Collaborative Research: Capturing 4D Variations in Stress, Slip, and Fault-Zone Material Properties: The 2019-2021 Gofar Transform Fault Earthquake Prediction Experiment

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This project will determine the fault properties that control the magnitude and timing of earthquakes on Gofar Transform Fault in the equatorial Pacific Ocean. With an advanced array of ocean bottom seismometers (OBSs), rock collection, and fault imaging, this research will produce a multifaceted understanding of two magnitude 6 earthquake zones and the regions that separate them. To ensure a continuous earthquake dataset over the period that next large earthquakes are expected, this project involves three research cruises: 1) to deploy the OBSs and collect rock samples; 2) to recover, refurbish, and redeploy the OBSs and to survey the fault-zone with an autonomous underwater vehicle (AUV); and 3) to recover the OBSs. The two-year dataset is expected to record hundreds of thousands of microearthquakes in addition to hopefully capturing the next two magnitude 6 mainshocks. This research will reach 6-12th grade students, by taking two teachers from low-income school districts in New England to sea and working with these teachers to develop curricula that can be used by teachers throughout the US. The teachers will work with project scientists and the University of New Hampshire's Center for Mathematics, Science, and Engineering Education to develop a "Curriculum Kit" - a web-based set of resources that will include classroom-based earthquake investigations, background information, and periodic classroom video chats with the Gofar experiment scientists. This project will also enhance earthquake research at the university level both nationally and internationally through the support of graduate students and postdocs. Gofar fault was chosen for this project because previous work there allows for the planning of a precise experiment aimed at imaging transitions in fault behavior over just a few kilometers. This experiment will capture the temporal evolution of the fault in unprecedented detail and link these variations to the underlying geology and fault mechanics. Specifically, the aim of this project is to understand why oceanic transform faults are dominated by aseismic slip, have such repeatable seismic cycles, and nucleate hundreds of thousands of small earthquakes in the rupture barriers that stop large earthquakes. In particular, previous work at Gofar indicates that rupture barrier behavior cannot be explained by basic bimodal frictional properties and requires more sophisticated rheological descriptions of the fault zone. This project approaches these questions by recording the time dependence of earthquake stress drops with a strong-motion array and by estimating seismic velocities within the rupture barrier using 4 mini arrays of short-period OBSs that will allow us to use a seismic technique known as double beam forming to determine the space-time evolution of shear velocity. The project will determine if the pre-seismic changes in S-velocity are contained to a narrow fault-zone or spread throughout the wider damage zone and to what extent they extend to seismogenic depths. This research will compare the migration of the velocity anomalies, or lack thereof, with fault mechanics models to try to constrain the underlying rheology of the rupture barrier and investigate the role of dilatancy in stopping large ruptures. The project will also conduct the most comprehensive, high-resolution mapping of a RTF to date through a combination of AUV based bathymetry, backscatter, photomosaic imaging, and water column surveys along with rock dredging. This suite of studies will help clarify the roles of dilatancy and hydrothermal alteration in producing the contrasting seismic behavior between the rupture zones and rupture barrier regions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.