Temporal Changes in Fault damage Zones: Effects of Coseismic Damage Accumulation and Interseismic Healing on Earthquake Cycles

Abstract:

Active faults are usually surrounded by several hundred meters wide zones of localized deformation across the fault. This region of deformation consisting of a dense fracture network is macroscopically viewed as an elastic layer with low seismic wave velocities and referred to as a fault damage zone. The strength of the fault damage zone evolves throughout the seismic cycle, but the details of the mechanism and the nature of this evolution remain elusive. Understanding the structural evolution of the fault damage zone is the key to unravel the location, recurrence, the stressing history, and the probability of subsequent earthquakes in an active fault zone.

We use fully-dynamic numerical simulations to understand the effects of the coseismic damage accumulation through multiple earthquakes and the rate at which the fault damage zone regains strength during the interseismic periods. The fault damage zone is modeled as an elastic layer with a lower shear wave velocity compared to the surrounding host rock. We model the damage accumulation and the interseismic healing as changes in the shear wave velocity of an elastic layer surrounding a strike-slip fault. We use two-dimensional earthquake cycle models of strike-slip faults with mode III rupture. Using observations from Wenchuan, Landers, and Nojima, we constrain the changes in shear wave velocity and the rate of interseismic healing. Our results demonstrate how the coseismic damage accumulation, the interseismic healing rate, and the level of permanent damage after each earthquake influence the surface and subsurface slip accumulation and the recurrence intervals of strike-slip earthquakes. We show that faster healing rate implies longer period between earthquakes, whereas a slower healing rate exhibits shorter interevent durations. Furthermore, permanent damage restricts the size of earthquakes and influences the size and location of subsequent events by adding complexity to fault stress redistribution. We attribute the results to temporal changes in the earthquake nucleation size as the fault zone rigidity evolves. Our results will help quantify the partitioning of damage and healing during seismic cycles and unveil the role of the structural evolution of fault damage zones in the nucleation of small and large earthquakes.