DESCRIPTION:

RSAM - Real-Time Seismic-Amplitude Measurement System
SSAM - Seismic Spectral-Amplitude Measurement System

Two new systems, the Real-time Seismic-Amplitude Measurement (RSAM) and the Seismic Spectral-Amplitude Measurement (SSAM), have been developed by the USGS to summarize seismic activity during volcanic crises. These techniques for characterizing a volcano's changing seismicity in real time (as it is occurring) rely on the amplitudes and frequencies of seismic signals rather than on the locations and magnitudes of the earthquakes. During a volcanic crisis, seismicity commonly reaches a level at which individual seismic events are difficult to distinguish. Analog seismic records (seismograms) provide some information, but rapid quantitative analysis is not always possible without substantially disturbing the continuity of recording. Although several real-time earthquake-detection and recording systems exist, most fail to provide quantitative information during periods of intense seismicity, which is a common situation before a volcanic eruption. Yet it is precisely during such periods that the need for timely quantitative seismic information becomes most critical. To fill this need a simple and inexpensive real-time seismic-amplitude measurement system (RSAM) was developed.

The RSAM computes and stores the average amplitude of ground shaking caused by earthquakes and volcanic tremor over 10-minute intervals. Increases in tremor amplitude or the rate of occurrence and size of earthquakes cause the RSAM values to increase. Rather than focusing on individual events, RSAM sums up the signals from all events during 10- minute intervals to provide a simplified but still very useful measure of the overall level of seismic activity (Figure 9). This information is easy to plot and convey to public officials.

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Figure 9 -- Real-Time Seismic-Amplitude Measurement (RSAM) plot. Comparison of RSAM data (top) and seismograms (bottom) shows how RSAM reduces complex seismic data to a simple line graph that correlates with ground-shaking energy. Eruptions (heavy dark lines) from Mount Redoubt occurred at 09:47 am, and 10:15 am on the 14th and 15th of December, 1989.

The Seismic Spectral-Amplitude Measurement (SSAM) system takes this approach one step further by computing in real time the average amplitude of the seismic signals in specific frequency bands (Figure 10). This permits seismologists to evaluate the nature of seismicity at a volcano and recognize subtle shifts in frequency that are related to changing dynamics of magma movement.

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Figure 10 --Seismic Spectral-Amplitude Measurement (SSAM) plot (Mount Pinatubo, the Philippines, June 15, 1991). This plot shows the average relative seismic amplitude in specific frequency bands over 15 minute intervals. This type of seismic data is available in real time, and permits seismologists to detect and evaluate a change in the type of earthquake activity occurring beneath an active, restless volcano. In the figure, the time scale refers to Greenwich mean time (G.m.t.). Brief episodes of intense seismicity in the 0.5-1.5 Hz frequency band between approximately 0200 and 0530 were associated with explosive eruptions. Intense tremor during the first part of the climactic eruption began at about 0540 and gave way after approximately 3 hours, as the eruption waned, to higher-frequency seismicity related to structural readjustments of the volcano. Data gaps result from loss of power to the system during the evacuation of Clark Air Base.

The delay in CVO's learning about the January 6, 1990, event, coupled with increased concerns about the hazards of volcanic ash triggered by the Boeing 747 incident in Alaska on December 15, 1989 (Brantley, 1990), prompted CVO to develop a seismic-alarm system that is activated by small, as well as large, volcanic events. CVO also made a few adjustments in the notification and call-down procedures to improve communication of hazards information during non-working hours.

Because seismicity is one of the main tools used to monitor volcanoes, CVO and UW 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.

RSAM alarms that have been triggered at Mount St. Helens between March 1, 1990, and September 20, 1991
Type of event	Number of alarms
Explosion-like seismic events (four of which had confirmed ash plumes)	16
Rockfalls (some with dust plumes)	22
Mount St. Helens earthquakes	1
Regional earthquakes	12
Telemetry problems	6

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.

Although several real-time detection and recorder systems exist, few address the problem of continuously measuring the amplitude of seismic signals during volcano-crisis conditions, when individual events are difficult to recognize. We developed a real-time seismic-amplitude measurement system (RSAM) that uses an inexpensive eight-bit analog-to-digital converter controlled by a laptop computer to provide 1-minute-averaged, absolute-amplitude information for eight seismic stations near Mount St. Helens. The absolute voltage level for each station is digitized at 50 samples/second, averaged, and immediately transmitted to a host computer for analysis. The RSAM proveds a convenient-to-access, continuous time history of seismic activity at the volcano. RSAM systems calculating 10-minute amplitude averages have been installed at the Cascades, Alaska, and Hawaiian Volcanoes Observatories. The RSAM has been a useful tool in predicting eruptive activity at Mount St. Helens and Redoubt Volcano, Alaska.

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