Non-volcanic Tremor in the Northern San Andreas Fault System

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This project seeks to provide a clearer understanding of the relationship between non-volcanic tremor (a recently discovered seismic phenomenon) and earthquakes within the San Andreas Fault System. Along the San Andreas Fault, tremor has been studied in only a few locations near Parkfield, California and for a limited time period. This has effectively limited the association of tremor to only one larger earthquake--the 2004 Parkfield magnitude 6 event. Notable changes in the rate and migration pattern of tremor leading up to this earthquake were observed, however, this single example is insufficient for establishing precursory tremor as a general phenomena leading up to larger events. This project will significantly expand the geographic and temporal base of tremor observations and will allow for a greater number of associations between tremor and larger earthquakes to be studied. Included in the project are detailed analyses of these associations with the aim of establishing a more robust understanding of the relationship between tremor and earthquakes. The project will also search for and characterize tremor on a greater diversity of faults within the San Andreas fault system, which will provide additional constraints on ?transform system? tremor for comparisons with subduction zone tremor. The project?s analyses of the special properties of tremor (i.e., greater depths, sensitivity to small stress change, correlation with deformation rates, and associations with deep fault fluids and stressing from earth-tides and distant large earthquakes) will also impact other fields of study by providing information on the genesis of tremor, the relationship of tremor with deep faulting, on earth-structure, deep earth properties, and on active tectonic processes. The project includes significant student participation (both graduate and undergraduate) that will support their training and research experience in science and technology, and it increases partnerships with international institutions and participation of early-career and underrepresented groups through collaboration with female early-career geophysicist Aurelie Guilhem (CEA/DAM/DIF, France) and early-career professor Haijiang Zhang (University of Science and Technology, China). Detections of ambient tremor events in northern California have relied primarily on sensitive borehole recordings of continuous seismic observations from the local-scale High Resolution Seismic Network (HRSN) at Parkfield, CA. This has resulted in a parochial view of tremor on California faults. Additional observations of tremor on a broader array of fault-systems and in association with a greater number of larger earthquakes are needed to accurately characterize the process of tremor in California, to accurately compare transform fault and subduction zone tremor, and to more clearly understand the relationship of tremor to larger earthquakes. The objective of this project is to expand the geographic and temporal scope of tremor observation and to analyze the expanded dataset in terms of tremors? relationships to a greater number of fault systems and their structures, process, inferred composition, and stress-change through time. This will be done through a comprehensive and systematic search for ambient and triggered tremor throughout much of northern California going back to the mid-1990s using, primarily, a recently available dataset of continuous seismic records from the regional USGS Northern California Seismic Network (NCSN). Continuous records from other networks in the region will also be analyzed. Envelope cross-correlation and percentile stacking methods will be used to deal with the lower sensitivity of the surface NCSN stations. Corresponding high-resolution LFE catalogs will also be developed when possible using waveform stacking and pattern match scanning techniques. Proven methods for inferring mechanics and process in the tremor-genic fault zones will be employed, and velocity and attenuation tomography using tremor sources and station-pair double-differencing methods will be performed to improve tremor locations and to enhance understanding of deep fault properties and structure.