Earthquake cycle simulations for long-term damage evolution and healing
in low velocity fault zones
Abstract
Most major strike slip fault systems are surrounded by narrow zones of
damaged rocks that can have a crucial effect on earthquake dynamics.
Owing to the limited timescale of seismic observations, the structural
evolution of this damaged zone and its long-term effects are not well
understood. We study the mechanical response of damage evolution and
healing over multiple earthquake cycles using fully dynamic earthquake
cycle simulations in a 2D vertical strike-slip fault. We use a spectral
element method to discretize the domain and a rate-state dependent
friction on the fault to simulate all the stages of the seismic cycle,
including interseismic slip, earthquake nucleation, rupture propagation
and postseismic slip. A narrow, compliant fault-parallel elastic layer
with low seismic wave velocities is introduced to emulate near-fault
damage. The low-velocity layer reflects waves during the seismic period
giving rise to stress heterogeneities that persist through multiple
seismic cycles. We introduce a scalar damage multiplier ‘d (01)‘ that
reduces the effective shear modulus during the earthquake and increases
it during the interseismic period. We study different realizations of d
and h through time: the simplest model consists of a constant increase
in damage and healing over each seismic cycle and the more complex model
includes a heterogeneous damage proportional to the peak slip velocity
along the fault. The distribution and evolution of dynamic parameters
(shear stresses and slip velocities) and static earthquake parameters
(cumulative slip and static stress drops) as a function of the damage is
shown and compared to the existing continuum damage rheology models and
field geologic observations. These simulations will provide a better
insight into the partitioning of damage and healing during seismic
cycles and the saturation of damage in mature fault zones.