EAR-PF: What is the role of metasomatic alteration in subduction zone episodic tremor and slip?

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2). Dr. William Hoover has been awarded an NSF EAR postdoctoral fellowship to carry out research and educational plans at the University of Washington alongside mentor Dr. Cailey Condit. Dr. Hoover will investigate the role of water-rock reaction in producing the enigmatic seismic phenomena known as episodic tremor and slip (ETS) that can precede large, damaging earthquakes, particularly along subduction zone faults. This study will focus on rocks from Catalina Island (Southern California) exhumed from depths characteristic of ETS in modern subduction zones (e.g., Cascadia in Washington and Oregon). Integrated macro- and micro-scale structural and geochemical analysis will reveal how rock alteration by fluid flow yields deformation behavior characteristic of ETS. This work will provide important direct observations of rocks from ETS source regions and inform modern subduction zone monitoring and seismic research. During field work on Catalina Island, Dr. Hoover will teach a field course to students from Hispanic-serving institution Cal State Channel Islands and mentor capstone research projects. He will also develop and present educational activities as part of the University of Washington Rockin’ Out K-12 outreach program. Lastly, this research will be carried out on land of the Gabrielino-Tongva Tribe and he will return tangible benefits of the research through outreach events held for the tribe. Rheological modeling of ETS suggests that it may be hosted within metasomatic talc and chlorite schists, and Dr. Hoover will test this hypothesis using the exhumed rock record. He will identify geologic equivalents of the structures and fluid-rich conditions inferred from geophysical studies of ETS source regions through field mapping of the Catalina Schist (CA). He will connect outcrop-scale deformation partitioning to metasomatic changes with bulk major and trace element and Sr and Li isotope geochemistry. Using spatially-resolved micro-scale chemical and structural analysis with electron microbeam techniques, he will determine deformation mechanisms and connect them to compositional changes across all scales. These observations will be placed in the temporal context of the seismic cycle through bulk Li and intra-crystalline diffusion chronometry. Using this multi-scale approach Dr. Hoover will document how metasomatism controls deformation partitioning and mechanisms at the conditions of ETS in modern subduction zones. These results will inform future geophysical, theoretical and experimental studies of subduction zone seismicity.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.