CRUSTAL VELOCITIES, STRAIN RATES, AND FAULT SLIP RATES IN THE WESTERN US: SOLUTIONS TO SOME OLD PROBLEMS OF CALIFORNIA NEOTECTONICS FROM APPLIED RESEARCH

Neotectonic deformation models can merge geologic constraints (fault traces, dips, rake-senses, & dated offset features) with geophysical constraints (GPS velocities & stress azimuths). The necessary algorithms and programs are objective, transparent, and reproducible; this is an improvement over reliance on expert opinion.

A solution for the western conterminous U.S. was computed with program NeoKinema [Bird, 2009] for the UCERF3 (California) and NSHM (national) seismic hazard estimation programs. The 2-D F-E grid had 11000 nodes and 21500 triangular elements. Many prior geologic estimates of fault offset rates with 95%-confidence limits were obtained from automated statistical analysis of dated offset features with program Slippery [Bird, 2007]. Other inputs included: traces, dips, & rakes of 606 active faults; locations of creeping fault patches; 6675 interseismic GPS velocities; & 640 azimuths of horizontal principal stress from the World Stress Map. Fault offset rates constrained by dated offset features were kept within their geologic 95%-confidence bounds, and no rakes were allowed to reverse. Yet GPS velocities were fit with RMS residual of only 2.3 standard deviations.

Three longstanding problems of California neotectonics have been resolved: (1) Excess crustal area around the left step of the San Andreas fault is not resolved by overthrusting alone; half the area-rate problem is solved by the transtensional right step in a new fault system that begins in Joshua Tree NP and continues to the Walker Lane. (2) Rapid left-slip of the Garlock fault, which does not continue past its ends, occurs because the Mojave Desert region is moving NE-ward (relative to NA) to transfer excess crust away from the compressive left-step and into the extensional right-step. (3) Kinematic conflicts at the unstable Mendocino triple junction are solved by: (a) clockwise rotation of the Pacific Northwest about a pole in Montana; (b) reduction in San Andreas slip rate near the 3-J, with rapid distributed deformation and faulting in the Coast Ranges; and (c) deformation of the S part of the Juan de Fuca plate. California neotectonics has little resemblance to idealized plate-tectonic models. Estimating these flows requires merging diverse constraints; understanding them requires consideration of the prior tectonic history.