Dynamic Simulations of Earthquake Cycles in Strike-Slip Fault Zones

Abstract

Earthquakes are a complex phenomena occurring over a variety of spatial and temporal scales. The fault zones hosting such earthquakes are also structurally complex networks of fractures that evolve through time. This thesis presents novel numerical simulations of the spatial and temporal complexities of fault zones over multiple earthquake sequences, to uncover the physics of long-term earthquake cycles that include earthquakes and aseismic events. We use seismic, geological, and geodetic observations as well as laboratory experiments to constrain the dynamics of earthquake wave propagation and quasi-static strain accumulation in strike-slip fault systems.

In chapter 2, we 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 shallow fault damage zone produces shallower earthquakes with earthquake depths centered around two locations (i.e., bimodal depth distribution), as opposed to a deep fault damage zone with the earthquake depths centered around a single location (i.e., unimodal depth distribution). Our study suggests that imaging the geometry and physical properties of fault damage zones could affect the depths of future earthquakes with implications for earthquake probabilistic hazard assessment.

In chapter 3, we simulate the evolution of fault damage zones by prescribing coseismic damage accumulation and interseismic healing as constrained by seismic and geodetic observations. Depending on the compliance (i.e., the ability to accommodate deformation) of the damage zone with respect to the surrounding host rock, the maturity of fault zone can vary, with higher compliance corresponding to a more mature fault zone. We find that our models with immature fault zone tends to produce smaller earthquakes whose slip does not reach the surface of the earth, and the duration between earthquakes is irregular. As fault zones become more mature in the models, earthquakes can rupture to the surface and occur more regularly. Our results highlight a link between regional seismic hazard and fault structural maturity.

Finally, chapter 4 investigates the predictive behavior of precursory seismic signals by simulating earthquake sequences with prescribed seismic velocity changes as precursors, under different homogeneous and heterogeneous stress conditions. We find that such precursory velocity changes can affect how fast and how early the earthquakes nucleate, with earlier onset of precursors leading to earlier earthquake nucleation. The slow-slip events seen in the earthquake cycle simulations with fault zones grow into dynamic earthquakes in the presence of velocity precursors. Precursory velocity changes also amplify the shear stress heterogeneities along the fault, which may result from fault damage zones or self-similar background stresses.