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.

Research Highlights
► Analysis of fault-plane solutions from the VT earthquake swarm immediately preceding the 2004–2008 eruption of Mount St. Helens (MSH), Washington, United States indicates that magma migration in the shallow subsurface significantly affected the type of faulting and shallow stress field orientation during the early vent-clearing phase.
► 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.
► It is likely that future episodes of shallow dike intrusion preceding eruption at MSH will result in ~ 90° sub-horizontal rotations of the local σ1 axis similar to that observed on September 24, 2004.