Shortly after being recorded to disk on the central site HRSN computer, event triggers for the individual station data are also determined, and a multi-station trigger association routine then processes the station triggers and generates a list of potential earthquakes. For each potential earthquake that is detected, a unique event identification number (compatible with the NCEDC classification scheme) is also assigned. Prior to the 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 and relatively distant and small amplitude nonvolcanic tremor events. For example, spot checks of aftershocks following the M6.5 San Simeon earthquake of December 22, 2003 using continuous data and HRSN event detection listings have revealed that the overwhelming majority of HRSN detections following San Simeon resulted from seismic signals generated by San Simeon's aftershocks, despite the HRSN's 50 km distance from the events. Data from the California Integrated Seismic Network (CISN) show that there were 1,150 San Simeon aftershocks with magnitudes 1.8 in the week following San Simeon, and during this same period, the number of HRSN event detections was 10,500 (compared to an average weekly rate before San Simeon of 115 detections). This suggests that, despite the 50 km distance, the HRSN is detecting San Simeon aftershocks well below magnitude 1.
Current Processing. Since the beginning of the network's data collection in 1987, and up until recently, 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. However, because of the networks extreme sensitivity to the large swarm of aftershocks from the San Simeon and M6 Parkfield earthquakes of September 2004 (e.g., in the first 5 months following the San Simeon mainshock, over 70,000 event detections were made by the HRSN system, compared to an average 5 month detection rate of 2500 prior to San Simeon) and because of ever declining funding levels, this approach has had to be abandoned.
The dramatic increase in event detections vastly exceed 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 continuous 250 sps waveform data is currently being periodically uploaded from the DLT tape archive onto the NCEDC for access to the research community.
Funding to generate catalogs of local events from the 10s of thousands of aftershock detections has not been forthcoming, and, as a consequence, major changes in our approach to cataloging events have had to be implemented, which involves integration of HRSN data into NCSN automated event detection and cataloging (with no analyst review) combined with a high resolution procedure now being developed to automatically detect, pick, locate, and determine magnitudes for similar and repeating events down to very low magnitudes (i.e., below magnitude -1.0Ml). These new schemes are discussed in more detail in the activities section below.
Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
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