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Real-time detection and location of local seismic events in central California

OFFICE OF EARTHQUAKE STUDIES U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD MENLO PARK, CA 94025

Abstract

A computer-based system dedicated full time to automatic detection and location of local seismic events in central California has been developed. The system monitors 108 short-period vertical-component stations from the U.S. Geological Survey central California and Oroville seismic networks. Locations and magnitudes, when determined, are printed out along with first arrival times, within 2 to 5 min after an event occurs. Wave onsets must be clear and impulsive for best results. For this reason, regional events and teleseisms are usually rejected.

The best results have been obtained for the relatively dense, 16-station Oroville network. For the month of October 1975, 107 (91 per cent) of the 118 events timed by hand were also timed and located by the real-time system. An additional eight events (7 per cent) were detected in real-time but were not successfully located. Of the 107 events for which both on-line and hand-timed locations are available, 92 per cent of the on-line locations are within 2 km of the epicenters determined by hand-timing.

During October 1975 the real-time system monitored 91 of the 150 stations of the central California network. Of the 260 events located by hand-timing, 225 (86 per cent) were detected by the real-time system. Magnitudes of detected events range from 0.8 to 2.9. Approximately 95 per cent of the events of magnitude 1 and greater detected and located by hand-timing methods were also detected by the real-time system. Differences between hypocentral locations based on hand-timed and computer-timed arrivals may vary from 0.1 to 5 min of latitude or longitude.

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