Collaborative Research: An Earthscope Magnetotelluric Survey of the Southern Cascadia Subduction System, Washington

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The Pacific Northwest, and Washington State in particular, are characterized by a subduction zone environment called the Cascadia subduction system. The Cascadia subduction system is an area of the Earth where the seafloor is colliding with, and is being pushed below the overriding crust of the North American continent. The recycling of the seafloor beneath the continent is a complex process: the collision and descent of the seafloor can cause major earthquakes, slow slip and tremor, and volcanic eruptions such as that seen at Mount St Helens and Mount Redoubt. In order to understand these potentially devastating processes, the internal structure of the Earth needs to be imaged at the point where earthquakes, fluid release and melting are triggered, to depths of up to 200 kilometers beneath the surface. Seismic techniques for looking into the Earth's interior are familiar to most people, and they have been used in the Pacific Northwest as part of the USA wide Earthscope program. However, other methods of imaging the Earth, such as the magnetotelluric (MT) method, can provide information regarding the thermal structure of the Earth and can pinpoint areas containing deep released fluids and molten rock. MT uses naturally occurring electric currents generated by lightning activity and by solar wind-ionospheric interactions, to estimate Earth's electrical conductivity. Conductivity in turn depends partly on composition and temperature, but it can be dramatically increased by small amounts of fluid or melt provided that they form an interconnected network. The EarthScope program has also established an array of MT stations over the U.S. Cascadia subduction system, but the existing coverage lacks the spatial resolution to address some of the key issues related to the dynamics of the subduction zone. To rectify this situation, scientists from the University of Utah and Woods Hole Oceanographic Institute are collecting a dense profile of MT stations across west-central Washington State coincident with a previously recorded seismic profile, to rectify this situation. The co-located seismic data will allow the development of techniques for jointly interpreting the two complementary techniques and will provide insights into the understanding of the evolution of the Cascadia subduction system. Societal benefits associated with this research include applications to understanding of processes that contribute to seismic and volcanic hazards associated with the Cascadia subduction system, contributions to research infrastructure, and training of graduate students.