SCEC Award Number 19178 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Distribution of Fault Slip and Off-fault Deformation with Focus on Cajon Pass
Investigator(s)
Name Organization
Kaj Johnson Indiana University
Other Participants Jake Dorsett, graduate student
SCEC Priorities 2a, 1a, 1e SCEC Groups SDOT, SAFS, Geodesy
Report Due Date 04/30/2020 Date Report Submitted 11/20/2020
Project Abstract
 An important problem cited in the SCEC Science Plan is resolving how much present-day deformation across southern California occurs as slip on known faults and how much deformation is distributed through the crust. One of the surprising results of the UCERF3 kinematic deformation models is that all the models required 20-30% of the total deformation across California to occur as distributed, off-fault strain. To address this issue, graduate student Jake Dorsett and PI Johnson attempted to compute elasto-plastic deformation models of faulting in southern California using the finite element code PyLith. Unfortunately, this project had to be abandoned because PyLith was not able to compute the plastic deformation required to address our problem. Jake travelled to the PyLith workshop in Colorado during the 2019 summer and learned that the plasticity solver is currently not capable of handling the large, localized strains around fault tips. The PyLith developers plan to improved the plasticity implementation in a future release of PyLith, but not in time for us to complete this project.
Intellectual Merit An important problem cited in the SCEC Science Plan is resolving how much present-day deformation across southern California occurs as slip on known faults and how much deformation is distributed through the crust. One of the surprising results of the UCERF3 kinematic deformation models is that all the models required 20-30% of the total deformation across California to occur as distributed, off-fault strain. Johnson (2013) reached the same conclusion comparing several kinematic deformation models with GPS data from southern California. Herbert (2014) used elastic boundary element models to show that about 40% of the shear strain across the Eastern California Shear Zone (ECSZ) is expected to occur as distributed deformation off the main faults. They suggest that well-known discrepancies between geologic slip rates and kinematic model slip rates in the ECSZ can be attributed to this off-fault strain that is not accounted for in kinematic models.

The implications of the on-fault/off-fault deformation budget for seismic hazard are clear; inferring too little off-fault deformation in models will lead to overestimates of fault slip rates and earthquake hazard on known faults. Yet, these inferences of off-fault deformation to date are limited to models that cannot fully address the problem. In the kinematic deformation models of UCERF3 and Johnson (2013), the ratio of on-fault to off-fault deformation is not controlled by physics; the rates are inferred entirely from data fitting. The elastic boundary element model in Herbert (2014) does not explicitly incorporate off-fault inelasticity.
Broader Impacts This project helped fund a graduate student at Indiana University.
Exemplary Figure Figure 1. Illustration of “thin sheet” finite element model. (a) Mesh generated with Trelis. Red lines show faults that are active in this simulation. Velocity boundary conditions are shown. Other boundaries (without imposed velocities) are traction free for this demonstration.