Postdoctoral Fellowship: EAR-PF: Slow earthquakes in continental transform faults - Rheology and architecture of the brittle-ductile transition zone

Dr. Drew A. Levy has been awarded an EAR Postdoctoral Fellowship to investigate slow earthquakes along continental fault systems and conduct professional development under the mentorship of Dr. Cailey Condit at the University of Washington. The research will characterize the type of earthquakes at continental plate boundary fault zones. Earthquakes along these fault zones occur in the top 15 km of the crust known as the seismogenic zone. Brittle fault rocks fracture and slip when stress is released as seismic waves. Deeper in the crust, rocks deform ductilely, and stress is released without creating seismic waves. This is called aseismic fault slip. Ductile fault zones move independently of the brittle part during rapid increases in slip velocity known as slow slip events. Slow slip events dissipate stress in the deep seismogenic zone. This may increase the stress on brittle faults above leading to an earthquake. Most observations of slow slip come from subduction plate boundaries. Similar phenomena have recently been reported in continental transform fault settings. Along part of the San Andreas fault in California, co-located seismic and slow slip events occur in the deep seismogenic zone. It is challenging to observe slow slip events with seismological instruments and GPS stations. We have a rapidly growing understanding of the physics that control aseismic fault slip during transient events. Discovering the rock types that experience seismic and slow slip events is important for understanding the seismic hazard of active faults. This research aims to characterize the architecture and rheology of continental transform faults at the deep seismogenic zone, or the brittle-ductile transition, where major earthquakes nucleate through feedbacks between seismic and aseismic processes. Recent discovery of slow slip events (SSE) at major continental transform faults indicates our current models of the earthquake cycle do not account for slip and energy released through transient aseismic events. Using field and analytical study of an ancient transform fault system analogous to the SSE environment of the modern San Andreas fault, this project examines the significance of fault zone geometry and structure, lithologic and fluid-related heterogeneities, and temperature in controlling slip mode throughout the brittle-ductile transition. The methodology of this project is to test whether geological mechanisms of SSEs proposed for subduction zones are active in continental settings, and if the observed structures and deformation mechanisms can be related to predicted geological expressions of geophysically observed phenomena. The Norumbega fault system in the northern Appalachians of Maine, USA offers a natural laboratory for exploring deep to shallow levels of a fossil brittle-ductile transition zone. This research applies a field-based investigation of the geometry and structure of this ancient fault system, a high-resolution microanalytical examination of the microstructures and deformation mechanisms that accommodated variable slip modes, and an analysis of how the geological phenomena this study characterizes relates to geophysical observations of variable slip modes on active transform faults. This research will be translated into educational curriculum communicated through a video series on YouTube, which will feature the Dr. Levy and students working in the field and laboratory. This video series will focus on bridging the gap between STEM research and public understanding of geoscience. The student mentees and Dr. Levy will work in collaboration with the Rockin’ Out, a K-12 outreach program in the Earth and Space Sciences department of UW, to develop modules on faulting and earthquakes related to this research for K-12 programs and help them be implemented in the Puget Sound area. 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.