iMUSH-aided fault-plane studies at Mount St. Helens, Washington: Evidence for magma recharge

Abstract:

Background seismicity has been relatively low at Mount St. Helens (MSH) following its last eruption in 2004-2008, with an average of 95 located M > 0 events per year. This is in marked contrast to the five years immediately following the 1980-86 eruptive period, when the yearly average rate was about 400 events. During that time there was clear evidence, in the form of rotated fault-plane solutions (FPS), that magma recharge was occurring at depths > 2 km. Despite lower seismicity rates and generally smaller earthquakes, an improved seismic network recorded data sufficient to allow for computation of 88 FPS for the period 2008-2013. These FPS show that stress fields at depths > 2 km were rotated in a manner similar to that seen post-1980-86, providing evidence that magma recharge is again occurring at MSH. A subtle trend towards slightly deeper earthquakes since 2011 is consistent with this hypothesis, as is previously reported outward motion on GPS stations that has been modeled with an inflationary source beneath the volcano at ~8-9 km depth.

In the summer of 2014, 70 broadband seismometers were installed within 50 km of MSH as part of the iMUSH experiment, greatly increasing the number of stations close enough to MSH to obtain good recordings of MSH-generated events. By the Fall AGU meeting we expect to have several months of data collected and processed from iMUSH stations. These data should greatly improve constraints on first-motion FPS and/or the number of events for which well-constrained FPS can be computed. In addition, the density of three-component stations may allow for computation of moment tensor solutions for larger events (M > 1), which typically occur ~20 times per year. This would allow us to assess whether recent MSH events have significant non-double-couple components, something that could indicate fluid involvement and that has previously only been seen during eruptive periods at MSH via short-term deployments of broadband stations.