Off-fault deformation, stress field rotation, and the mechanical conditioning for rupture through an earthquake gate along the Altyn Tagh fault, northwest China

Veronica B. Prush, Michael E. Oskin, & Jing Liu

Submitted August 15, 2020, SCEC Contribution #10764, 2020 SCEC Annual Meeting Poster #014

Historical rupture mapping shows that fault complexities, such as bends, stepovers, and branches, are associated with the endpoints of earthquake rupture. Modeling studies show that stress heterogeneities arising from prior earthquake ruptures terminating within a complexity change the probability that a future event may break through. We term these types of complexities ‘earthquake gates’ that exhibit a probability of stopping ruptures conditioned on past recent events. We use Quaternary slip rates and mechanical analysis as tools to understand this conditionally dependent behavior for the Akatengneng Shan earthquake gate along the Altyn Tagh fault in northwest China. We find that slip rates decline from east to west along the main strand of the Altyn Tagh fault through the gate. That slip rate is lowest where the main strand exits the gate to the west suggests that the amount of distributed deformation within the bend is strongly variable along strike, possibly due to contrasting basement lithology. We explore possible causes for the observed slip rate variation by inverting for a stress field that allows for observed kinematic behavior of the Altyn Tagh fault and surrounding secondary structures. Using Coulomb modeling, we show that there is no single stress field that can allow for the observed slip directions on all active faults within the Aketengneng Shan earthquake gate. We find that there must be rotation of the stress field near the fault to allow for left-lateral slip along its more misoriented sections. Stress rotation could be accommodated by elevated pore pressure or a weakened fault core relative to a stronger surrounding crustal volume, as has been argued along other mature continental strike-slip faults, such as the San Andreas fault. Our results suggest that future studies evaluating earthquake gate-like rupture behavior should account for the orientation and activity of secondary structures that accommodate deformation away from the main fault. Our results also show that rotation of near-fault stresses is important to consider in multi-cycle earthquake rupture models.

Key Words
Earthquake Gate, Stress, Slip Rate, Off-Fault Deformation

Citation
Prush, V. B., Oskin, M. E., & Liu, J. (2020, 08). Off-fault deformation, stress field rotation, and the mechanical conditioning for rupture through an earthquake gate along the Altyn Tagh fault, northwest China. Poster Presentation at 2020 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology