Time‐dependent recovery of microcrack damage and seismic wave speeds in deformed limestone

Nicolas Brantut

doi:10.1002/2015jb012324

Abstract

AbstractLimestone samples were deformed up to 5% inelastic axial strain at an effective confining pressure Peff=50 MPa in the cataclastic flow regime and subsequently maintained under constant static stress conditions (either isostatic of triaxial) for extended periods of time while elastic wave speeds and permeability were continuously monitored. During deformation, both seismic wave speeds and permeability decrease with increasing strain, due to the growth of subvertical microcracks and inelastic porosity reduction. During the static hold period under water‐saturated conditions, the seismic wave speeds recovered gradually, typically by around 5% (relative to their initial value) after 2 days, while permeability remained constant. The recovery in wave speed increases with increasing confining pressure but decreases with increasing applied differential stress. The recovery is markedly lower when the samples are saturated with an inert fluid as opposed to water. The evolution in wave speed is interpreted quantitatively in terms of microcrack density, which shows that the post‐deformation recovery is associated with a decrease in effective microcrack length, typically of the order to 10% after 2 days. The proposed mechanism for the observed damage recovery is microcrack closure due to a combination of backsliding on wing cracks driven by time‐dependent friction and closure due to pressure solution at contacts between propping particles or asperities and microcrack walls. The recovery rates observed in the experiments, and the proposed underlying mechanisms, are compatible with seismological observations of seismic wave speed recovery along faults following earthquakes.

Highlights

In the Earth’s upper crust, rocks accommodate deformation by fracturing and faulting
It has been shown that seismic wave speeds recover significantly after deformation in the cataclastic flow regime in limestone
The evidence for the latter were mostly inferred from the large increase in recovery rate observed in the presence of water by contrast with an inert fluid, but a direct microstructural signature remains elusive

Summary

Introduction

In the Earth’s upper crust, rocks accommodate deformation by fracturing and faulting. Pervasive microcrack networks are commonly found in rocks surrounding major faults, where they form the so-called “damage zones,” generated during the faulting process and due to dynamic loading during earthquakes propagation along seismogenic faults [e.g., Faulkner et al, 2010]. Damaged rocks have higher permeability, lower elastic moduli, and lower elastic wave speeds than their intact counterparts [e.g., Faulkner et al, 2006; Mitchell and Faulkner, 2008; Rempe et al, 2013], which has an impact on the stress state on the fault core and on the dynamics of earthquake propagation [e.g., Huang et al, 2014]. The timescales associated with these recovery processes control how quickly damaged rocks regain strength and stiffness and drive fluid flow around faults

Objectives

Methods

Findings

Discussion

Conclusion