Subevents of long-period seismicity during the 2004-2008 eruption of Mount St. Helens
Abstract
One of the most striking aspects of seismicity during the 2004-2008 eruption of Mount St. Helens (MSH) was the precise regularity or ‘periodicity’ in occurrence of repetitive long-period (LP) or ‘drumbeat’ events over sustained time periods. However, this periodicity was not always observed, and at times the temporal occurrence of LP events became more random. In addition, accompanying the dominant LP class of events during the 2004-2008 MSH eruption, there was a near-continuous, randomly occurring series of smaller seismic events. These subevents were usually only observable at stations deployed in or close to the MSH crater, i.e., in the immediate vicinity of the LP seismic source region. The subevents are not always simply small-amplitude versions of the dominant LP class of events, but appear instead to result from a separate random process only loosely-coupled to the main LP source mechanism. We present an analysis of the inter-event time and amplitude distributions of the subevents, using waveform cross-correlation to separate LP events from the subevents. We also discuss seismic tremor that accompanied the 8 March 2005 phreatic explosion event at MSH. This tremor consists of a rapid succession of LPs and subevents triggered during the explosion, in addition to broadband noise from the sustained degassing. Immediately afterwards, seismicity returned to the pre-explosion occurrence pattern. This triggering in relation to the rapid ejection of steam from the system, and subsequent return to pre-explosion seismicity, suggest that both seismic event types originated in a region of the subsurface hydrothermal system that was 1) in contact with the reservoir feeding the 8 March 2005 phreatic explosion, but 2) not destroyed or drained by the explosion event.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V23D2114M
- Keywords:
-
- 7280 SEISMOLOGY / Volcano seismology;
- 8419 VOLCANOLOGY / Volcano monitoring;
- 8424 VOLCANOLOGY / Hydrothermal systems