Seismicity and infrasound associated with explosions at Mount St. Helens, 2004-2005

Six explosions occurred during 2004-5 in association
with renewed eruptive activity at Mount St. Helens, Washington. Of four explosions in October 2004, none had precursory
seismicity and two had explosion-related seismic tremor that
marked the end of the explosion. However, seismicity levels
dropped following each of the October explosions, providing
the primary instrumental means for explosion detection during the initial vent-clearing phase. In contrast, explosions on
January 16 and March 8, 2005, produced noticeable seismicity in the form of explosion-related tremor, infrasonic signals,
and, in the case of the March 8 explosion, an increase in event
size ~2 hours before the explosion. In both 2005 cases seismic
tremor appeared before any infrasonic signals and was best
recorded on stations located within the crater. These explosions demonstrated that reliable explosion detection at volcanoes like Mount St. Helens requires seismic stations within
1-2 km of the vent and stations with multiple acoustic sensors.