Local earthquake Vp and Vs tomography in the Mount St. Helens region with the iMUSH broadband array

Geochemistry, Geophysics, Geosystems
By: , and 



We present new 3-D P wave and S wave velocity models of the upper 20 km of the Mount St. Helens (MSH) region. These were obtained using local-source arrival time tomography from earthquakes and explosions recorded at 70 broadband stations deployed as part of the imaging Magma Under St. Helens (iMUSH) project and augmented by several data sets. Principal features of our models include (1) low P wave and S wave velocities along the St. Helens seismic zone to depths of at least 20 km corresponding to high conductivity imaged by iMUSH magnetotelluric studies. This delineates a zone of weakness that magma can exploit at the location of MSH; (2) a 5- to 7-km diameter, 6–15 km deep, 3–6% negative P wave and S wave velocity anomaly beneath MSH, consistent with previous estimates of the source region for recent eruptions. We interpret this as a magma storage region containing up to 15–20 km3 of partial melt, which is about 5 times more than the largest documented eruption at MSH; (3) a broad region of low P wave velocity below 10-km depth extending between Mount Adams and Mount Rainier along and to the east of the main Cascade arc, which is likely due to high-temperature arc crust and possible presence of fluids or melt; (4) several anomalies associated with surface-mapped features, including high-velocity igneous units such as the Spud Mountain and Spirit Lake plutons and low velocities in the Chehalis sedimentary basin and the Indian Heaven volcanic field. Our results place further constraints on the geometry of these features at depth.

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Publication type Article
Publication Subtype Journal Article
Title Local earthquake Vp and Vs tomography in the Mount St. Helens region with the iMUSH broadband array
Series title Geochemistry, Geophysics, Geosystems
DOI 10.1029/2019GC008888
Volume 21
Issue 3
Year Published 2020
Language English
Publisher American Geophysical Union
Contributing office(s) Volcano Science Center
Description e2019GC008888, 19 p.
Country United States
State Washington
Other Geospatial Mount St. Helens
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