Real-time Seismic-Amplitude Measurement (RSAM) was developed by the USGS to summarize seismic activity during heightened volcanic activity. This technique for
characterizing a volcano's changing seismicity in real time (as it is occurring) relies on the average amplitude of the signal from individual seismometers
rather than on the locations and magnitudes of the earthquakes. During heightened volcanic activity, seismicity commonly reaches a level at which individual
seismic events are difficult to distinguish. Analog seismic records (such as the webicorders) provide some information, but do not provide rapid quantitative
analysis. 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, very useful measure of the overall level of seismic activity.
The RSAM does not differentiate between volcanic activity and other sources of ground vibration such as wind, someone walking nearby, or a large regional
earthquake. Thus one must be careful to not mistake wind noise or other non-volcanic signals for volcanic activity. RSAM works best during times of high
activity when the volcanic signal dwarfs most noise.
We also make no instrumental corrections when we calculate RSAM, so it has only relative units. The units should not be compared from station to station. We calculate RSAM values in real-time from our seismic data, and do not archive the RSAM values, as they can always be recalculated.