Temporal changes in stress preceding the 2004-2008 eruption of Mount St. Helens, Washington

The 2004-2008 eruption of Mount St. Helens (MSH), Washington, was preceded by a swarm of shallow volcano-tectonic earthquakes (VTs) that began on September 23, 2004. We calculated locations and fault-plane solutions (FPS) for shallow VTs recorded during a background period (January 1999 to July 2004...

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Bibliographic Details
Published in:Journal of volcanology and geothermal research 2010-12, Vol.198 (1-2), p.129-142
Main Authors: LEHTO, Heather L, ROMAN, Diana C, MORAN, Seth C
Format: Article
Language:English
Subjects:
Crystalline rocks
Earth sciences
Earth, ocean, space
Exact sciences and technology
Igneous and metamorphic rocks petrology, volcanic processes, magmas
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Summary:The 2004-2008 eruption of Mount St. Helens (MSH), Washington, was preceded by a swarm of shallow volcano-tectonic earthquakes (VTs) that began on September 23, 2004. We calculated locations and fault-plane solutions (FPS) for shallow VTs recorded during a background period (January 1999 to July 2004) and during the early vent-clearing phase (September 23 to 29, 2004) of the 2004-2008 eruption. FPS show normal and strike-slip faulting during the background period and on September 23; strike-slip and reverse faulting on September 24; and a mixture of strike-slip, reverse, and normal faulting on September 25-29. The orientation of [sigma] sub(1) beneath MSH, as estimated from stress tensor inversions, was found to be sub-horizontal for all periods and oriented NE-SW during the background period, NW-SE on September 24, and NE-SW on September 25-29. We suggest that the ephemeral ~ 90 change in [sigma] sub(1) orientation was due to intrusion and inflation of a NE-SW-oriented dike in the shallow crust prior to the eruption onset.
ISSN:0377-0273
1872-6097
DOI:10.1016/j.jvolgeores.2010.08.015