Collaborative Research: Marine EM Survey of Fluids in the Alaskan Megathrust
DESCRIPTION: The goal of this project is to map water inside deep faults beneath the Alaska Peninsula where the Pacific tectonic plate is being subducted underneath the North American tectonic plate. Water trapped between the plates is thought to play an important role in controlling friction along the plate boundary and may explain large variations in earthquake activity along the Alaska Peninsula. This study will collect seafloor electromagnetic geophysical data to image the distribution of water in the sediments and crust along the plate boundary, with the goal of understanding how the transport and release of water in this critical region may regulate earthquake behavior and plate slip. This project will advance electromagnetic imaging as a new tool for studying subduction zones, and one that will help to image fluid volumes in key areas of the megathrust fault. The project supports the training of undergraduate and graduate students at sea where they will learn how to collect and analyze marine electromagnetic data. This project will use new marine electromagnetic (EM) imaging technology to study the fluid content of the crust and mantle along the Semidi and Shumagin sections of the Aleutian-Alaska subduction zone. These sections of the subduction system have markedly different seafloor fabric entering the subduction zone trench and commensurately different seismicity. Data from a 2011 active seismic survey at this location show variations in sediment thickness and crustal fracturing along the incoming plate but are not able to fully constrain the amount and distribution of fluid. Passive and active EM data will be collected during a month-long cruise at 160 ocean-bottom stations along three trench-crossing profiles and one along-strike profile. Controlled-source EM data will image the electrical conductivity of the sediments and crust, while passive magnetotelluric data will map deep crustal and upper mantle conductivity. Together these data will constrain the hydration state of the incoming plate at the trench outer-rise, the distribution of fluids along the plate-interface beneath the forearc, and the release of fluids into the overlying forearc crust. The new EM data will be interpreted in conjunction with the existing seismic survey results to: (1) test the hypothesis that along strike variations in seismicity and plate locking are related to the fluid content of the incoming oceanic plate, which in turn is related to the pre-existing fabric in the plate and the orientation of this fabric with respect to the trench; (2) image and quantify fluids released from sediment dehydration along the surface of the down-going plate; (3) image and quantify zones of over-pressurized fluids and hydrofacturing of the overriding plate related to fluid release from the slab; (4) compare the conductivity structure at the Aleutian-Alaska subduction zone with a similar profile at the Middle America Trench, an end-member system where seafloor faults are aligned parallel to the trench, potentially maximizing fluid-flux into the system.