On the determination of self-consistent strain rate fields within zones of distributed continental deformation

Abstract

We present a method to estimate self-consistent, continuous velocity field models given geologic or seismic strain rate and space-based geodetic observations. In a least-squares inversion, geologic or seismic strain rates are matched with continuous functions, subject to the constraint that GPS or VLBI observations are also matched by the model velocity field within a defined frame of reference. We investigate the active deformation field in the western US and central and east Asia using geologic strain rates, GPS and VLBI data, and the Aegean using earthquake strain rates and SLR data. Integrated Quaternary rates of strain across the western U.S. define a Pacific-North America (PA-NA) motion that deviates counterclockwise by 6 degrees from the NUVEL-1A velocity direction, but velocity magnitudes are equal to NUVEL-1A. Inversion of recent geodetic data alone yields a PA-NA velocity direction that is 2-3 degrees counterclockwise from the NUVEL-1A PA-NA direction, but the difference between the two is not significant at the 95% confidence level. The discrepancy between total PA-NA motion obtained from the geologic data and NUVEL-lA indicates that a marginally significant amount of NE-SW directed shortening (possibly as much as 5 mm/yr both onshore and offshore) is missing in the geologic data. From the continuous velocity field solution we also determine rotation rates and seismic moment deficits within the western US. Significant moment deficits exist on the southern San Andreas Fault and in the LA-Basin-Western Transverse Range region. A joint inversion of Quaternary strain rates and GPS velocities in Asia for a self-consistent velocity field yields a solution in which south China moves at an almost uniform velocity E-SE (azimuths of 110 ± 15 degrees) relative to Eurasia at 14 ± 3 mm/yr (95 % confidence). The Sunda block moves eastward with respect to Eurasia at 16 ± 4 mm/yr with azimuth of 96 ± 20 degrees (95 % confidence). Seismic strain rates in the Aegean, from earthquakes this century, are compared with total strain rates inferred from SLR data. The SLR measurements do not require strain different in orientation from that observed in earthquakes, but do require more of it. On average, the seismic strain rates are roughly two thirds of the inferred total strain rate that accommodates all relative motions in the Aegean.