High Resolution Geodetic Measurements of Interseismic Deformation Across the Ventura Basin, CA

Abstract:

A portion of the total motion across the Pacific-North American plate boundary in southern California is accommodated by a complex network of seismically-active oblique-reverse slip faults in the Transverse Ranges region. Most notably, existing geologic and geophysical data support the presence of a large seismogenic reverse fault, the Ventura fault, which has a previously estimated slip rate of 4.4-6.9 mm/yr. The Ventura fault appears to link with several other faults and may represent a potential source for future ~M8.0 earthquakes. A reverse fault of this size and slip rate should be detectable with modern geodetic techniques; however, our past attempts to use continuous GPS data to map out the interseismic strain patterns did not clearly delineate the fault’s slip rate and/or locking depth. Accurate measurement of interseismic deformation across the Ventura fault requires further examination with a spatially denser data set. We, therefore, combine GPS velocities from the current Plate Boundary Observatory Network and the Southern California Earthquake Center’s Crustal Motion map into a single local reference frame to provide a dense GPS network for strain rate inversions. Additionally, we process InSAR data from the Envisat and ERS satellites using the persistent scatterer method resulting in >1 million line of sight velocity estimates throughout the region. Velocity profiles across the InSAR data do not clearly delineate any significant line of sight strains associated with the Ventura fault, but strain rate inversions of the GPS network reveal clear horizontal contraction gradients along the fault structure. Mechanical models of the Ventura fault predict average slip rates between 2.7-3.2 mm/yr, but with maximum slip rates near the coast, where the existing long term slip rate estimates were made. Thus, the existing slip rate estimate may have been made in a location with nearly maximum slip rates for the Ventura fault and may overestimate the seismic hazard.