Since their discovery nearly a decade ago, Episodic Tremor and Slip (ETS) events have been intensively studied along the Japan margin and in the Cascadia subduction zone of the US Pacific Northwest. These deformation events are now understood to occur in a transitional zone between the shallower, colder, locked seismogenic region and the down-dip, hot region where stable sliding of the subducting plate occurs. There is considerable interest in the role that fluids may play in ETS, from simply controlling rheological properties in the subducting and overriding plates to, possibly, explaining ETS dynamics through competing models ranging from permeability pumping/hydrofracturing to a continuum of rate and state frictional slip. Observed ETS temporal patterns can be replicated by numerical models with low confining pressures, consistent with elevated pore pressure along the plate interface. However, a lack of high-resolution geophysical imaging at depth has inhibited our ability to map the baseline distribution of fluids along subduction zones exhibiting ETS. Since magnetotelluric (MT) data are highly sensitive to the presence and connectivity of fluids, they offer the best strategy for mapping the fluid distribution within the subduction zone, and for testing the competing ETS hypotheses (e.g. friction vs. fluids).
The Magnetotelluric Observations of Cascadia using a Huge Array (MOCHA) project entails acquiring high-resolution onshore and offshore MT soundings that can be used to image deep electrical conductivity structure. Images thus derived impart knowledge of the fluid distribution and the segmentation of the convergent margin that is associated with the subducting slab along the coasts of Oregon and Washington. MOCHA consists of 60 marine and 75 land MT stations covering a 400 km section of the forearc along the north to south Siletzia terranes. The MOCHA data will ultimately constrain 3D models of the crust and upper mantle of the subduction system, from the incoming plate to the magmatic arc, revealing the distribution of fluids in the subduction system in unprecedented detail. These models will detail the offshore fluid input to the system and the distribution of fluids released from the down-going slab, including along the transitional zone where ETS occurs.
