Linda A. Reinen

The frictional behavior of lizardite and antigorite serpentinites: experiments, constitutive models, and implications for natural faults

Laboratory studies of the frictional behavior of rocks can provide important information about the strength and sliding stability of natural faults. We have conducted friction experiments on antigorite and lizardite serpentinites, rocks common to both continental and oceanic crustal faults. We conducted both velocity-step tests and timed-hold tests on bare surfaces and gouge layers of serpentinite at room temperature. We find that the coefficient of friction of lizardite serpentinite is quite low (0.15–0.35) and could explain the apparent low stresses observed on crustal transform faults, while that of antigorite serpentinite is comparable to other crustal rocks (0.50–0.85). The frictional behavior of both types of serpentinite is well described by a two-mechanism model combining state-variable-dominated behavior at high slip velocities and flow-dominated behavior at low velocities. The two-mechanism model is supported by data from velocity-step tests and timed-hold tests. The low velocity behavior of serpentinite is strongly rate strengthening and should result in stable fault creep on natural faults containing either antigorite or lizardite serpentinite.

The frictional behavior of serpentinite: Implications for aseismic creep on shallow crustal faults

Serpentine is common in many active faults and may be responsible for aseismic creep along segments of these faults. To test this, we have conducted a series of velocity stepping experiments to determine the frictional velocity dependence of serpentinite. We slid initially bare, rough surfaces of antigorite serpentinite at room temperature, with velocities from 0.0032 to 10.0 μm/s (1.0×102 to 3.2×105 mm/yr) and normal stresses from 25 to 125 MPa. We find that the velocity dependence of serpentinite undergoes a transition from velocity weakening at fast loading velocities to velocity strengthening at slow velocities and that this change is accompanied by other changes in the constitutive behavior. These results suggest that serpentinite should not be the site of instability initiation during sliding at plate velocities, but may permit propagation of unstable slip initiated elsewhere.

Determination of rock friction constitutive parameters using an iterative least squares inversion method

In order to understand the behavior of slip in fault zones that can lead to earthquakes, a detailed description of the constitutive behavior of the slipping rocks is needed. Rate- and state-variable constitutive laws have been very successful in describing results of laboratory studies of rock friction, but the actual determination of constitutive law parameter values has been limited to forward trial-and-error methods only. This paper presents a method of inverting rock friction experimental data to determine the parameters in the Dieterich-Ruina friction constitutive model. We use an iterative least squares method to solve the inverse problem, and we describe the solutions to several difficulties that arose owing to the nonlinearity of both the model and the iterated solution. These solutions include (1) using a finite differences method to estimate the derivatives of the constitutive model, (2) incorporating a vector to weight the relative “importance” of the friction observations, (3) using singular-value decomposition to solve the inverse problem, and (4) creating a damped version of the inverse routine to enhance the ability of the program to converge on a final solution. We explore the effects of machine stiffness and added noise on the covariances and correlations between model parameters. We find that increasing stiffness reduces parameter variances, covariances, and the strong correlations between some model parameters; increasing noise increases parameter variances and covariances without affecting the correlation between model parameters.

Two‐mechanism model for frictional sliding of serpentinite

Room-temperature experiments on antigorite serpentinite indicate that at least two mechanisms accommodate frictional sliding of the serpentinite: a rate-weakening, history-dependent mechanism which dominates at fast slip velocities and a rate-strengthening mechanism with no apparent history dependence which dominates at slow velocities. We present a two-mechanism model that successfully describes the two types of behavior observed in the friction experimental data without changing the constitutive parameters over the range of experimental conditions. Multiple mechanisms have been identified in higher-temperature experiments by previous workers and may be required for proper representation of all rock types at low slip rates and elevated temperatures.

Seismic and aseismic slip indicators in serpentinite gouge

Structures observed within experimentally deformed serpentinite gouges provide information that may be used to identify the seismic behavior of natural fault zones. In laboratory friction experiments serpentinite exhibits two modes of behavior: one can only result in stable fault creep, and the other may result in stable slip, but has the potential for earthquakes. The microstructures that form during these experiments reflect the deformation style of the serpentinite: distributed deformation results from aseismic fault creep, and localized deformation results from conditions favorable for seismic slip. Distributed deformation produces a crystallographic preferred orientation of the serpentine grains (S foliation). Localized deformation forms Riedel shears. Similar structures occur within a natural serpentinite shear zone from Monterey County, California, and suggest a history of stable fault creep with intermittent seismic events.

Slip styles in a Spring-Slider Model with a laboratory-derived constitutive law for serpentinite

A spring-slider model with a laboratory-derived constitutive law for serpentinite generates seismic slip, aseismic slip, or a combination of both. Such behavior is observed on oceanic ridge-transform systems and portions of the San Andreas Fault, and may be due to the presence of serpentinite. The constitutive law combines two sets of equations: a rate-dependent flow equation which dominates at low velocities, and a rate- and state-dependent set of equations which dominate at high velocities. The combined model produces transient behavior with maximum slip velocities well in excess of the loading velocity. Metastable slip prior to the onset of high-speed slip is produced in many simulations and may be analogous to the slow precursors observed for some oceanic ridge-transform earthquakes. This precursory slip results from the double-valued nature of the steady-state curve of the two-mechanism model in which the state-variable equations are rate weakening.