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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS009 Poster presentation 2054
Magnetotelluric study of mount ST. Helens Washington, USA: phase tensor
analysis preliminary results
Prof. Ray Cas
School of Geosciences Monash University IAVCEI
G. J. Hill, J. P. Cull, T. G. Caldwell, H. M. Bibby, W. Heise, M.K. Burgess, L.G. Mastin
Our attempts to understand the processes that drive volcanism can be greatly enhanced by imaging the
location and geometry of magma storage systems and by imaging the internal structure of volcanic
complexes. The majority of efforts have been seismically based, using technologies adapted from
petroleum and mineral exploration. Recently, however, there have been efforts to incorporate additional
survey and data types, GPS and gravity successfully at Okmok, thermal and acoustic studies of
Stromboli, self potential at Misti, and recently magneto-tellurics at Ruapehu. These multidisciplinary
studies unequivocally add to the understanding of these volcanic systems, either by providing
supporting evidence or suggesting alternative viable interpretations. We have undertaken a magnetotelluric
study of Mount St. Helens a quaternary stratovolcano located in south-western Washington,
USA, lying along the western front of the Cascade Range between Mt. Hood to the south and Mt.
Rainier to the north, Mt. St. Helens is located in a region of transition both geologically and
geophysically. Phase tensor and induction arrow analysis from 37 broadband magnetotelluric (~0.01
2000 s) sounding sites show the regional conductivity structure in a ~1000 km2 area around the
volcano is 3-D at all period scales. Phase tensor analysis indicates that Mount St. Helens lies on the
boundary of a large regional conductor to the north east of the volcano which begins at a depth of ~25
km and extends into the lower crust. The phase tensor analysis indicates a shallow response (
Keywords: magnetotellurics, mtsthelens