USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Seismic Identification of Gas-and-Ash Explosions at Mount St. Helens --
Capabilities, Limitations, and Regional Application
-- Chris Jonientz-Trisler, Bobbie Myers, and John A. Power, 1994,
Seismic Identification of Gas-and-Ash Explosions at Mount St. Helens --
Capabilities, Limitations, and Regional Application
IN: Casadevall, Thomas J., (ed.), 1994,
Volcanic Ash and Aviation Safety:
Proceedings of the First International Symposium on Volcanic Ash and Aviation
Safety:
U.S. Geological Survey Bulletin 2047, 450p., p.351-356.
Abstract
Since 1980, analysis of seismic signals recorded by the University of Washington
(UW) and the U.S. Geological Survey (USGS) Mount St. Helens seismic network have
been analyzed and correlated with observations made by field crews, pilots,
radar operators, and the general public. These analyses have led to the
development of criteria that enable identification of most kinds of seismic
events that occur at Mount St. Helens. These events include gas-and-ash
explosions, volcanic earthquakes, volcanic tremor, rockfalls, debris flows,
dome-building eruptions, and many types of man-made seismic noise.
Identification criteria are based on comparison of signal envelope, dominant
frequencies, and relative amplitudes and timing among stations.
In general, gas-and-ash explosions (vigorous venting of steam and (or) other
gases and volcanic ash) at Mount St. Helens are characterized by emergent,
low-frequency signals of extended duration, often with pulsating amplitude
changes. The signal amplitudes from stations in the crater are much higher than
those from stations on the volcano's flanks. The amplitudes from different
flank stations are approximately equal.
Several factors may complicate signal identification. These include:
(1) evolution of the volcanic system with time, (2) overlap of signals that are
recorded close together on seismograms, and (3) the temporary loss of data from
key stations. Nevertheless, we have had considerable success identifying
explosions within minutes of their occurrence. The criteria used to identify
gas-and-ash explosion signals at Mount St. Helens and the techniques used to
develop those criteria provide a foundation for signal identification at other
active volcanoes.
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06/20/02, Lyn Topinka