Imaging Magma Under St. Helens (iMUSH): Details of passive-source seismic deployment and preliminary 3-D velocity structure

Monday, 15 December 2014
Carl W Ulberg1, Kenneth C Creager1, Seth C Moran2, Geoffrey A Abers3, Roger P Denlinger4, Alicia J Hotovec-Ellis1, John Emilio Vidale1, Eric Kiser5, Alan Levander5 and Adam Schultz6, (1)University of Washington, Seattle, WA, United States, (2)USGS, Vancouver, WA, United States, (3)Cornell University, Ithaca, NY, United States, (4)USGS, Cascades Volcano Observatory, Vancouver, WA, United States, (5)Rice University, Earth Science Department, Houston, TX, United States, (6)Oregon State University, Corvallis, OR, United States
The imaging Magma Under St. Helens (iMUSH) experiment aims to delineate the extent of the magmatic system beneath Mount St. Helens (MSH) in Washington State. The experiment involves active- and passive-source seismology, magnetotellurics, and geochemistry/petrology. Seventy passive-source broadband seismometers were deployed in a 100-km-diameter array centered on MSH, with an average spacing of 10 km, and a planned duration of two years. The deployment over two weeks in June 2014 involved a group of 18 people split into 6 teams. Approximately half of the seismic stations have aircell batteries and/or pole-mounted solar panels in order to maintain power through deep snow at higher elevations during the winter months. Data will be retrieved 2-4 times a year throughout the duration of the experiment. The first service run performed in mid-July 2014 had a 98.4% data recovery. This is one of the largest wide-aperture two-dimensional arrays covering a volcano anywhere.

The active-source portion of the experiment successfully set off 23 shots in late-July 2014. These were recorded clearly at permanent stations run by the Pacific Northwest Seismic Network up to 200 km away, and are expected to be well-recorded on many of the 70 broadband seismometers in addition to the 2500 Reftek “Texans” deployed temporarily for this purpose. For the 2-4 weeks of broadband data collected in July, local earthquakes down to magnitude 0 are recorded across the array, with clear P- and S- arrivals. Earthquakes of this size occur daily within 50 km of MSH. We are keeping a careful catalog of all activity in the region for the duration of the iMUSH experiment.

We will pick P- and S-wave travel times at the 70 broadband stations from local earthquakes and active shots, for available data from between June and October 2014. We will also use a tomographic code (Preston et al, 2003, Science) to invert the travel times to obtain preliminary earthquake location and 3-D velocity structure.