Combining Active- and Passive-Source Seismic Observations to Image Magma Storage Beneath Mount St. Helens

Date: 4/26/2019

Time: 02:15 PM

Room: Puget Sound

In the summer of 2014 we initiated passive and active seismic tomography experiments at Mount St. Helens (MSH), Washington, as part of the collaborative imaging Magma Under St. Helens (iMUSH) project. The passive experiment used local earthquake data from 70 broadband seismometers that were deployed in a 100-km aperture array around MSH from 2014 to 2016, and the active experiment used recordings of 23 explosions in the summer of 2014 at ~6000 locations using Texan and Nodal seismometers. The passive and active data sets were inverted separately to obtain 3-D velocity models of the upper crust around MSH, yielding many similar features and interpretations. These include: (1) High P- and S-wave velocities near the surface corresponding to surface-mapped plutons; (2) Low velocities near the surface corresponding to the Chehalis sedimentary basin and the Indian Heaven volcanic field; (3) Low velocities along the St. Helens seismic zone, striking NNW-SSE north of MSH from near the surface to at least 10 km depth; (4) Low P- and S-wave velocities beneath MSH at depths of 6-15 km below sea level, probably indicating a magma storage region. However, there remain some areas which differ between the two models, including the details of the MSH magma storage region. In this work, we present the results of a joint inversion of the active and passive data sets, evaluate the reasons for any differences in the separate inversions, and make further interpretations using results of iMUSH magnetotelluric inversions and petrologic/tectonic models of the area.

Presenting Author: Carl W. Ulberg

Authors