Exploring Deep Fault Mechanics by Identifying Non-Volcanic Tremor on Southern California Faults

University Of California-Riverside, Riverside CA

Investigators

Abstract

The investigator will undertake a study of non-volcanic tremor in southern California using continuous seismic data made available by the Southern California Seismic Network (SCSN). Initially, they will focus our search for tremor near Anza, California in a densely instrumented region of southern California that has both surface and borehole continuous seismic data. These high-quality data will be used to determine the frequency content and duration of tremor that occurs near Anza allowing for tuning of future searches to these features. An automatic tremor detection scheme will then be used to search for tremor across all SCSN stations over a 5-year time period. These efforts provide the foundation to address fundamental questions about physical conditions necessary for non-volcanic tremor in California. They will also determine whether tremor is correlated with local earthquakes, teleseismic wave arrivals, Earth tides, or observations of slow slip or aseismic creep. And, they will determine whether tremor occurrence correlates with known fault geometries or material properties. The major questions to be addressed in this proposal are: 1) Does tremor occur primarily along well-developed, through-going faults such as the San Andreas or San Jacinto fault zones?; 2) Does non-subduction zone tremor occur primarily in regions of a fault that are likely to be in conditionally stable such as: a) at depths less than 5 km in shallow sediments and/or b) deep (>15 -20 km) on crustal faults beneath the seismogenic zone?, and 3) Is timing of tremor in California controlled by stressing events such as the passage of teleseismic waves, changes in local seismicity rates, or tidal forces? If so, what is the stress amplitude and period required to trigger tremor? Tremor are low amplitude, long duration seismic signals that have been only recently discovered in seismic data recorded continuously. The first observations of tremor were made in Japan and found to occur deep on the subducting plate interface. Tremor has since been observed at multiple plate boundaries around the globe, most recently along several faults in California. Tremor typically occurs deep on faults, below the portion of the crust that ruptures during an earthquake. It turns out that tremor is part of a suite of seismic and geodetic phenomena likely associated with deep slip on faults. By studying the details of tremor occurrence, including when and where tremor is typically observed, under what conditions tremor can be triggered, and the relationship between tremor and regular earthquakes, we can investigate the frictional properties of faults to greater depths than was previously possible using seismic data.