogonal horizontal traces (H1 and H2) below. The bottom trace (H2) was clipped in the velocity recording and has been replaced by the integrated accelerometer trace. The offset traces between 15:32:50 and 15:32:53 are scaled 100,000 times the traces for the mainshock. There is clearly a very small event 1.5 s before the mainshock onset showing the range of sizes of the events in this sequence and the similarities in the waveform of both large and small events.

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 $M_{d}$ 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

Figure 16.2: Calibration of ``manual magnitude'' to catalog magnitude ($M_{L}$ or $M_{d}$). Diamonds denote events for which the NCSN catalog gives magnitudes. Triangles show how events for which only manual magnitudes exist. They have been placed on the regression line to determine their corresponding catalog magnitude.

horizontal components of the borehole velocity sensor and multiplied by the period and by the magnification of a Wood-Anderson instrument at that period. The manual magnitude is defined as $M_{m} = log_{10} [(N_{max} + E_{max})/2]$. This numerical result is comparable to the measurement used to determine $M_{L}$. As the distances of these events from the station BRIB are all nearly the same, no distance correction is necessary. Figure 16.2 shows a comparison between the catalog and manual magnitudes (diamonds). The line shows the regression using of the catalog magnitudes and manual magnitudes and allows me to project manual magnitudes to a corresponding catalog magnitude (triangles). The sequence includes still smaller events for which I have not yet determined manual magnitudes, thus the earthquakes in the sequence recorded at station BRIB range over more than 5 magnitudes in size.

Perspectives

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.

References

Richter, C.F., An instrumental earthquake magnitude scale, Bull. Seismol. Soc. Am., 25, 1-32, 1935.