Block kinematics of the Pacific - North America plate boundary in the southwestern US from inversion of GPS, seismological, and geologic data

Robert McCaffrey

Published 2005, SCEC Contribution #1827

The active deformation of the southwestern US (30° to 41°N) is represented by a finite num-ber of rotating, elastic-plastic spherical caps. GPS-derived horizontal velocities, geologic fault slip rates, transform fault azimuths and earthquake-derived slip vector azimuths are inverted for block angular velocities, creep on block-bounding faults, permanent strain rates within the blocks and the rotations of 11 published GPS velocity fields into to a common North American reference frame. GPS velocities are considered to be a combination of rigid block rotations, re-coverable elastic strain rates resulting from friction on block bounding faults, and non-recoverable strain rates resulting from slip on faults within the blocks. The resulting Pacific – North America angular velocity is similar to some published estimates and satisfies spreading rate and transform azimuths in the Gulf of California, earthquake slip vectors in the Gulf of Cal-ifornia and Alaska, as well as GPS velocities in coastal California and the Pacific Basin. Pub-lished fault slip rates are satisfied except in the southern Mojave Desert where the motion of the Mohave block relative to North America is faster than can be explained by mapped faults. The largest blocks, the Sierra Nevada – Great Valley and the eastern Basin and Range, show perma-nent strain rates, after removing elastic strain, of only a few nanostrain/year, demonstrating ap-proximately rigid behavior. Observed horizontal strain rates correlate strongly with predicted strain rates from known faults suggesting that the short-term strains evident in GPS velocities are largely elastic. In only about 20% of the region is distributed deformation needed to match the data, indicating that a plate tectonic style description of the deformation of the western US is plausible. Most blocks rotate about vertical axes at approximately the same rate as the Pacific (relative to North America) suggesting that locally, spin rates are communicated from block to block, arguing against both floating-block and ball-bearing mechanisms of block rotation. The similarities of the blocks’ spin rates to that of the Pacific suggests that the Pacific strongly influ-ences their motions through edge tractions. However, it is shown that the blocks cannot rotate about the Pacific – North America pole without spinning counter to the sense of Pacific – North America shear. Unlike some other broad plate boundaries, in the western US, vertical axis rota-tions take up very little of the slip rate budget across the region.

McCaffrey, R. (2005). Block kinematics of the Pacific - North America plate boundary in the southwestern US from inversion of GPS, seismological, and geologic data. Journal of Geophysical Research, 110. doi: 10.1029/2004JB003307.