Effects of Low-Velocity Fault Damage Zones on Long-Term Earthquake Behaviors on Mature Strike-Slip Faults

Mature strike-slip faults are usually surrounded by a narrow zone of damaged rocks characterized by low seismic wave velocities. Observations of earthquakes along such faults indicate that seismicity is highly concentrated within this fault damage zone. However, the long-term influence of the fault damage zone on complete earthquake cycles, that is, years to centuries, is not well understood. We simulate aseismic slip and dynamic earthquake rupture on a vertical strike-slip fault surrounded by a fault damage zone for a thousand-year timescale using fault zone material properties and geometries motivated by observations along major strike-slip faults. The fault damage zone is approximated asan elastic layer with lower shear wave velocity than the surrounding rock. We find that dynamic wave reflections, whose characteristics are strongly dependent on the width and the rigidity contrast of the fault damage zone, have a prominent effect on the stressing history of the fault. The presence of elastic damage can partially explain the variability in the earthquake sizes and hypocenter locations along a single fault, which vary with fault damage zone depth, width and rigidity contrast from the host rock. The depth extent of the fault damage zone has a pronounced effect on the earthquake hypocenter locations, and shallower fault damage zones favor shallower hypocenters with a bimodal distribution of seismicity along depth. Our findings also suggest significant effects on the hypocenter distribution when the fault damage zone penetrates to the nucleation sites of earthquakes, likely being influenced by both lithological (material) and rheological (frictional) boundaries. Plain Language Summary Large strike-slip earthquakes tend to create a zone of fractured network surrounding the main fault. This zone, referred to as a fault damage zone, becomes highly localized as the fault matures, with a width of few hundred meters. The influence of this fault damage zone on earthquake characteristics remains elusive since we do not have enough long-term observations along a single fault. We use numerical simulations to examine the behavior of earthquake nucleation and rupture dynamics on a fault surrounded by a damage zone over a thousand-year timescale. Our simulations reveal that the reflection of seismic waves from the fault damage zone boundaries leads to complexity in earthquake sequences, such as variability in earthquake locations and sizes. We also show that a shallowfault damage zone produces shallower earthquakes with the earthquake depths centered around two locations (bimodal), as opposed to a deep fault damage zone with the earthquake depths centered around a single location (unimodal). Our study suggests that imaging the geometry and physical properties of fault damage zones could potentially give us clues about depths of future earthquakes and improve earthquake probabilistic hazard assessment.