Off-fault deformations and shallow slip deficit from dynamic rupture simulations with fault zone plasticity

Daniel Roten, Kim B. Olsen, & Steven M. Day

Published August 14, 2017, SCEC Contribution #7661, 2017 SCEC Annual Meeting Poster #086

Fault slip distributions obtained from source inversions of major (M > 7) strike-slip earthquakes (e.g., M 7.3 Landers, M 7.1 Hector Mine, M 7.6 Izmit, M 6.5 Bam, M 7.2 Baja California) indicate that coseismic slip near the surface is significantly smaller than slip at 4-6 km depth. Although this shallow slip deficit (SSD) is thought to represent the result of distributed plastic failure in the shallow crust, previous numerical models were not able to reproduce the 30–60% of SSD inferred from slip inversions. Subpixel correlations of aerial and satellite images from recent coseismic ruptures (e.g., Landers, Hector Mine, M 7.7 Balochistan, Pakistan) reveal significant distributed, inelastic off-fault deformation, which supports the interpretation that the SSD is caused by fault zone plasticity.

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. Simulations are carried out with the AWP-ODC finite difference code using a slip-weakening fault friction law, a Drucker-Prager (DP) yield condition and depth-dependent stress. We generate three rupture models with the largest slip in the same general areas as published slip distributions for the Landers earthquake. Friction angles and cohesions are derived from the Hoek-Brown criterion for fractured rock masses, where the rock quality is systematically varied to represent fault damage zones of very good to poor quality.

We calculate the SSD as the ratio of the mean simulated coseismic slip at the surface to the mean coseismic slip at depth. The simulated OFD is defined as the relative difference between the total displacement across the fault and the displacement between split-nodes on either side of fault. While linear simulations clearly underpredict observed SSD and OFDs, nonlinear simulations for good quality rocks predict a SSD of 44–53% and OFDs of 39–48%, consistent with the 30–60% SSD derived from source inversions and 46 ± 10% (1σ) OFD inferred from image correlations for the 1992 M 7.3 Landers earthquake (Milliner et al., 2015). Both SSD and OFDs are sensitive to the quality of the fractured rock mass inside the fault damage zone, and surface rupture is almost entirely suppressed in poor quality material.

Key Words
Fault zone plasticity, off-fault deformation, shallow slip deficit, dynamic rupture simulation

Citation
Roten, D., Olsen, K. B., & Day, S. M. (2017, 08). Off-fault deformations and shallow slip deficit from dynamic rupture simulations with fault zone plasticity. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy