Geologic Publications for Mount Rainier

A real-time seismic amplitude measurement system (RSAM)

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Author(s): Thomas L. Murray, Elliot T. Endo

Category: PUBLICATION
Document Type: Open-File Report 89-684
Publisher: United States Geological Survey
Published Year: 1989
Volume:
Number:
Pages: 26
DOI Identifier:
ISBN Identifier:
Keywords:

Abstract:
Although several real-time earthquake detection/recorder systems exist, few address the specific problem of continuous seismic amplitude measurements under conditions where individual events are difficult to recognize, such as those which occur prior to volcanic eruptions. Yet it is during these conditions, when most conventional systems saturate, that seismic information needs to be processed most rapidly. To fill this need, a simple and inexpensive Real-time Seismic Amplitude Measurement System (RSAM) was developed.

Each minute RSAM computes the average amplitude for each of 8 seismic signals for that minute. From this information, seismicitity can continue to be monitored even during periods of intense tremor. Data akin to earthquakes/hour can computed by comparing sucessive two-second amplitudes. If the latest exceeds the earlier by a set ratio (typically 2) it is considered an "RSAM event". Even energy release can be monitored by simply squaring the average amplitude (It is proportional to the electrical energy generated by the geophone. How that relates quantitatively to seismic energy release is still unclear). Though the method almost seems too simple to be effective, RSAM has been a useful tool in predicting the May 1985, May 1986 (figures 1 and 2), and October 1986 dome building eruptions at Mount St. Helens. Both figures show the RSAM data from stations close to the lava dome (Yellow Rock and St. Helens West) beginning to rise above background noise some 48 hours before the time of extrusion of a new lobe. The amplitudes continue increasing and peak near the probable time of extrusion (exact time of extrusion is not known). The spikes and high amplitudes following extrusion are due to surface activity resulting from the emplacement of a new lobe (fig. 1) or the forming of a graben (fig. 2).

Generally, data from RSAM is shown in "RSAM UNITS". "RSAM UNITS" are the direct ouput of the eight-bit analog to digital converter in the system. In a system set up for discriminators with a ±2.5 volt output, one volt peak-topeak discriminator output equals roughly 38 RSAM UNITS. The program in appendix A multiplies the RSAM units by 10 when sending the data to a host computer. Data transfered to a host computer using the program in appendix A should be divided by 10 to get RSAM UNITS, or 380 to get volts peak-to-peak. For more precise measurements, each unit should be individually calibrated. RSAM is not meant to be a replacement for a conventional seismic system. It is to be used as a complement to the conventional system, giving real-time information on tremor/amplitude levels while earthquake locations and magnitudes are being computed by other systems. During times of little or moderate activity, RSAM may be only marginally useful. But during times of tremor or when the earthquake activity is high enough such that the conventional seismic system fails to keep up with activity, RSAM can become the primary monitor of seismicity, simply because the data is continuing to be available in real-time.

Although RSAM can be used as a "stand alone" unit, it is highly recommended that it be configured to periodically transfer its data to at least an IBM XT class computer for data archival and analysis, thus enabling its data to be integrated with the conventional seismic data.

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Suggested Citations:
In Text Citation:
Murray and Endo (1989) or (Murray and Endo, 1989)

References Citation:
Murray, T.L. and E.T. Endo, 1989, A real-time seismic amplitude measurement system (RSAM): Open-File Report 89-684, United States Geological Survey, 26 p..