Shortly after being recorded to disk on the central site HRSN computer, event triggers for the individual station data are determined, and a multi-station trigger association routine then processes the station triggers and identifies potential earthquakes. For each potential earthquake that is detected, a unique event identification number (compatible with the NCEDC classification scheme) is assigned. Prior to San Simeon earthquake of December 22, 2003, 30 second waveform segments were then collected for all stations and components and saved to local disk as an event gather, and event gathers were then periodically telemetered to BSL and included directly into the NCEDC earthquake database (dbms) for analysis and processing.
Because of its mandate to detect and record very low magnitude events in the Parkfield area, the HRSN is extremely sensitive to changes in very low amplitude seismic signals. As a consequence, in addition to detecting very small local earthquakes at Parkfield, the HRSN also detects numerous regional events. Since the beginning of the network's data collection in 1987, the local and regional events were discriminated based on analyst assessment of S-P times, and only local events with S-P times less than 2.5 sec at the first arriving station were picked and located as part of the HRSN routine catalog.
Following the occurrence of the M6.5 San Simeon earthquake on December 22, of 2003, our long-standing data handling procedure was no longer viable due to the enormous rate of San Simeon aftershock detections (Figures 6.5) In the first 5 months following the mainshock, over 70,000 event detections were made by the HRSN system (compared to a yearly average detection rate of 6000 prior to San Simeon), and spot checks of the continuous 20 sps data revealed that the overwhelming majority of these detections resulted from seismic signals generated by San Simeon's aftershocks.
Data from the California Integrated Seismic Network (CISN) show that there were 1,150 San Simeon aftershocks with magnitudes 1.8 occurring in the week following the mainshock. During this same period, the number of event detections from the HRSN was 10,500 (compared to an average weekly for the year prior to San Simeon of 115 detections/per week). This suggests that the HRSN is detecting San Simeon aftershocks well below magnitude 1, despite the network's 50 km distance from the mainshock.
The dramatic increase in event detections vastly exceeded the HRSN's capacity to process both the continuous and triggered event waveform data. To prevent the loss of seismic waveform coverage, processing of the triggered waveform data has been suspended to allow archiving of the 250 sps continuous data to tape to continue uninterrupted. Cataloging of the event detection times from the modified REDI real-time system algorithm is also continuing, and the 250 sps waveform data is currently being periodically uploaded from the DLT tapes onto the NCEDC for access to the research research community, and its NCEDC accessibility will also play a vital role in the automated similar event catalog processing that we are currently developing.
Berkeley Seismological Laboratory
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