EAR-PF: The spatiotemporal scales of transient slow slip on the San Andreas fault near San Juan Bautista, central California, and the implications for seismic hazard

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Dr. Heather Shaddox has been awarded an NSF EAR Postdoctoral Fellowship to combine seismic and geodetic observations to characterize the nature of fault slip and earthquake hazard for the San Andreas fault near San Juan Bautista in central California. This work will be conducted at University of California Berkeley in collaboration with mentor Roland Burgmann. Slip on faults occurs as a spectrum, ranging from slow, continuous sliding (i.e., aseismic slip) to rapid movement during earthquakes (i.e., seismic failure). Aseismic slip can temporarily relieve stress on a fault but may also trigger large, damaging earthquakes on adjacent segments of the fault. Therefore, detecting and understanding this temporary release of stress (called aseismic slip transients) is key in seismic hazard evaluation and has direct societal impacts. Because aseismic slip transients do not generate seismic waves, geodetic instruments like land Global Positional System (GPS) stations with observational limits are required to detect these movements. This project will use related small earthquakes (microseismicity) to infer the location of aseismic slip transients, taking advantage of existing and newly installed instrumentation to create a catalog of seismicity at unprecedented resolution. The results of this project will provide information about spatial and temporal scales of fault slip and have implications for determining seismic hazard. This detailed analysis may also uncover answers to fundamental fault slip questions including how often aseismic slip precedes earthquakes and how often earthquakes trigger aseismic slip transients. We can also gain insights into other tectonic environments where direct observations are limited (e.g., the offshore portion of subduction zones) by studying this natural laboratory. This project also involves educational materials about earthquakes for elementary school students in the San Francisco Bay Area of California, a region with high seismic hazards. The general objective of this project is to study the spatiotemporal scales of transient aseismic slip on the San Andreas fault near San Juan Bautista in central California at unprecedented resolution to ultimately gain a broader understanding of the nature of fault slip. San Juan Bautista, located at the northwest boundary of the creeping section of the San Andreas fault and the southeast termination point of the Mw 7.9 1906 San Francisco earthquake, forms a creeping-to-locked transition of the fault. The dense instrumentation and historic record of transient aseismic slip near San Juan Bautista make it an excellent natural laboratory to study the interplay of seismic and aseismic slip. We will create a catalog of spontaneous and trigged aseismic slip transients on the San Andreas fault near San Juan Bautista by combining seismic (general seismicity, near-repeating earthquakes) and geodetic (cGPS, InSAR, borehole strainmeter, creepmeter) observations to help answer the following questions: What is the spatial scale, temporal scale, and magnitude of aseismic slip transients? How often and by what mechanism(s) does aseismic slip lead to small and moderate-sized earthquakes? How often do these earthquakes lead to aseismic slip? Based on the scales of transient aseismic slip, is the San Andreas fault near San Juan Bautista capable of large (Mw 6) earthquakes? We will further study the relationship between shallow and deep aseismic slip, spatiotemporal changes in creep rate in this locking transition, and evaluate the utility of near-repeating earthquakes as a proxy for transient aseismic slip. We additionally hope to gain insights into other tectonic environments, particularly the offshore portion of subduction zones where direct observations are limited, by studying this natural laboratory.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.