Deformation kinematics in the western United States determined from Quaternary fault slip rates and recent geodetic data

Abstract

We estimate the horizontal velocity gradient tensor field from Quaternary fault slip rates, and recent Global Positioning System (GPS) and very long baseline interferometry (VLBI) velocity solutions in the western United States transform plate boundary zone. The total velocity obtained from the Quaternary fault slip rate data across the entire plate boundary is within 1 mm/yr of the NUVEL‐1A predicted Pacific (PA)‐North American (NA) plate motion velocity, but directions are 5°–6° anticlockwise of directions given by NUVEL‐1A. The total velocity obtained from inversion of recent geodetic data is 2°–3° anticlockwise from the NUVEL‐1A NA‐PA velocity, but the difference between the two is not significant at the 95% confidence level. The discrepancy between the total PA‐NA motion obtained from the geological data and NUVEL‐1A indicates that a marginally significant amount of NE‐SW shortening (possibly as much as 5 mm/yr) is missing overall in the geologic data. Shortening may occur in the long‐term in the offshore and coastal areas of California where such shortening is required in the shorter‐term geodetic solution. The seismic moment released in the last 148 years is ∼59% of the total moment release rate expected from long‐term strain rate field (including both seismic and aseismic deformation) derived from the inversion of geological data with NUVEL‐1A far‐field PA‐NA motion constraints. The accumulated strain in the areas containing the southern San Andreas fault‐San Jacinto fault, the San Francisco Bay area, and the area containing the Ventura basin in the Western Transverse Ranges in the last 148 years is the equivalent to that which could be released by an Mw>7.0 earthquake in each 50 × 100 × 15 (km3) crustal volume if strain is to be released seismically in these areas.