SCEC Award Number 16238 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Fault zone plasticity and shallow slip deficit from 3D nonlinear dynamic rupture simulations
Investigator(s)
Name Organization
Daniel Roten San Diego State University Kim Olsen San Diego State University Steven Day San Diego State University
Other Participants
SCEC Priorities 6b, 3c, 6e SCEC Groups CS, FARM, GMP
Report Due Date 03/15/2017 Date Report Submitted 06/02/2017
Project Abstract
 Kinematic source inversions of major (M > 7) strike-slip earthquakes show that the slip at depth exceeds surface displacements measured in the field, and it has been suggested that this shallow slip deficit (SSD) is caused by distributed plastic deformation near the surface. We perform dynamic rupture simulations of M 7.2-7.4 earthquakes in elastoplastic media and analyze the sensitivity of SSD and off-fault deformation (OFD) to rock quality parameters. While linear simulations clearly underpredict observed SSD and OFDs, nonlinear simulations for a moderately fractured fault damage zone predict a SSD of 44-53% and OFDs of 39-48%, consistent with the 30-60 SSD and 46% OFD reported for the 1992 M 7.3 Landers earthquake. Both SSD and OFDs are sensitive to the quality of the fractured rocks mass inside the fault damage zone, and surface rupture is almost entirely suppressed in poor quality material.
Intellectual Merit The shallow slip deficit (i.e., the observation that fault slip measured in the field is lower than geodetically inferred slip at depth) is central to our understanding of the earthquake process and has been addressed in many high-profile publications. It has long been postulated that the shallow slip deficit is caused by distributed plastic deformation near the surface. Improvements in the acquisition and processing of high-resolution satellite and aerial imagery have resulted in detailed models of surface deformation during recent earthquakes, which are supporting the interpretation that near-surface plasticity controls shallow fault zone deformation. To our knowledge, however, no simulations have yet been presented with fully reproduce observed shallow slip deficit and off-fault deformations. Simulations of the 1992 M 7.3 Landers earthquake performed in the framework of this project are able to explain both the shallow slip deficit (30 - 60%) and the amount of off-fault deformation (~46%) reported from slip inversions and aerial image correlations, respectively. These results support the common interpretation that the shallow slip deficit and off-fault deformation are caused by distributed plastic yielding, implying that field observations of fault slip underestimate geodetic slip rates.
Broader Impacts The sensitivity of off-fault deformations and shallow slip deficit to rock strength parameters may open the possibility to validate dynamic rupture models with fault zone plasticity against geodetic observations, and to calibrate rock strength parameters from such observations for improved deterministic ground motion prediction. Improved ground motion models accounting for fault zone plasticity could lead to more accurate physics-based seismic hazard maps, and help society to better prepare for future large earthquakes.
Exemplary Figure Figure 4: (a) Off fault deformation (OFD) for Landers earthquake obtained by Milliner et al. (2015) from aerial image correlations (blue bars) and normal distribution with mean of 46 ± 10% (1σ, red line, modified from Milliner et al., 2015). Simulated OFD in the (b) linear case and in the nonlinear case for a rock mass of (c) very good, (d) good, (e) average and (f) poor quality in the three rupture models. P.D. = Probability distribution.