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.
This grant funded collection and analysis of geophysical Magnetotelluric (MT) data across a portion of the Cascadia subduction system, just to the south of Seattle, Washington. The profile crossed the Cascadia volcanic arc just to the north of Mount Rainier. The project formed the core of a PhD dissertation for a student in the WHOI/MIT Joint Program. Offshore Washington State, the seafloor is in collision with the continent and, as a result, is pushed beneath the continent and down into the mantle. This process of subduction results in the generation of large earthquakes, some of them capable of generating tsunamis, and is also responsible for the volcanism along the Cascadia arc at volcanoes such as Mount St. Helens. The volcanic process begins deep beneath the Earth’s surface when fluids carried down by the seafloor plate are released causing the mantle to melt. Magnetotellurics (MT) is a geophysical technique used to image deep into the Earth. The method measures naturally occurring electric and magnetic fields at the Earth’s surface, generated by the interaction of the Earth’s magnetic field and the charged particles of the solar wind. These charged particles create electric currents in the ionosphere which, in turn, induce current flow within the Earth. The pattern of current flow beneath Earth’s surface is dependent on the electrical conductivity structure, a physical property sensitive to temperature, composition and particularly the presence of fluids, in essence properties that are important for understanding key fluid release and melting processes beneath the Cascadia system. MT data were collected in an East-West profile through Washington state, along a line roughly coincident with the previous Café seismic experiment (Figure 1). The PhD student working on the MT data also analysed a subset of the seismic data and used constraints from the seismic image to inform the MT data modeling. The MT data have provided a cross sectional image of the downgoing slab and overlying mantle (Figure 2) and shows where melt is generated in the mantle (region A in Figure 2) and how it travels from its source region to the base of the continental crust (regions B and C in Figure 2). This is one of the first such images to be collected. It contrasts with another image generated during the project through reanalysis of existing data further south in Oregon (Figure 3) which shows much weaker evidence for substantial melt generation at depth. We already know that there are significant differences in earthquake episodicity along the Cascadia system and we are now beginning to link that variability to differences in fluid release from the downgoing slab.
