Investigating the role of fluids at three subduction zones along the Ring of Fire with electromagnetic data

It is widely recognized that fluids play an important role in tectonic processes at subduction zones, and they are often invoked as the dominant factor responsible for differing styles of earthquake slip, such as slow slip events and tsunami earthquakes. Yet there are other important factors that influence the range of observed earthquake behavior, including the composition, geometry, and thermal structure of the subduction zone. Distinguishing which mechanism, or combination thereof, that ultimately governs earthquakes and related transient phenomena requires constraints from independent methods. Here we describe the controlled-source electromagnetic (CSEM) method and its application to mapping the distribution of fluids in the incoming plate and forearc prism. Electromagnetic (EM) data are used to image electrical resistivity, which is highly sensitive to the presence of fluids and hence provides a proxy for porosity. We performed the first CSEM survey of a subduction zone in 2010, at the Middle America Trench offshore Nicaragua, where we deployed 50 seafloor EM receivers across a 280 km profile. This was followed by two new subduction zone surveys in the past year (2019) offshore the Alaska Peninsula and Hikurangi margin. Each survey involved over 160 receiver deployments across multiple profiles. We present 2-D inversions of bulk electrical resistivity from select profiles spanning all three survey regions, which show the capacity of CSEM data to constrain the fluid budget of the shallow megathrust and the porosity structure of the overriding plate. Results from Nicaragua exemplify the value added from integrating collocated observations to investigate the interplay between fluids and tectonics.