SCEC Award Number 18228 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title 2018 SCEC Proposal: Monitoring Seasonally-Driven Stress Changes on Faults within the Plate Boundary Zone in California using cGPS Observations
Investigator(s)
Name Organization
William Holt Stony Brook University
Other Participants Alireza Bahadoir and Jeonghyeop Kim
SCEC Priorities 1e, 1d, 2d SCEC Groups Geodesy, SDOT, FARM
Report Due Date 03/15/2019 Date Report Submitted 11/13/2019
Project Abstract
 Using horizontal cGPS (Plate Boundary Observatory, PBO) observations of crustal displacements processed by University of Nevada Reno’s (UNR) Geodetic Laboratory and NSF’s GAGE Facility at UNAVCO, we quantify non-steady-state horizontal deformation fields within the plate boundary zone in California for the last thirteen years, leading up to the 2019 Magnitude (Mw) 7.1 Ridgecrest Earthquake. We use the geodetic network processing tool of Holt and Shcherbenko (2013) and Kraner et al. (2018). We also determine the associated Coulomb stress changes on existing fault structures through time. Our compilation of the thirteen-year transient strain history in Southern California highlights a remarkable long-wavelength pattern of seasonal anomalies that impact the horizontal strains in parts of Northern, Central, and Southern California by as much as ± 10-20 x 10-9 and Coulomb stress changes on faults by ± 1 kPa. During most summers a seasonal positive dilatational strain anomaly develops along the San Andreas Fault (SAF) zone between 34° N – 37.5° N, resulting in positive Coulomb stress changes on the SAF. During the winter, on the other hand, dilatational compressional strain rates are observed along the SAF. Opposite to the patterns on the San Andreas, the right-lateral strike-slip faults in the Eastern California Shear Zone (ECSZ), east of the Sierra Nevada and north of the Garlock Fault, undergo extensional dilatation during the winter, and vice versa. We show that between 2008-2019 a consistent positive Coulomb stress change on faults of >1kPa peaks every May and June in the Ridgecrest region.
Intellectual Merit Our compilation of the thirteen-year transient strain history in Southern California highlights a remarkable long-wavelength pattern of seasonal anomalies that impact the horizontal strains in parts of Northern, Central, and Southern California by as much as ± 10-20 x 10-9 and Coulomb stress changes on faults by ± 1 kPa. During most summers a seasonal positive dilatational strain anomaly develops along the San Andreas Fault (SAF) zone between 34° N – 37.5° N (Figure 1b). The Great Valley and the Sierra Nevada move up to 2 mm toward the Great Basin with respect to the Pacific plate reference during the summer, resulting in positive Coulomb stress changes on the SAF. During the winter, on the other hand, the horizontal displacement patterns reverse and dilatational compressions are observed along the SAF. Opposite to the patterns on the San Andreas, the right-lateral strike-slip faults in the Eastern California Shear Zone (ECSZ), east of the Sierra Nevada and north of the Garlock Fault, undergo extensional dilatation during the winter (Figure 1a), and vice versa. In our previous results we show that just prior to the 2014 Mw 6.0 South Napa Earthquake of August 24, there was a Coulomb stress increase of 5.1 ± 1.6 kPa on right-lateral faults in the region of the event, caused by a seasonal positive dilatational strain anomaly there. We examined non-tectonic stress changes in South Napa from 2007–2014 and find non-tectonic positive Coulomb stress changes in each summer. With a similar damping level used to analyze Napa region, we investigate the Ridgecrest region. We find that a positive Coulomb stress change of ~1kPa develops on faults every May and June between 2008 and 2019 in the region. The long-wavelength transient strain/stress model (which matches the hydrologic loading models) can only explain up to ~15% of the Coulomb stress increases observed every May –June in the Ridgecrest region. The source of these positive Coulomb stress anomalies needs to be investigated further.
Broader Impacts The project has contributed to the training of Ph.D. students Jey Kim and Ali Bahadori. This work also provides the basis for future research that will be conducted by Lajhon Campbell, who is a student from an underrepresented group.
Exemplary Figure Figure 5. Seasonal anomalies: shorter-wavelength model strain field (a, b) and the associated Coulomb stress change (c, d) inferred from horizontal cGPS data for a month before the Mw 6.0 South Napa event (a) and (c) and a month before the Mw 7.1 Ridgecrest event (b) and (d). The hotter colors indicate dilation and positive Coulomb stress changes.

Figure by Jey Kim and William Holt