SCEC Award Number 17248 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Contributions to the SCEC CSM and CRM: Deformation Modeling and Coordination
Investigator(s)
Name Organization
Elizabeth Hearn Independent Contractor
Other Participants
SCEC Priorities 1c, 1e, 2a SCEC Groups Geodesy, CXM, SDOT
Report Due Date 06/15/2018 Date Report Submitted 08/02/2018
Project Abstract
 For year 2 of SCEC5, my research contributions spanned two domains: completing a kinematic modeling study of southern California, and contributing to SCEC community models. The kinematic finite-element model described in my past progress reports was expanded and densified. Several suites of thousands of models were re-run to identify present-day and longer term fault slip rates and assess any differences; to explore the effects of a “propeller” geometry for the San Andreas Fault (SAF), and to estimate patterns and quantities of “off-fault” deformation. I find that the models cannot resolve any effects of a propeller-like SAF geometry, and that about 38% of the present-day moment accumulation rate is associated with “off-fault” deformation. Present-day and long-term slip rates for the Mojave SAF are comparable and lower than UCERF3 preferred rates, but a robust discrepancy remains between present-day and long-term slip rates for the Imperial Fault and the Coachella SAF. My postseismic deformation model for 1990’s Mojave earthquakes showed that the preliminary Community Rheology Model (CRM) for this region is not consistent with far-field postseismic deformation following the Hector Mine event. Specifically, the CRM mantle effective viscosity is too high, possibly because the assumed mantle temperatures (from the preliminary Community Temperature Model) were too cool. This issue was discussed at the September 2017 workshop, and the Mojave CRM was updated by CRM TAG leaders during several conference calls later in the grant period.
Intellectual Merit The Community Rheology Model links to a multitude of SCEC5 endeavors, for example characterizing absolute stresses and forces driving southern California deformation, inferring fault slip and moment accumulation rates from surface deformation data, and accurately simulating rupture propagation and strong ground motions. My kinematic finite-element models provide estimates of long-term and present-day fault slip rates, and inform active debates on discrepancies between geologic and geodetic fault slip rates, possible millennial-scale variations in fault slip rates, and the extent of inelastic, "off-fault" deformation in the southern California crust.
Broader Impacts One goal of the CRM is to provide a consensus-based resource to modelers wishing to represent more faithfully the rheological properties of the southern California lithosphere and asthenosphere. This is a service to the community because it enables computationally sophisticated researchers who are not "in the know" with regard to California geology, thermal state or laboratory deformation studies to avoid rookie mistakes and loss of time in the development of their strong motion, rupture propagation or static deformation models. Some of these models are employed in estimating fault slip rates for seismic hazard studies (e.g. UCERF3), so their improvements benefit society. As more accurate brittle rheologies are folded in to the CRM, it will become a resource for strong motion modelers as well, further enhancing our understanding of seismic hazard in southern California.
My kinematic models are also relevant to assessing seismic hazard in southern California, as they provide estimates of fault slip rates and off-fault deformation which are used in rupture forecasts such as UCERF3. The finite-element meshes for my kinematic models, which have been densified further since the end of the grant period, will enable me to assess the effect of 3D rheological variations on inferred slip rates for southern California faults.
Exemplary Figure Figure 2. Community Rheology Model activities. (a) Calculated effective viscosity profiles for different geotherms and strain rates. Yellow shaded region shows the range of effective viscosity values in the mantle that are consistent with far-field postseismic velocities (e.g., Freed et al 2007; Pollitz and Thatcher, 2010). (b) Forward modeled Landers and Hector Mine postseismic displacement field seven years after the Hector Mine earthquake. (c) Comparison of viscosity profiles (calculated two ways) with xenolith and seismic / BDT constraints.