SCEC Award Number 16291 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title 2016 SCEC Proposal: Tracking Seasonal Influences on Stress Changes and Predicted Seismicity Rates using Estimates of Anomalous Geodetic Strains in Southern California
Investigator(s)
Name Organization
William Holt Stony Brook University
Other Participants Yuanyuan Liu
SCEC Priorities 5b, 2c, 1d SCEC Groups Transient Detection, SDOT, Geodesy
Report Due Date 03/15/2017 Date Report Submitted 11/16/2017
Project Abstract
 We use horizontal cGPS positions for stations in the National Science Foundation’s Earthscope Plate Boundary Observatory (PBO) (Williams et al., 2010) and the University of California Berkeley Bay Area Regional Deformation Network (BARD) to estimate the potential transient strains in Southern California. The PBO station data we use are processed by NSF’s GAGE Facility at UNAVCO, while the BARD station data are processed by University of Nevada Reno’s (UNR) Geodetic Laboratory. We use continuous GPS time series to determine a time-dependent crustal strain model using the geodetic network processing tool of Holt and Shcherbenko (2013). We compute full covariance estimates of transient strain and Coulomb stress changes on faults through time throughout the entire plate boundary zone in California (Kraner et al., 2017; Bahadori et al., 2017). Just prior to the 2014 Magnitude (M) 6.0 South Napa Earthquake, we detect an anomalous dilatational strain signal and positive Coulomb stress change on right lateral faults in the region. Between 2007–2014 we find a consistent seasonal dilatational strain anomaly averaging 76±17x10-9 in the broader region around South Napa, which typically peaks around August. The averaged associated Coulomb stress change on right lateral faults in this area is 1.9±0.8 kPa, which compares well with the South Napa preseismic anomaly. Over the broader plate boundary, we measure a long-wavelength periodic displacement (± 1.5 mm E.) of the Great Valley and Sierra Nevada, which produces seasonal Coulomb stress changes of 1.5±1.0 kPa on parts of the San Andreas fault between 34.5°N – 37.5° N.

Intellectual Merit Broader Impact. Analysis of the cGPS data have revealed significant transients on a variety of time scales. Some of these are linked with known earthquakes (post-seismic relaxation in El Mayor-Cucapah [Pollitz et al., 2012]; Brawly Swarm, Hauksson et al., 2013), whereas others are linked with unknown, and potentially slow-slip, processes (Parkfield and San Simeon regions) (Holt and Shcherbenko, 2013). We show that these anomalous strains have occurred on a variety of time scales, they involve heterogeneous distributions, and they have impacted stress rates and coulomb stress changes on faults. Current efforts are showing promise for quantifying links between measured strain changes, model stress changes, and seismicity rates. We applied the network processing tool in northern California where seasonal transient strains prior to the 2014 South Napa earthquake show a focused anomally within the South Napa region [Kraner et al., 2017]. The transient analysis tool has been expanded to delineate seasonal anomaly patterns throughout the entire plate boundary zone in California. This analysis has revealed seasonal large-scale periodic displacements of the Great Valley and Sierra Nevada block of ±1-2 mm, with accompanied Coulomb stress changes on sections of the San Andreas fault of ± 1kPa. We are currently developing a data product that will enable the tracking of strain and stress change evolution, along with expected seismicity rate evolution through time. The work performed to date fulfills the SCEC Science Objective 5b “Application of geodetic detectors to the search for aseismic transients across southern California” and also supports 2c, 1d, and 1e and 2d. Moreover, this research fulfills the recommendations under Research Strategies in Tectonic Geodesy to: (a) “Improve our understanding of the processes underlying detected transient deformation signals and/or their seismic hazard implications through data collection and development of new analysis tools.”
Broader Impacts The Broader Impact of this work involves the training of graduate students Alireza Bahadori and Jeonghyeop Kim. The development of a data product, enabling automated detection of anomalous strain from CGPS data also constitutes a broader impact. In 2014-2015 the funding supported the training of undergraduate student Meredith Kraner. Meredith went on to graduate school at UNR and a JGR manuscript summarizing the results and method applied to the South Napa region analysis is submitted for review.
Exemplary Figure Figure 3. Figure 3. A. Model displacements relative to Pacific frame obtained from smoothed fit to seasonal components of CGPS data, with dilatational strains plotted in background for October, 2008, when positive dilatation dominates along parts of the San Andreas Fault and the Sierra Nevada and Great Valley displace eastward. B. Model displacements relative to Pacific frame for contraction period (winter), with westward motion of Great Valley and Sierra Nevada and much of San Andreas in compressional dilatational strain.

Figure and results produced by Ali Bahadori, Jeonghyeop Kim, and William Holt