Field Report on the iMUSH Active Source Seismic Experiment

Eric Kiser, Rice University, Earth Science Department, Houston, TX, United States, Alan Levander, Rice University, Earth, Environmental and Planetary Sciences, Houston, United States, Brandon Schmandt, Rice University, Earth, Environmental, and Planetary Sciences, Houston, TX, United States, Imma Palomeras, Rice University, Earth Science Department, Houston, United States, Steven H Harder, University of Texas at El Paso, El Paso, TX, United States, Kenneth C Creager, Univ Washington, Earth and Space Sciences, Seattle, United States, Prof. John Emilio Vidale, University of Washington, Seattle, United States and Stephen D Malone, University of Washington, Department of Earth and Space Sciences, Seattle, United States
Abstract:
In the second half of July we completed the iMUSH active source seismic experiment, one component of the Imaging Magma Under Saint Helens project. A team of ~75 volunteers deployed 3500 seismographs to ~5920 locations on and around Mount St. Helens over the course of 3 weeks. This instrument deployment was accompanied by 23 shots distributed around the volcano. Instrumentation consisted of ~2550 Reftek 125A (Texan) seismographs with 4.5 Hz geophones, and 920 Nodal Seismic recorders with 10 Hz geophones. The shots were also recorded by the permanent stations of the Pacific Northwest Seismograph Network and 70 iMUSH broadband seismographs. Fifteen of the shots, 424 kg each, formed two rings around Mount Saint Helens at 15 km and 30 km radius from the summit. Eight of the shots, 828 kg each, were fired at distances of 50 to 80 km from MSH on NW-SE and NE-SW azimuths.

The deployment geometry consisted of two lines oriented NW/SE and NE/SW, and three arrays. The offset of the lines ranged from 150 km to 190 km with an average spacing of 200 m. The first array was centered on the volcano with a radius of 30 km, and required both driving and hiking to deploy. Arrays two and three were set out with, and centered on, the NW/SE line. These arrays had a distance range from MSH of 30-75 km and an azimuth range of about 100 degrees. In addition to this large-scale deployment, we set out 7 beamforming arrays approximately collocated with iMUSH broadband seismographs, and above clusters of seismicity in the region. The aperture of these arrays was about 1 km with an instrument spacing of 100 m.

The final deployment ended only days before the AGU abstract deadline, so we have not yet examined all of the data. However, the preliminary indications are that signal to noise is excellent: The shots, several of which registered on PNSN as ML>2.1, carried across the entire array, and were recorded as far away as Seattle and Corvallis on permanent stations. The array also recorded a number of local and regional earthquakes, and micro-seismicity at 3-4 km depth beneath the MSH summit.

Figure Caption: Blue dots are ~2500 Texan locations for the first deployment, which recorded 15 shots (blue stars). Red are ~2570 Texan locations for the second deployment, which recorded 8 shots (red stars). Purple are ~920 Nodal Seismic seismographs which recorded all the shots.

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