Autocorrelation Reflectivity Imaging of the Magmatic Plumbing System Under Mount St. Helens

Date: 4/25/2019

Time: 02:00 PM

Room: Cascade II

We present autocorrelation reflectivity imaging (Claerbout, 1968) of Mount St Helens (MSH) using ~5900 short period and 75 broadband seismographs from the iMUSH project (imaging Magma Under Saint Helens). Short period data include ~40-72 hours of noise from Texan and 2 weeks of noise from Nodal seismographs. These autocorrelations provide only P-wave reflectivity due to use of vertical geophones. The 3-component broadband array recorded noise from 2014-2016, and provides P and S reflectivity. Different normalization and filtering schemes were tested including windowing, temporal balancing (Bensen et al, 2007), spectral balancing, and/or sign bit normalization (Oren and Nowack, 2018) followed by band pass filtering. Changes in normalization and filtering resulted in minor image differences. Time-to-depth conversion used 2D and 3D tomography velocity models derived from the active source (Kiser et al, 2016, 2018) and local earthquake travel times (Ulberg et al, 2017).

The images show shallow layering in the Chehalis, Portland and other sedimentary basins, and volcanic stratigraphy around MSH and the Indian Heaven volcanic field. They also show reflection events at shallow to mid-crustal depths that we interpret as the suture between Siletzia and the modern Cascades arc.

In the upper crust under MSH, the reflectivity images are well correlated with complex sill-like structures in the magma storage zone at ~7-12 km depth estimated to have low volume partial melt from P-wave tomography. At the base of the crust beneath MSH, the images are consistent with the high-velocity (Vp > 7.5 km/s) potential restite body identified by 2D tomography. To the southeast, sill-like structures are found across the Moho in a ~10-15 km thick, 20-30 km wide low velocity zone. We identify this feature as a possible MASH (magma assimilation, storage, and homogenization) zone. The autocorrelation reflectivity observed here is consistent with PmP reflectivity (Hansen et al, 2016) and sub-Moho reflections observed in active source data.

Presenting Author: Alan Levander

Authors