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| Titel |
Preliminary Results from the iMUSH Active Source Seismic Experiment |
| VerfasserIn |
Alan Levander, Eric Kiser, Imma Palomeras, Colin Zelt, Brandon Schmandt, Steve Hansen, Steven Harder, Kenneth Creagar, John Vidale, Geoffrey Abers |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107834
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| Publikation (Nr.) |
 EGU/EGU2015-7550.pdf |
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| Zusammenfassung |
| iMUSH (imaging Magma Under Saint Helens) is a US NSF sponsored multi-disciplinary
investigation of Mount Saint Helens (MSH), currently the most active volcano in the
Cascades arc in the northwestern United States. The project consists of active and passive
seismic experiments, extensive magnetotelluric sounding, and geological/geochemical studies
involving scientists at 7 institutions in the U.S. and Europe. The long-term goal of the seismic
project is to combine analysis of the active source data with that of data from the 70 element
broadband seismograph operating from summer 2014 until 2016. Combining seismic and MT
analyses with other data, we hope to image the MSH volcanic plumbing system from the
surface to the subducting Juan de Fuca slab. Here we describe preliminary results
of the iMUSH active source seismic experiment, conducted in July and August
2014.
The active source experiment consisted of twenty-three 454 or 908 kg weight shots
recorded by ~3500 seismographs deployed at ~6,000 locations. Of these instruments, ~900
Nodal Seismic instruments were deployed continuously for two weeks in an areal array
within 10 km of the MSH summit. 2,500 PASSCAL Texan instruments were deployed twice
for five days in 3 areal arrays and 2 dense orthogonal linear arrays that extended from MSH to
distances > 80 km. Overall the data quality from the shots is excellent. The seismograph
arrays also recorded dozens of micro-earthquakes beneath the MSH summit and
along the MSH seismic zone, and numerous other local and regional earthquakes. In
addition, at least one low frequency event beneath MSH was recorded during the
experiment.
At this point we have begun various types of analysis of the data set: We have determined
an average 1D Vp structure from stacking short-term/long-term average ratios, we have
determined the 2-D Vp structure from ray-trace inversions along the two orthogonal
profiles (in the NW-SE and NE-SW directions), and we have made low-fold CMP
stacks of the profile data. The 1-D average model was made from the modulus of
short-term average/long term average ratios of all traces from all shots, with traces
sorted into offset bins and summed. In addition to strong Pg and Sg phases, the
STA/LTA stack clearly shows PmP, PmS, and SmS. The 2-D ray-trace inversions have
identified a -15% dlnVp anomaly at 4-10 km depth bsl beneath the MSH summit. We
identify this anomaly as the feeder magma chamber to the shallowest (1-3 km depth)
magma chamber previously imaged by local earthquake tomography (Waite and
Moran, 2009, Journal of Volcanology and Geothermal Research, 182, 113-209). Pre-
and post-critical PmP phases image the Moho, at depths varying from 35-40 km,
directly beneath the MSH plateau and to the east beneath the Cascadia backarc. |
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