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1 Lamont-Doherty Earth
Observatory
Columbia University
61 Route 9W
Palisades, New York
10964
We processed the complete digital seismogram database for northern California
to measure accurate differential travel times for correlated earthquakes
observed at common stations. Correlated earthquakes are earthquakes that occur
within a few kilometers of one another and have similar focal mechanisms, thus
generating similar waveforms, allowing measurements to be made via
cross-correlation analysis. The waveform database was obtained from the Northern
California Earthquake Data Center and includes about 15 million seismograms from
225,000 local earthquakes between 1984 and 2003. A total of 26 billion
cross-correlation measurements were performed on a 32-node (64 processor) Linux
cluster, using improved analysis tools. All event pairs with separation
distances of 5 km or less were processed at all stations that recorded the pair.
We computed a total of about 1.7 billion P-wave differential times from
pairs of waveforms that had cross-correlation coefficients (CC) of
0.6 or larger. The P-wave differential times are often on the order of
a factor of ten to a hundred times more accurate than those obtained from
routinely picked phase onsets. 1.2 billion S-wave differential times
were measured with CC 
0.6, a phase not routinely picked at the
Northern California Seismic Network because of the noise level of remaining
P coda. We found that approximately 95% of the seismicity includes
events that have cross-correlation coefficients of CC 0.7 with
at least one other event recorded at four or more stations. At some stations
more than 40% of the recorded events are similar at the CC 0.9
level, indicating the potential existence of large numbers of repeating
earthquakes. Large numbers of correlated events occur in different tectonic
regions, including the San Andreas Fault, Long Valley caldera, Geysers
geothermal field and Mendocino triple junction. Future research using these data
may substantially improve earthquake locations and add insight into the velocity
structure in the crust.
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