Slicing up the San Francisco Bay Area: Block kinematics and fault slip rates from GPS‐derived surface velocities

Abstract

Observations of surface deformation allow us to determine the kinematics of faults in the San Francisco Bay Area. We present the Bay Area velocity unification (BV, “bay view”), a compilation of over 200 horizontal surface velocities computed from campaign‐style and continuous Global Positioning System (GPS) observations from 1993 to 2003. We interpret this interseismic velocity field using a three‐dimensional block model to determine the relative contributions of block motion, elastic strain accumulation, and shallow aseismic creep. The total relative motion between the Pacific plate and the rigid Sierra Nevada/Great Valley (SNGV) microplate is 37.9 ± 0.6 mm yr−1 directed toward N30.4°W ± 0.8° at San Francisco (±2σ). Fault slip rates from our preferred model are typically within the error bounds of geologic estimates but provide a better fit to geodetic data (notable right‐lateral slip rates in mm yr−1: San Gregorio fault, 2.4 ± 1.0; West Napa fault, 4.0 ± 3.0; zone of faulting along the eastern margin of the Coast Range, 5.4 ± 1.0; and Mount Diablo thrust, 3.9 ± 1.0 of reverse slip and 4.0 ± 0.2 of right‐lateral strike slip). Slip on the northern Calaveras is partitioned between both the West Napa and Concord/Green Valley fault systems. The total convergence across the Bay Area is negligible. Poles of rotation for Bay Area blocks progress systematically from the North America‐Pacific to North America‐SNGV poles. The resulting present‐day relative motion cannot explain the strike of most Bay Area faults, but fault strike does loosely correlate with inferred plate motions at the time each fault initiated.

Restraining bend tectonics in the Santa Cruz Mountains, California, imaged using 10Be concentrations in river sands

“…The time spans integrated by the denudation rates range between ~1.5 and 6.6 k.y. (Table DR2), during which the SAF would slip ~25-120 m at a slip rate of 15-18 mm/yr (d'Alessio et al, 2005). These samples are thus a near-instantaneous record of denudation with respect to the advection of crust through the zone of active uplift.…”

The Mw 6.0 24 August 2014 South Napa Earthquake

“…The West Napa fault system is a relatively minor strike-slip fault in the Contra Costa shear zone, which transfers slip between the Northern Calaveras, West Napa, and Concord faults (Unruh and Kelson, 2002a,b;Kelson et al, 2004Kelson et al, , 2005Wesling and Hanson, 2008;Brossy et al, 2010). Block modeling of Global Positioning System (GPS) data estimated a slip rate of 4:0 3:0 mm=yr on the West Napa fault (d'Alessio et al, 2005). In the Uniform California Earthquake Rupture Forecast (UCERF 3) model, the entire Contra Costa shear zone was assigned a maximum slip rate of 1 mm=yr (Field et al, 2013, appendix C, 18 pp).…”

Widespread Fault Creep in the Northern San Francisco Bay Area Revealed by Multistation Cluster Detection of Repeating Earthquakes

“…Between them, the San Andreas, Maacama-Rodgers Creek, and Bartlett-Springs-Green Valley fault zones accommodate 38-43 mm/year of plate boundary-parallel motion (d'Alessio et al, 2005;Murray et al, 2014) (Figure 1). Between them, the San Andreas, Maacama-Rodgers Creek, and Bartlett-Springs-Green Valley fault zones accommodate 38-43 mm/year of plate boundary-parallel motion (d'Alessio et al, 2005;Murray et al, 2014) (Figure 1).…”