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Because seismicity is one of the main tools used to monitor volcanoes, CVO and
UW (University of Washington)
maintain a network of 18 seismic stations within 16 kilometers of Mount St.
Helens, including three stations in the crater. These stations provide a
detailed record of seismic activity at Mount St. Helens, including earthquakes,
tremor, rockfalls, explosions, and mudflows (Jonientz-Trisler and others, 1994).
Most of these seismic events, including many of the small ash-producing
explosions, are too small to record on the State-wide network at UW. However,
the events cause significant local ground motions that are detected on the Mount
St. Helens network by the CVO real-time seismic-amplitude monitoring system
(RSAM) as peaks in the time-averaged seismic amplitude (Endo and Murray, 1991).
An RSAM-based seismic-alarm system was developed and installed for testing 2 days after the January 6, 1990, event and was fully functional by the end of February 1990. With the RSAM system, a computer program compares the amplitude of a station's average seismic signal during a 1-minute interval with empirically determined threshold values. If thresholds are exceeded during the same 1-minute interval at several (usually three) of the stations in the crater and on the volcano flanks, an RSAM alert is generated. The computer is set to automatically dial the duty scientist's 24-hour beeper and transmit a number code indicating an RSAM alert. The computer redials the beeper every time a new 1-minute RSAM alert is generated. Many types of events, including explosions, rockfalls, earthquakes, and telemetry problems, can generate alerts. Because an alert does not indicate the nature of the event, the duty scientist must examine the seismic signature of the event recorded on the seismographs at CVO to determine the basis for the alert. Explosion signals can usually be identified by careful evaluation of the signal character (Jonientz-Trisler and others, 1994). Once an alert is received, the speed with which notification is issued will depend on the time it takes for the duty scientist to reach CVO and the scientist's skill at recognizing explosion signals. To date, all known ash-producing explosions since March 1, 1990, have generated alerts. However, failure of any component (key seismic stations, computer, computer programs, phone system, beeper, or beeper-pager system) would prevent an alert from getting through. As a precaution, a daily test alert is sent through the system to the beeper. Example of initial CVO response to a seismic event that occurred without precursors during nighttime, November 5, 1990.
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