Collaborative Research: Evaluating fault creep in California using geodetic and seismic observations

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There is a 99.7% chance a magnitude 6.7 earthquake or larger will strike California within the next 30 years. Earthquakes are a mode of fault slip that cause seismic waves to be radiated into the Earth. Within earthquake cycle, however, faults may also undergo aseismic slip (or creep), which radiates no seismic wave. The occurrence of seismic versus aseismic slip depends on the initial frictional properties of the fault zone and their variation as a function of fault slip rate. Understanding, why, when, where and how creep rate varies on a fault is essential for quantifying earthquake potential on California's fault systems. The knowledge of spatial distribution of fault creep allows estimating location and size of future earthquakes, while the temporal variation of creep rate can be used to determine frictional properties of the fault zone. In this project we integrate measurement of ground surface deformation obtained from space-borne Interferometric Synthetic Aperture Radar and Global Positioning System with seismic observations through numerical and analytical models to constrain spatially and temporally variable creep rates along the Central and Southern San Andreas Faults. We will analyze large data sets of Synthetic Aperture Radar images acquired by several radar satellites spanning period 1992 - 2020, to generate maps of surface deformation time series at unprecedented resolution and accuracy. The results from this project will be used to investigate active crustal deformation and provide new insight into the its underlying mechanisms and dynamics, and allows better recognition and assessment of earthquake hazard and its associated risk in California. In particular, we will work to answer important questions: How much elastic vs. permanent strain occurs adjacent to the San Andreas Fault? Does this proportion change along the length of the fault? How do fault slip rates change and evolve over time? How do short-term geodetic measurements match with long-term geological measurements? How do earthquakes initiate? How do fault geometry, rheology, and history combine to determine the propagation, size, and location of earthquakes? What is the friction on a fault at the depths and conditions at which big earthquakes rupture? What role do fluids play in the generation of silent slip events? This project will also bring together young early career scientists, including one female, from American and British universities and will provide them with partial support. It will also provide valuable research experience for a graduate student. The results from this project will be incorporated in undergraduate teaching, including Physical Geology as well as graduate courses Crustal Deformation and Radar Remote Sensing, which both include numerous case examples from the San Andreas Fault.An improved knowledge of the spatially and temporally variable surface deformation field and the link to seismic and aseismic slips on faults are critically important for understanding active tectonics, mechanics of faulting and triggering large earthquakes. Unique to the San Andreas Fault is the combination of rich historic data sets, the recent deployment of EarthScope instrumentation, fault complexities and variety of natural transient phenomena, making it a natural laboratory for studying faulting processes. This 3-year research project is a collaboration between 3 early career scientists from 3 universities to advance understanding of aseismic faulting processes and underlying mechanisms in California. The study is inspired by seismic and geodetic observations of interseismic creep rate variations along the Central and Southern San Andreas Fault. Through this study, the full capacity of vast seismic, geodetic and geologic data sets provided through EarthScope will be explored. An advanced multitemporal interferometric synthetic aperture radar (InSAR) algorithm will be applied to large data sets of SAR images acquired by several radar satellites (e.g., ERS1,2, Envisat, ALOS, TerraSAR-X, CosmoSkyMed and Sentinel-A,B) spanning the period 1992 - 2020. In combination with Global Positioning System (GPS) observations, this effort provides observations of surface deformation time series at unprecedented resolution and accuracy. Time-dependent kinematic models will be applied to constrain spatiotemporal distribution of fault creep, integrating InSAR, Creepmeter, GPS and repeating earthquakes. Dynamic models informed by creep time series and lab measurements allow linking fault transient and long term behaviors to its frictional properties, evolution of effective normal stress and crustal lithology. Lastly, the link between rate changes on creeping segments and occurrence of major earthquakes on the adjacent locked sections will be investigated through static stress transferring.