Characteristics of earthquake ruptures and dynamic off-fault deformation on propagating faults

Abstract:

Natural fault networks are geometrically complex systems that evolve due to dynamic earthquake ruptures and aseismic deformation. To better understand each contribution we simulate both earthquake rupture dynamics and long-term deformation in a 2.5D visco-elasto-plastic crust subjected to rate- and state-dependent friction. Localized fault growth is simulated for four bulk rheologies ranging from persistent velocity weakening to velocity strengthening. In each bulk rheology faults predominantly localize and grow due to aseismic deformation. Yet, episodic fault growth at more realistic growth rates is obtained when friction transitions from velocity-strengthening to velocity-weakening with increasing plastic strain. Fault growth occurs under Riedel and conjugate angles and transitions towards wing cracks. Distributed off-fault deformation and localised splay faults are typically formed during earthquake ruptures. We observe that the fault-normal width of the outer damage zone saturates with increasing fault length due to the finite depth of the seismogenic zone. We also observe that dynamically and statically evolving stress fields from neighboring fault strands affect primary and secondary fault growth and thus that normal stress variations affect earthquake sequences. Finally, we find that the amount of off-fault deformation distinctly depends on the degree of optimality of a fault with respect to the prevailing but dynamically changing stress field. Simulation of off-fault deformation on faults parallel to the loading direction produces a 6.5-fold higher off-fault energy dissipation than on an optimally oriented fault, which leads to a 1.5-fold larger stress drop. The misalignment of the fault with respect to the static stress field thus facilitates off-fault deformation. These results that converge with grid size imply that fault geometries bend, individual fault strands interact, and optimal orientations and off-fault deformation vary through space and time.