The Mechanics of Frictional Healing and Slip Instability During the Seismic Cycle

Abstract

The seismic cycle of earthquake failure and strength recovery attests to the frictional instability and healing of tectonic faults. However, our understanding of these processes and the detailed underlying mechanics is limited. Here, we discuss frictional strength recovery and stability in the broad historical context of early studies. These studies introduced the concepts of static and dynamic friction, applied to a wide range of materials, and led to modern friction laws that account for constitutive relations between friction, slip, and strain. Friction memory effects and the processes that govern the nucleation of instability, creep relaxation, and frictional strength recovery during the seismic cycle are discussed in the context of rate and state friction (RSF) laws. One key underlying concept is that to host repeated earthquakes, tectonic faults must exhibit both frictional healing and frictional and elastic properties that allow instabilities to nucleate. A particular interest for frictional healing is the independent roles of contact time and slip as related to frictional aging and contact rejuvenation, including the effects of granular consolidation and dilation. We address the conditions under which frictional healing is dictated primarily by either time-dependent contact junction aging, as described by the Dieterich law for friction state evolution, or slip-dependent healing, as described by the Ruina law. We present new data from slide-hold-slide (SHS) friction tests and show that the loading rate effect on frictional healing can be accounted for by considering the effective healing displacement given by the product of hold time and loading velocity. The full time series of aging tests, including relaxation and reloading during ‘SHS’ tests, are used to recover friction parameters for comparison to data from velocity step tests and SHS tests at a range of slip velocities. We combine friction data from several types of tests and use them to critically evaluate the internal consistency of the RSF laws. We find that the Ruina law provides a closer match to laboratory friction data than the Dieterich law although neither law matches the data fully.