How off-fault damage and splay faulting modulate shallow deformation in extensional detachment systems: new insights from 3D dynamic rupture simulations

James Biemiller, Alice-Agnes Gabriel, & Thomas Ulrich

Submitted September 11, 2022, SCEC Contribution #12542, 2022 SCEC Annual Meeting Poster #175

Despite their apparent misorientation to regional tectonic stresses, normal-sense detachment faults dipping < 30° are globally common structures that can help accommodate kilometers of crustal extension. Recent paleoseismic, geodetic, microstructural and experimental evidence suggests that active detachment faults can host infrequent large (Mw 7.0+) earthquakes that rupture neighboring and subsidiary fault segments, including the more steeply-dipping splay faults that typically dissect the hanging walls of detachments. In the absence of modern instrumentally well-recorded ruptures, untangling their mechanics and slip patterns from observations alone remains challenging. Here, we develop data-constrained 3D dynamic rupture simulations of the Mai’iu fault in Papua New Guinea to analyze the competition between dynamic rupture, splay fault reactivation, and off-fault damage in large detachment earthquakes. We detail how pre-existing splay fault geometry, sediment strength, and the thickness of overlying sediments impact shallow coseismic slip and deformation during large detachment ruptures. Splay fault rupture and an associated reduction of shallow detachment slip occurs in all simulations with synthetic or antithetic splay faults dipping 45°, 60°, or 75°. The largest splay fault slip spontaneously occurs in models with shallowly-dipping synthetic splay geometries and is accompanied by increased deep detachment slip and the elimination of all shallow detachment slip. Models accounting for coseismic off-fault inelastic damage via Drucker-Prager plasticity suggest that distributed and localized off-fault yielding may accommodate a major component of shallow rupture energy in seismogenic detachments overlain by thick deposits of weak sediments and involve dynamic initiation of new splay faults. Our results suggest that both splay faulting and off-fault inelastic damage can redirect shallow deformation away from the main detachment, which would shift, enhance and localize surface displacements above the hanging wall and thus may warrant further consideration in future hazard potential estimations of seismogenic detachments. Finally, we use these models to examine longer-term structural processes including rider block formation and detachment abandonment.

Biemiller, J., Gabriel, A., & Ulrich, T. (2022, 09). How off-fault damage and splay faulting modulate shallow deformation in extensional detachment systems: new insights from 3D dynamic rupture simulations. Poster Presentation at 2022 SCEC Annual Meeting.

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