SCEC2022 Plenary Talk, Tectonic Geodesy

Northern Exposure: Why you should consider studying the faults of northern California

Gareth J. Funning

Oral Presentation

2022 SCEC Annual Meeting, SCEC Contribution #12523
In its next incarnation, SCEC plans to expand its region of interest in all directions, to become a truly statewide earthquake center. Given SCEC’s past priorities and future goals, I believe the faults of the northern San Andreas system provide compelling targets for research, in the next Center and beyond, that will be of interest to many members of the collaboration. I will highlight some of the most interesting aspects of these faults, from my admittedly biased perspective as a tectonic geodesist who has worked in the region for over a decade.

Widespread, shallow aseismic fault creep: Several of the major faults in northern California creep. The development of precise time...
series InSAR methods, along with the advent of frequent SAR satellite acquisitions, have allowed us to start to map the spatial extent of the shallow creep on each fault with increasing confidence as data volumes increase. Combining InSAR analysis with seismological data mining for repeating earthquakes, which are also markers of creep, allows us to extend this mapping to greater depths. This knowledge has led us to identify potential lithological causes for creep, such as the presence of ultramafic rocks. In some cases, such as for the Hayward fault, excellent geodetic and seismic data coverage allows us to invert for the creep distribution and fault coupling at depth, constraining the dimensions of likely asperities that could generate future damaging earthquakes.

Persistent geologic and geodetic slip rate differences: We have been collecting campaign GNSS data north of San Francisco for over 15 years. Inversions of our horizontal velocities by multiple methods reveal persistent differences between our contemporary geodetic slip rates and geologic rates – e.g. <20 mm/yr vs 24 mm/yr (respectively) for the northern San Andreas, with similar amplitude discrepancies on faults inland. It remains to be determined whether this is due to earthquake cycle effects, biases in the geodetic or geologic data, and/or temporal variation.

Seismic hazards with statewide implications: The water supply for over 25 million Californians, plus over 4 million acres of irrigated farmland, passes through the Sacramento-San Joaquin Delta, drawing a sharp focus on potential seismic hazards there. Putative blind fault structures and up to 4 mm/yr of residual tectonic motion between the Bay Area faults and the Great Valley block motivate a more detailed investigation of hazards in that area.