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) 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 σ1 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 σ1 orientation was due to intrusion and inflation of a NE–SW-oriented dike in the shallow crust prior to the eruption onset.