Seismicity associated with renewed dome building at Mount St. Helens, 2004-2005

Abstract

The reawakening of Mount St. Helens after 17 years and 11 months of slumber was heralded by a swarm of shallow (depth <2 km) volcano-tectonic earthquakes on September 23, 2004. After an initial decline on September 25, seismicity rapidly intensified; by September 29, M d >2 earthquakes were occurring at a rate of ~1 per minute. A gradual transition from volcano-tectonic to hybrid and low-frequency events occurred along with this intensification, a characteristic of many precursory swarms at Mount St. Helens before dome-building eruptions in the 1980s. The first explosion occurred October 1, 2004, 8.5 days after the first earthquakes, and was followed by three other explosions over the next four days. Seismicity declined after each explosion and after two energetic noneruptive tremor episodes on October 2 and 3. Following the last explosion of this series, on October 5, seismicity declined significantly. Over the next ten days seismicity was dominated by several event families; by October 16, spacing between events had become so regular that we dubbed the earthquakes “drumbeats.” Through the end of 2005 seismicity was dominated by these drumbeats, although occasional larger earthquakes ( M d 2.0-3.4) dominated seismic energy release. Over time there were significant variations in drumbeat size, spacing, and spectra that correlated with changes in the style of extrusion at the surface. Changes in drumbeat character did not correspond to variations in magma flux at the conduit, indicating that drumbeat size and spacing may be more a function of the mechanics of extrusion than of the extrusion rate.