An infrasound array study of Mount St. Helens

The ongoing activity of Mount St. Helens provides an opportunity to study the infrasonic wavefield produced by an active, silica-rich volcano. In late October 2004, as a pilot experiment for the Acoustic Surveillance for Hazardous Eruptions (ASHE) project, we deployed two infrasound arrays, each co-located with a broadband seismometer and weather station, to continuously record seismo-acoustic signals from Mount St. Helens. The nearest array, Coldwater, was deployed on the northern flank of the volcano, ∼ 13 km from the summit. The second array, Sacajawea, was deployed ∼ 250 km east of the volcano, at a distance where stratospherically ducted acoustic waves may be expected during the winter. This paper presents an overview of the experimental setup, and preliminary results from this unique data set. Eruptions on January 16th 2005 and March 9th 2005 produced strong infrasonic signals. The aseismic January 16th eruption signal lasted ∼ 9.4 min beginning at ∼ 11:20:44 01/16/05 UTC, while the March 9th eruption signal lasted ∼ 52.8 min beginning at ∼ 01:26:17 03/09/05 UTC, with the main steam and ash venting stage probably lasting ∼ 7.2 min. The March 9th signal was an order of magnitude larger than the January 16th signal, and was clearly recorded 250 km east at the Sacajawea array. Infrasonic expressions of long period (LP) seismic events (‘drumbeats’) have also been intermittently observed, and are recorded as acoustic waves mimicking the waveform and temporal sequence of their seismic counterparts. These acoustic observations provide important constraints for source models of long period events and eruptions.