Evolutions of fault damage zones and consequences for earthquake triggering

Abstract

Studying the interactions between underground fluid and faults is crucial to help us understand the earthquakes which occur at relatively low driving stresses. In this thesis, I investigate the permeability evolution in fault damage zones and how it influences the triggering of fluid induced earthquakes. In addition, I use ambient noise auto-correlation to detect changes of seismic velocity which are associated with such fluid induced earthquakes, to obtain a better constraint of properties changes in the damage zones.

In Chapter 1, I focus on the modeling and prediction of the hydromechanical response of faults due to fluid injections. In order to couple the mechanical and hydraulic responses, I propose to use poroelastic modeling while considering the evolution of permeability due to mechanical deformations. I demonstrate the importance of applying a more realistic fault architecture that includes a damage zone and considering permeability evolution by comparing simulation results from different models. I show that the high-permeable damage zone, the poroelastic response, and the permeability evolution can accelerate the pore pressure diffusion process during and after the wastewater injection. Then, I investigate a wastewater induced earthquake sequence called Guy-Greenbrier sequence in Arkansas, United States. By using a geologically based model, I simulated the injection processes and show that the existence of highly permeable damage zones could facilitate the diffusion process to greater depth and induced earthquakes in the basement layer.

In Chapter 2, I continue to explore the effect of permeability evolution in fault damage zones in longer time scales. During long-term earthquake cycles, the micro-crack density, porosity, and permeability in damage zones change with time, which are influenced by coseismic damaging and interseismic healing. During earthquakes, existing fractures in the off-fault media could expand and new cracks will be opened, which will result in an increase in the porosity of the damage zones, and the permeability will increase accordingly. On the contrary, the cracks will gradually heal during interseismic periods, which will result in a decrease in the porosity. In this study, I use a single degree of freedom spring slider fault model to simulate the earthquake cycles in long time scales. By considering the effect of coseismic damaging and interseismic thermal enhanced healing, I show some cases where the change of permeability in the damage zones could alter the effective normal stress on the fault plane and change the occurrence patterns of the earthquake cycles. The results reveal the importance of considering permeability evolution in earthquake simulations and give more insights into earthquake risk assessments.

In order to validate the previous simulation results of the induced Guy-Greenbrier earthquake sequence with observation data, in Chapter 3, I use seismic ambient noise auto-correlation to detect seismic velocity changes near three seismic stations in the Guy-Greenbrier area. By using one-year-long continuous waveform records, I measure time and depth dependent changes in seismic velocities. By comparing the relative velocity changes with the seismic catalog and peak ground accelerations in three seismic stations, I find that several decreases in seismic velocities are caused by coseismic damages. Given the relatively small ground shaking but the relatively large velocity decreases, I find that the pore pressure in the basement layer has been increased due to wastewater injection, which implies the highly permeable fault damage zones could lead injected wastewater into deeper depth and induce earthquakes in the basement layer.