In this project, an integrated field experiment is undertaken in the Cascadia subduction zone to elucidate the relationship between water transport, aseismic slip, episodic tremor, and arc magmatism. The ultimate goal is to explore H2O processes in subduction zones using the tools of seismology, geodesy and petrology, and to integrate these results with complementary constraints from geodynamics and geochemistry. Seismic imaging is employed to illuminate (i) the descending oceanic plate where it metamorphoses, and (ii) the mantle wedge where fluids may be producing hydrous phases such as serpentine or, beneath the volcanic arc, primary magmas. The experiment is designed to traverse the one part of the Cascadia system where earthquakes extend to nearly 100 km depth, thus permitting an investigation of the relationship between the release of fluids and the generation of Wadati-Benioff-zone earthquakes. The transport of fluids may be also a primary driver for episodic tremor and slip (ETS), a phenomenon observed in Cascadia perhaps better than anywhere else on the planet, including source regions within this experiment. Measurements of tremor from known source regions are integrated with slip distributions derived from GPS data and existing long-baseline tiltmeters. Together with the seismic imaging, these observations yield an unparalleled data set for determining the relationship between tremor, slip and the regions where imaging indicates metamorphism of the down-going plate or hydration of the overlying mantle wedge.
The basic experiment has four components: a broadband imaging array of flexible-array instruments integrated with Bigfoot, three small-aperture seismic arrays near sources of non-volcanic tremor, analysis of the PBO and PANGA GPS data sets to define the details of episodic slip events, and integrative modeling. The broadband array features a dense transect across the part of the Cascadia subduction system that includes intermediate-depth earthquakes and the Nisqually earthquake hypocenter, in a staggered configuration to allow along-strike effects to be tested. This is complemented by 2 cross lines, one crossing the slab where the crust appears to be dehydrating, and one in the Cascades foothills to sample the roots of the arc. The tremor and GPS arrays are collocated with the broadband imaging as much as possible, to allow simultaneous location of tremor and slip and imaging of their source region. These data are subject to the gamut of analyses appropriate to such data, including array analysis for wave-front orientation of tremor waves, migration of teleseismic scattered waves, tomographic images of Vp, Vs and Q, shear-wave splitting, earthquake relocation, investigation of high-frequency phases interacting with the slab, and specialized GPS processing designed for the detection and quantification of transient events. The results are interpreted in conjunction with detailed petrological-thermal models of the Cascadia subduction system. These results are used to place new constraints on the dehydration pathways within the down-going plate, the relationship between structure and seismicity at intermediate depths, the relationship between transient strain events and structure, the temperature, melt and volatile content of the mantle wedge, and the growth of continental crust.
