SCEC Award Number 13023 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative Research: Relating fault-slip gradients to distributed deformation in the Eastern California Shear Zone
Name Organization
Michael Oskin University of California, Davis Michele Cooke University of Massachusetts Amherst
Other Participants Jacob Salender (UC Davis) and Justin Herbert (UMass Amherst)
SCEC Priorities 1, 4, 4 SCEC Groups Geology, SDOT, USR
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
Using a combined modeling and observational approach, we have investigated the amount and style off-fault deformation from a very well exposed, active set of strike-slip faults in the central Mojave Desert portion of the Eastern California shear zone (ECSZ). Our investigations have been three-pronged: quantifying the role of disconnected faults in promoting off-fault deformation within the Mojave ECSZ, geologic and geomorphic assessment of off-fault deformation around the northern tip of the Gravel Hills fault, and understanding the role of thrust faults in accommodation deformation within the central Mojave Desert. Our new geologic observations and synthesis of prior mapping illuminates several mechanisms of active distributed deformation: (1) fault propagation, led by a zone of distributed active faulting and folding, and expressed as gradients in fault slip-rate (2) transpressional deformation between strike slip faults, and (3) long-wavelength warping and folding. Each of these processes provides information for validation of our Boundary Element Method modeling results. The BEM model results show that disconnected faults of the Mojave ECSZ produce fault slip rates that better match geologic data and produce off-fault deformation that could account for part of the observed discrepancy between geologic and geodetically-derived slip rates across the shear zone. The inclusion of the mapped thrust faults within the models produces upift patterns that match well many regions of active uplift. These results confirm that details of fault connectivity have a significant impact on partitioning of deformation and the details of fault geometry should be considered for accurate assement of seismic hazards.
Intellectual Merit Using a combined modeling and observational approach, we investigate the amount and style off-fault deformation from a very well exposed, active set of strike-slip faults in the central Mojave Desert portion of the Eastern California shear zone (Lenwood, Camp Rock, and Calico faults in the south, to the Blackwater and Gravel Hills faults in the north). Based on existing geologic maps we produced a revised model of the fault system and analyzed this model using the boundary element method (BEM) technique to re-assess the slip distribution. We find that slip rates are indeed strongly affected by the connectivity of faults, and that a more geologically accurate and disconnected fault system in the Mojave produces slip rates in closer agreement to geological observations. (Herbert et al., 2014 Geology). We estimate that as much as 40% of the regional deformation may be accommodated as off-fault deformation within the central Mojave Desert. Newly generated maps and balanced cross-sections reveal a set of east-west striking blind thrust faults and fault-related folds that act to transfer slip between strike-slip faults. All five of the strike-slip faults we studied terminate within this thrust system, which is located where our revised BEM model results predict a large amount of off-fault strain. We also mapped and modeled deformation within the active damage zone at the northwest tip of the Gravel Hills fault. We estimate that approximately 2 km of dextral-oblique slip is absorbed over a distance of 10 km. This deformation manifests as secondary faulting and folding within the damage zone, as well as long-wavelength flexure of the surrounding crust
Broader Impacts An important indirect outcome of this work is to quantify deformation away from major faults. This contributes to hazard assessment in understanding rare earthquakes on slow-slipping faults and, potentially, quantifying a contribution of aseismic deformation to the strain budget. The project has supported the training of two PhD students, Justin Herbert at the University of Massachusetts, Amherst and Jacob Selander at the University of California, Davis. The project has also supported UMass undergraduate, Karl Grette in his honors thesis. One of the PIs is a member of two underrepresented groups, as a woman with a hearing impairment.
Exemplary Figure Figure 1. Overview of mapping results as of March, 2014, showing revised fault geometry for the central Mojave Desert based on map and cross-sectional interpretations and recent field work. Stars note locations of slip-rate sites where 10Be depth profiles have been sampled for exposure-age dating.