ALBERT

Description: 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〉 or =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〉 or =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〉 or =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.

Description: Statistical analyses were conducted on the capability of correlation detectors for similar events. Semiempirical synthetic runs took a 50-sec window on an Lg wave recorded at 750-km distance filtered from 1 to 3 Hz and embedded it 300,000 times in real continuous background seismic noise. The noise was selected for 36 days spread throughout the year to capture diurnal and seasonal variations. No screening for random, unknown signals in the noise was performed. A correlation detector has a 50% probability of detection with 1.5 false alarms per day for a signal-to-noise ratio (SNR) of 0.32, which corresponds to a full magnitude unit reduction in detection threshold over a standard short-term average/long-term average (STA/LTA) technique. A scaled cross-correlation coefficient performs slightly better with one false alarm per day and has fewer false triggers on unknown, random signals. Summing the cross-correlation traces together for all three components enhances the detection signal similar to beamforming. A correlation detector summing the correlation traces for the three components together has a 96% probability of detection with zero false alarms in 36 days for an SNR of 0.32. The significant result of this study is that a correlation detector has more than an order of magnitude improvement in detection threshold for similar events with acceptably low false alarm rates to be used in practice.