2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 323-2
Presentation Time: 9:15 AM


BRIGGS, Richard W. and GOLD, Ryan D., Geologic Hazards Science Center, U.S. Geological Survey, 1711 Illinois St., Golden, CO 80401

An outstanding problem in earthquake science is the apparent disparity between geodetic and geologic slip rates in many regions. An example is the Walker Lane Belt (WLB), a zone of transtension east of the Sierra Nevada where geodetic strain rates can be 80% higher than observed geologic slip rates. Here, subparallel rangefront normal and basin strike-slip faults intersect at <5 km depth, such as along the Sierran, Wassuk, and Diamond Mountains rangefronts. We hypothesize that a significant portion of northwest-directed shear across the WLB occurs in the shallow- to mid-crust as strike-slip motion on reactivated, moderately-dipping normal faults, and that slip partitioning onto subvertical faults occurs primarily in the upper few km of crust in response to the free surface. A testable prediction of this hypothesis is that shallow, distributed strike-slip faulting occurs commonly in unconsolidated basin sediment of the WLB; this may help to explain the apparent disparity between geodetic and geologic slip rates. The kinematics we propose are consistent with 1) COCORP data across the northern WLB that imaged steep faults soling into moderately-dipping ramps at <5 km depth; 2) GPS velocity profiles implying strain accumulation on dipping structures rather than vertical faults; and 3) moment tensor analyses that find consistent P and T axis orientations across the WLB, rather than discrete populations predicted by pure partitioning. Large strike-slip ruptures of moderately dipping faults have been observed recently in Pakistan (2013 M7.7) and the Scotia Sea (2013 M7.7), and moderately-dipping strike-slip faults include portions of the Median Tectonic Line, the Garlock fault, and the San Andreas. The common Andersonian-Byerlee assumption that strike-slip faulting is limited to subvertical structures does not describe conditions in the WLB, where preexisting faults dominate and dynamic friction during fault rupture is likely low. Hazard implications of this revised view of WLB tectonics include the possibility that surface deformation is more widely distributed in basins than presently assumed; surface fault trace mapping may inadequately characterize deep seismogenic structures; and surface shaking from lateral slip on dipping structures may be more complex than that inferred from subvertical faults.