Fault slip rates and interseismic deformation in the western Transverse Ranges, California

Abstract

[1] To better constrain fault slip rates and patterns of interseismic deformation in the western Transverse Ranges of southern California, we present results from analysis of GPS and interferometric synthetic aperture radar (InSAR) data and three-dimensional mechanical and kinematic models of active faulting. Anthropogenic motions are detected in several localized zones but do not significantly affect the vast majority of continuous GPS site locations. GPS measures contraction rates across the Ventura Basin of~7 mm/yr oriented west-northwest with rates decreasing to the west and east. The Santa Barbara channel is accommodating~6.5 mm/yr in the east and~2.5 mm/yr in the western portions of N/S contraction. Inversion of horizontal GPS velocities highlights a zone of localized fast contraction rates following the Ventura Basin. Using a mechanical model driven by geodetically calculated strain rates, we show that there are no significant discrepancies between short-term slip rates captured by geodesy and longer-term slip rates measured by geology. Mechanical models reproduce the first-order interseismic velocity and strain rate patterns but fail to reproduce strongly localized contraction in the Ventura Basin due to the inadequate homogeneous elastic properties of the model. Existing two-dimensional models match horizontal rates but predict significant uplift gradients that are not observed in the GPS data. Mechanical models predict zones of fast contraction in the Santa Barbara channel and offshore near Malibu, suggesting that offshore faults represent a significant seismic hazard to the region. Furthermore, many active faults throughout the region may produce little to no interseismic deformation, making accurate seismic hazard assessment challenging.

Shelf evolution along a transpressive transform margin, Santa Barbara Channel, California

“…Farther west, where the fault tip is deeper and fault-related deformation is distributed more broadly, we used the elevation of the shelf break and buried shelf wave-cut platforms as uplift indicators to estimate fault slip rates of 2.0 ± 0.2 mm/yr. Westward-decreasing slip is consistent with westward-decreasing shortening based on geodesy (Marshall et al, 2013).…”

“…1 and 2). Modeling of geodetic data by Marshall et al (2013) indicates that this region is now undergoing ~2.5 mm/yr (in the west) to 6.5 mm/yr (in the east) of north-south contraction (Fig. 2).…”

The 2015 Fillmore Earthquake Swarm and Possible Crustal Deformation Mechanisms near the Bottom of the Eastern Ventura Basin, California

“…Fluid pressures higher than hydrostatic levels have been interpreted from well logs but are only reported in the western basin (Watts, 1948;Hubbard et al, 2014). The 12.5-13.5 km depth of the Fillmore swarm agrees with the geodetically determined locking depth of major faults in the ∼12-13-kmdepth range (Marshall et al, 2013). The effective normal stress needs to be significantly reduced by near-lithostatic pore pressure to accommodate episodic slow-slip events at the depths of the Fillmore swarm, according to recent numerical models of slow-slip events at depth in subduction zones (Liu and Rice, 2009).…”

“…The greater Ventura region is characterized by an elevated rate of background seismicity that is, in part, driven by rapid tectonic convergence (∼7 mm=yr) in the region (Marshall et al, 2013). Background seismicity from 1981 to 2015, with average rate of approximately five M ≥ 3 events per year, is mostly located on the north side of the Ventura basin (Fig.…”

“…The cross section shows the modified 3D V P model from Shaw et al (2015) and the approximate locations of late Quaternary faults in the cross section from the Southern California Earthquake Center-Community Fault Model (SCEC-CFM) (Plesch et al, 2007;Marshall et al, 2013;Nicholson et al, 2014). The 1989 and 2015 swarms are plotted in red, and the 2000 swarm is shown in purple in the map view.…”