MAGNETOTELLURIC STUDY OF MT ST HELENS WASHINGTON, USA: PHASE TENSOR ANALYSIS PRELIMINARY RESULTS

Mount St. Helens is 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.