Supershear transition on 3D faults with favorable heterogeneity

Mary Agajanian, & Nadia Lapusta

Published September 8, 2024, SCEC Contribution #13815, 2024 SCEC Annual Meeting Poster #135

Supershear ruptures are characterized by propagation speeds which exceed the shear wave speed of the surrounding bulk. These ruptures have a characteristic Mach front which carries large shaking velocities far from the fault. The traditional Burridge-Andrews mechanism of supershear transition (Andrews, 1976) requires faults to have sufficiently low seismic ratio to transition to supershear rupture speeds. For linear slip-weakening friction, this is synonymous with high prestress on the fault. Liu and Lapusta (2008) used a fully-dynamic 2D in-plane single-rupture model to study supershear transition. They showed that supershear transition can be triggered by favorable fault heterogeneity even when shear stresses are lower than required by the Burridge-Andrews mechanism.

Our study extends their work to 3D rupture sequences on faults governed by rate and state friction. We identify a parameter regime for a homogeneous seismogenic fault in which earthquake rupture sequences are all sub-shear. We demonstrate two mechanisms by which supershear transition may be achieved in the long-term sequence. First, we introduce a compact, instability-promoting heterogeneity. The resulting lower peak stress due to the patch decreases the local seismic ratio and allows for the rupture to transition to supershear. Then, by modifying the parameters of the velocity strengthening loading region, we increase the stressing on the seismogenic zone. The resulting increase in prestress also lowers the seismic ratio, thus promoting supershear transition. Lastly, we demonstrate that supershear transition cannot occur in (commonly used) quasi-dynamic sequence simulations which include a radiation damping approximation of inertial effects rather than the true wave-mediated stress changes. This approximation underestimates the shear stress ahead of the propagating rupture. Thus, ruptures travel much slower than the Rayleigh wave speed. We find that incorporating even limited stress-concentrating dynamic effects ensures realistic rupture speeds and allows for supershear transition. Although this case does not completely account for the stress redistribution on the fault, it allows for more accurate characterization of the long-term behavior of faults with only a marginal increase in computational cost.

Citation
Agajanian, M., & Lapusta, N. (2024, 09). Supershear transition on 3D faults with favorable heterogeneity. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)