SCEC Award Number 14023 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Contributions to the SCEC CSM: A finite-element model of the southern California lithosphere
Investigator(s)
Name Organization
Elizabeth Hearn Consulting Geophysicist
Other Participants
SCEC Priorities 2d, 1b, 1e SCEC Groups SDOT, Geodesy, Simulators
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 I am developing a finite element (FE) deformation model of the southern California lithosphere to estimate stresses and stressing rates for the SCEC Community Stress Model. Other modeling objectives include reconciling geological and geodetic slip rates, and better understanding how strain is accommodated away from known, major faults. Though the model mesh is largely complete, initial calculations have made it clear that plasticity and an alternative to the “split node” technique for modeling stress-driven slip along faults are required. These features have been implemented, and are being evaluated with test models. Models with plasticity produce higher and more uniform slip rates along discontinuous faults, and plasticity can profoundly influence modeled stresses. For elastic models, modified “slippery” nodes (with specified shear tractions) give surface velocities, stress rates and slip rates identical to those for conventional, traction-free “slippery” nodes, but absolute stresses are different for the two cases. Hence, using modified “slippery” nodes, stress tensor data (e.g. Yang and Hauksson, 2013) might be inverted for fault tractions and (given modeled or measured slip rates) shear zone properties.
Intellectual Merit Although several deformation modeling studies have targeted the southern California lithosphere, surprisingly few provide depth-dependent estimates of stresses, and address the dynamics of deformation throughout southern California. Given detailed GPS surface deformation data and compilations of fault geometries and slip rates, as well as new EarthScope constraints on the structure and properties of the lithosphere and asthenosphere, a comprehensive and detailed dynamic model of the region is overdue. I am developing a finite-element model to bridge the gap between this vision and the available, mostly fault-free dynamic models of the region. My phased approach involves developing both kinematic and dynamic models. Objectives include estimating stresses and stressing rates in the southern California lithosphere for the active tectonics and seismic hazard communities, reconciling geological and geodetic slip rates, and better understanding the mechanics of regional deformation and how much strain is accommodated away from known faults.
Broader Impacts Absolute stresses in the lithosphere are used in earthquake forecasts, and are also key to understanding likely maximum rupture depths in large earthquakes, which may be pertinent to scaling relationships and moment balancing. Stressing rates on known faults and estimates of off-fault stress accumulation are both required for earthquake forecasts such as UCERF3, which lead to improved building codes and zoning, reducing casualties and economic losses. Modeled stresses and stressing rates will be shared by way of the SCEC Community Stress Model website (https:// sceczero.usc.edu/projects/CSM), from which they may be downloaded by anyone.
Exemplary Figure Figure 1, top left panel (reproduced in the submitted report)
Caption for general audience:
Modeled surface velocities (pink arrows) and long-term San Andreas Fault slip rates in southern California (numerals and colored symbols), from a simplified computational model. Relative motion of the Pacific and North American plates is prescribed at the boundaries of the model (which is much larger than the area shown). The relative plate motion is almost parallel to the sides of the figure, so where the San Andreas fault bends, its slip rate decreases as shown. After we add more faults and other model refinements, fitting the pink vectors to measured (GPS) surface velocities allows us to estimate long-term slip rates, which go into rupture forecasts.
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