Collaborative Research: Cascadia2020: Investigating subduction zone segmentation with a 3D high-resolution Vp model

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The Cascadia subduction zone is known to generate infrequent (every ~500 years) but potentially very large (up to magnitude 9) earthquakes. The most recent such earthquake occurred on January 26th, 1700, and GPS data acquired during the last two decades suggest that strain is accumulating across the subduction zone fault in preparation for a future earthquake. However, there is considerable uncertainty about how the geologic structure of the subduction zone will impact strain release during a Cascadia earthquake. Different plausible earthquake scenarios result in quite different predicted ground shaking, which in turn influences building codes and emergency response plans. This project will image the structure of the crust in the region where large Cascadia earthquakes are thought to occur. The objective is to understand three-dimensional variations in seismic velocity in order to provide insights into the factors that control segmentation of slip during earthquakes and the resulting amplitude of ground shaking in populated areas and sedimentary basins. With its large footprint and involvement of many field hands, this project will provide student research experiences and opportunities for public outreach and education about geohazards in the Pacific Northwest. This project is a collaboration between Oregon State University, University of Oregon, Eugene, South Dakota School of Mines and Technology and the U.S. Geological Survey.This project has the ultimate goal of generating a high-resolution model of an active subduction system that will span the Cascadia plate boundary, including the transition between major along-strike segments, and cross the transition from the nominally locked zone down-dip to depths where plate motion is accommodated by episodic tremor and slip. A dense network of temporary short-period seismometers along the coast of the Pacific Northwest will extend high resolution imaging of the structure of the overlying and down-going plates, as well as the thickness and characteristics of material within the plate interface zone, to depths that are critical for understanding earthquake processes. The array design includes a single deployment of three-component instruments at 1-km intervals along the onshore extension of dip lines embedded within a sparser grid of seismometers spaced 7-10 km apart for a total of ~700 three-component seismic stations. To tie results from the seismogenic part of the plate boundary to larger-scale forearc and arc structure, two profiles will extend across the forearc. All stations will record natural sources as well as man-made sources for the duration of the deployment. The seismic data from this experiment will be made immediately available to the community. The resulting high-resolution 3D Vp model will also be made accessible and is critical for constraining analyses of seismic noise, magnetotelluric and potential field data, as well as for improving predictions of earthquake ground shaking and tsunami generation in the Pacific Northwest.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.