The main goal of this study is to determine whether the presence of fluids within Earth's mantle is a controlling factor determining where earthquakes occur within subduction zones, specifically along the fault that enables the down-going tectonic plate to slip deeper into surrounding viscous mantle. The fact that this seismicity is located within the crust at 'cool' subduction zones, such as Alaska and Tohoku, versus in the mantle at 'warm' subduction zones, such as Cascadia and Nankai, suggests that fluids play an important role. Directional dependence of seismic wave propagation speeds will be assessed, so that possible bias in earthquake locations can be accounted for. Simultaneously, information about deformation within the viscously flowing mantle will be obtained. Ratios of shear and compressional wave velocities will suggest where fluids are present or not. These constraints will guide computer modeling of mantle flow and temperature in the subduction zones. These results will be linked to petrologic models of mineral phase change associated with plate dehydration that introduce fluids near the plate interface and lead to the generation of arc volcanism.
In-depth collaboration by three geoscientists- a geodynamist, petrologist, and seismologist, serves as a model of interdisciplinary Earth System research. Graduate and undergraduate students will receive training in the use of state-of-the-art methods in their sub-discipline as well as learning how to address interplay with evolving results from complementary sub disciplines. The international collaboration with Japanese scientists provides access to the best instrumented and well-studied Japanese subduction system. The study leverages ongoing work in the NSF GeoPRISMS and EarthScope Programs at Cascadia and that planned for Alaska.
