SCEC Award Number 17136 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Crack models to constrain time-dependent interseismic slip-rate distributions from geodetic data
Name Organization
Paul Segall Stanford University Lucile Bruhat Stanford University
Other Participants
SCEC Priorities 1a, 1c, 3b SCEC Groups SDOT, Geodesy, FARM
Report Due Date 06/15/2018 Date Report Submitted 08/20/2019
Project Abstract
Most inversions consider the depth distribution of interseismic fault slip-rate to be time invariant. However, some numerical simulations show penetration of dynamic rupture into regions with velocity-strengthening friction, with subsequent interseismic up-dip propagation of the locked-to-creeping transition. We explored this hypothesis by developing and testing crack models to describe creep penetration upward into the locked region. Previous work from Bruhat and Segall (2017) developed a new method to characterize interseismic slip rates, that does not assume that the spatial distribution of interseismic slip is stationary. This simple model considers deep interseismic slip as a crack loaded at specified slip rate at the down-dip end. It provides analytical expressions for stress drop within the crack, slip, and slip rate along the fault.

This work extended this new class of solution to strike-slip fault environment. Unlike Bruhat and Segall (2017) which considered creep propagation in a fully elastic medium, we included here the long-term deformation due to viscoelastic flow in the lower crust and upper mantle. We improved the model presented in Bruhat and Segall (2017) to account for the coupling between creep and viscoelastic flow, and derived expression for viscoelastic response due to time-dependent creep. Finally, we employed this model to investigate the long-term rates along the Carrizo Plain section of the San Andreas fault.
Intellectual Merit This project explored a model of contemporary deformation along the San Andreas fault that allowed for non-stationary fault creep within an elastic crust underlain by a viscoelastic region. While the model allows for upward propagating creep, the best fit solutions do not appear to require this.
Broader Impacts This project contributed to the training of a PhD student. Her principal work had focused on subduction zones, so this work brought her into the SCEC community and introduced her to challenges of the strain-rate field in southern California.
Exemplary Figure Figure 6. Distribution of coseismic slip, slip rate, stress rate, $D$, $d$ and $H$ for our improved model. Median solutions are indicated in bold lines, the 2-$\sigma$ uncertainties are given by the shaded regions. We compare the obtained stress rate distribution to the cumulative moment from microseismicity between 1981 and 2016 along the Carrizo Plain \citep{lin2007applying, hauksson2012waveform}.