Tomographic Imaging of the Magmatic System at Mount St. Helens with the iMUSH Broadband Array

Thursday, 17 December 2015
Poster Hall (Moscone South)
Carl W Ulberg1, Kenneth C Creager1, Alan Levander2, Eric Kiser2, Seth C Moran3, Geoffrey A Abers4, Brandon Schmandt5, John Emilio Vidale1, Heidi Houston1, Roger P Denlinger3 and Mitchell Christian Blair Williams6, (1)University of Washington, Department of Earth and Space Sciences, Seattle, WA, United States, (2)Rice University, Earth Science Department, Houston, TX, United States, (3)USGS, Cascades Volcano Observatory, Vancouver, WA, United States, (4)Cornell University, Ithaca, NY, United States, (5)University of New Mexico Main Campus, Albuquerque, NM, United States, (6)University of California Santa Cruz, Santa Cruz, CA, United States
We deployed 70 broadband seismometers in the summer of 2014 to image the velocity structure beneath Mount St. Helens (MSH), Washington, USA as part of a collaborative project called imaging Magma Under St. Helens (iMUSH). Our goal is to illuminate the MSH magmatic system, using active- and passive-source seismology, magnetotellurics and petrology. Details of the velocity structure, coupled with other geophysical and geologic data, can help constrain the geometry and physical state of any bodies of melt beneath the volcano. The broadband array has a diameter of ~100 km centered on MSH with an average station spacing of 10 km, and will remain deployed through summer 2016. It is augmented by dozens of permanent stations in the area. We determine P-wave arrival times using Antelope software and incorporate permanent network picks for the region. We use the program struct3DP to invert travel times to obtain a 3-D seismic velocity model and relocate hypocenters, computing travel times using a 3-D eikonal-equation solver. There were more than 500 useable local events during the first year of iMUSH broadband recording, which to date have provided 5000 arrival times, with the number growing rapidly. The local events include 23 active shots that were set off in the summer of 2014 as part of the iMUSH experiment, which recorded with good signal-to-noise ratios across the entire array. The absolute P times will be augmented by differential times calculated by cross-correlation between observations at the same station for nearby event pairs. These will be incorporated into our model using double-difference tomography. We anticipate that our 3D velocity model will provide the highest resolution image of volcanic plumbing at MSH thus far. Our model interpretation will incorporate results from active-source and ambient noise tomography, receiver functions, magnetotellurics, and petrology.