SCEC Award Number 14066 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
Michele Cooke University of Massachusetts Amherst Michael Oskin University of California, Davis
Other Participants Jacob Selander, Justin Herbert
SCEC Priorities 4b, 4c, 1a SCEC Groups USR, Geology, Geodesy
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
Because the Earth’s crust is not purely elastic, it can accrue permanent, distributed deformation during and between earthquakes. This permanent off-fault deformation may be expressed in damaged, folded and uplifted rocks between active faults in southern California. Our study examines the rates of slip along faults in the Mojave Desert portion of the Eastern California Shear zone from both geologic and numerical modeling approaches. In this final year of this project we have (1) completed defining new fault geometries that include previously unrecgonized active thrust faults and reconfigured connections between strike-slip faults, (2) developed a boundary element method model that incorporates these new fault geometries and yields a distributed uplift signal very similar to that observed, and (3) determined five new fault slip that quantify gradients in fault displacement rate and help to test our model. One paper published in 2014 on our initial modeling efforts was lead authored by PhD student Justin Herbert who completed his thesis in June 2014. Also, one Ph.D. student, Jacob Selander, completed his thesis in February 2015. We anticipate publishing three chapters from this thesis, along with additional modeling results that incorporate the improved geologic information.
Intellectual Merit Quantifying off-fault deformation is needed to improve our understanding strain release at ends of earthquake ruptures, where coseismic slip must gradually diminish to avoid unrealistically high stresses. Similar processes may act at geometric irregularities along faults. Enforcement of uniform slip rate along faults leads to persistent slip deficits in these regions. These deficits may be realistic for very long, straight and mature faults, such as the central San Andreas, where overlapping earthquake ruptures can fill in the gaps left from previous events. However, it is unrealistic to expect such fill-in events at the ends of faults that may rupture end-to-end, and instead permanent off-fault deformation must accrue to accommodate gradients in fault slip. This study examines the distributed network of strike-slip and reverse faults present in the central Mojave Desert portion of the eastern California shear zone. According to our modeling and existing slip rate data, this fault network displays an unusually high proportion of off-fault deformation. This may explain geologic versus geodetic rate discrepancies in this area.
Broader Impacts By not accounting for permanent off-fault deformation, most geodetic inversions for fault slip rates may over-estimate seismic hazard if this deformation accrues aseismically. Hazard may also be under-estimated away from known fault traces if some off-fault strain energy is released by rare, seismogenic slip events on unrecognized secondary structures. Even the San Andreas fault may lose significant slip to permanent off-fault deformation in areas of structural complexity, such as the San Gorgonio Pass. If significant off-fault deformation occurs aseismically, as our preliminary results suggest, this lowers the expected seismic moment accumulation rate across California, especially in areas away from the principal strike-slip faults.
Exemplary Figure Figure 2: Uplift patterns within the Mojave change with the addition of thrust faults and revision of dip along other faults. Uplift rates increase within the hanging walls of dipping faults as these faults accommodate convergence via dip slip. The region of subsidence along the footwall of the Gravel Hills Hills fault is generally consistent with local areas of deposition.