Jianye Chen

Water vaporization promotes coseismic fluid pressurization and buffers temperature rise

We investigated the frictional properties of carbonate-rich gouge layers at a slip rate of 1.3 m/s, under dry and water-saturated conditions, while monitoring temperature at different locations on one of the gouge-host rock interfaces. All experiments showed a peak frictional strength of 0.4–0.7, followed by strong slip weakening to steady-state values of 0.1–0.3. Experiments which used a pore fluid with a constant drainage path to the atmosphere showed the development of a temperature plateau beyond 100 °C, contemporaneous with the dynamic slip weakening, and consistent with thermodynamic considerations of ongoing vaporization of pore water. Upon pore fluid vaporization, the pore pressure increases while the temperature is buffered endothermically, such that the pore water moves along the liquid-vapor transition curve in a pressure-temperature phase diagram. Pore fluid phase transitions of this kind are expected to occur in natural earthquakes at relatively shallow crustal levels, enhancing fluid pressurization while impeding the achievement of high temperatures. Therefore, the operation of vaporization may help explain the low downhole temperature anomalies obtained shortly after large earthquakes.

Rate and state frictional and healing behavior of carbonate fault gouge explained using microphysical model

Classical rate-and-state friction (RSF) laws are widely applied in modeling earthquake dynamics but generally using empirically determined parameters with little or no knowledge of, or quantitative account for, the controlling physical mechanisms. Here a mechanism-based microphysical model is developed for describing the frictional behavior of carbonate fault gouge, assuming that the frictional behavior seen in lab experiments is controlled by competing processes of rate-strengthening intergranular sliding versus contact creep by pressure solution. By solving the controlling equations, derived from kinematic and energy/entropy balance considerations, and employing a microphysical model for rate-strengthening grain boundary friction plus standard creep equations for pressure solution, we simulate typical lab-frictional tests, namely, “velocity stepping” and “slide-hold-slide” test sequences, for velocity histories and environmental conditions employed in previous experiments. The modeling results capture all of the main features and trends seen in the experimental results, including both steady state and transient aspects of the observed behavior, with reasonable quantitative agreement. To our knowledge, ours is the first mechanism-based model that can reproduce RSF-like behavior in terms of microstructurally verifiable processes and state variables. Since it is microphysically based, we believe that our modeling approach can provide an improved framework for extrapolating friction data to natural conditions.

Effects of healing on the seismogenic potential of carbonate fault rocks : Experiments on samples from the Longmenshan Fault, Sichuan, China

Fault slip and healing history may crucially affect the fault seismogenic potential in the earthquake nucleation regime. Here we report direct shear friction tests on simulated gouges derived from a carbonate fault breccia, and from a clay/carbonate fault-core gouge, retrieved from a surface exposure of the Longmenshan Fault Zone (LFZ) which hosted the 2008 Wenchuan earthquake. The experiments were conducted under dry and hydrothermal conditions, at temperatures up to 140°C, at an effective normal stress of 50 MPa, and involved sequential velocity-stepping (VS), slide-hold-slide (SHS), and velocity-stepping stages. Dry tests performed on breccia-derived samples showed no dependence of (quasi) steady state friction (μss) on SHS or VS history, and a log linear relation between transient peak healing (Δμpk) and hold time, or classical “Dieterich-type” healing behavior. By contrast, all experiments conducted under hydrothermal conditions were characterized by “non-Dieterich” healing behavior. This included (1) an increase in μss upon resliding after a hold period and (2) an increase in friction rate parameter (a − b), after SHS testing. Comparison with previous results suggests that the healing behavior seen in our wet tests may be attributed to solution transfer processes occurring during hold periods. Our findings imply that the shallow portions of faults with carbonate/clay-rich cores (e.g., the LFZ) can heal much faster than previously recognized, while the upper limit of the seismogenic zone may migrate to deeper levels during interseismic periods. These effects have important implications for understanding the seismic cycle in tectonically active carbonate terrains.

Microphysically derived expressions for rate-and-state friction parameters, a, b, and Dc

Rate-and-state friction (RSF) laws are extensively applied in fault mechanics but have a largely empirical basis reflecting only limited understanding of the underlying physical mechanisms. We recently proposed a microphysical model [Chen and Spiers, 2016] describing the frictional behavior of a granular fault gouge undergoing deformation in terms of granular flow accompanied by thermally activated creep and intergranular sliding at grain contacts. Numerical solutions reproduced typical experimental results well. Here, we extend our model to obtain physically meaningful, analytical expressions for the steady-state frictional strength and standard RSF parameters, a, b, and Dc. The frictional strength contains two components, namely grain boundary friction and friction due to intergranular dilatation. The expressions obtained for a and b linearly reflect the rate dependence of these two terms. Dc scales with slip band thickness and varies only slightly with velocity. The values of a, b and Dc predicted show quantitative agreement with previous experimental results, and inserting their values into classical RSF laws gives simulated friction behavior that is consistent with the predictions of our numerically implemented model for small departures from steady state. For large velocity-steps, the model produces mixed RSF behavior that falls between the Slowness and Slip laws, e.g. with an intermediate equivalent slip(-weakening) distance d0. Our model possesses the interesting property not only that a and b are velocity dependent but also that Dc and d0 scale differently from classical RSF models, potentially explaining behaviour seen in many hydrothermal friction experiments and having substantial implications for natural fault friction.

Vaporization of fault water during seismic slip

Laboratory and numerical studies, as well as field observations, indicate that phase transitions of pore water might be an important process in large earthquakes. We present a model of the thermo-hydro-chemo-mechanical processes, including a two-phase mixture model to incorporate the phase transitions of pore water, occurring during fast slip (i.e., a natural earthquake) in order to investigate the effects of vaporization on the coseismic slip. Using parameters from typical natural faults, our modeling shows that vaporization can indeed occur at the shallow depths of an earthquake, irrespective of the wide variability of the parameters involved (sliding velocity, friction coefficient, gouge permeability and porosity, and shear-induced dilatancy). Due to the fast kinetics, water vaporization can cause a rapid slip weakening even when the hydrological conditions of the fault zone are not favorable for thermal pressurization, e.g., when permeability is high. At the same time, the latent heat associated with the phase transition causes the temperature rise in the slip zone to be buffered. Our parametric analyses reveal that the amount of frictional work is the principal factor controlling the onset and activity of vaporization and that it can easily be achieved in earthquakes. Our study shows that coseismic pore fluid vaporization might have played important roles at shallow depths of large earthquakes by enhancing slip weakening and buffering the temperature rise. The combined effects may provide an alternative explanation for the fact that low-temperature anomalies were measured in the slip zones at shallow depths of large earthquakes.

Rock magnetic expression of fluid infiltration in the Yingxiu‐Beichuan fault (Longmen Shan thrust belt, China)

Fluid infiltration within fault zones is an important process in earthquake rupture. Magnetic properties of fault rocks convey essential clues pertaining to physicochemical processes in fault zones. In 2011, two shallow holes (134 and 54 m depth, respectively) were drilled into the Yingxiu-Beichuan fault (Longmen Shan thrust belt, China), which accommodated most of the displacement of the 2008 Mw 7.9 Wenchuan earthquake. Fifty-eight drill core samples, including granitic host rock and various fault rocks, were analyzed rock-magnetically, mineralogically, and geochemically. The magnetic behavior of fault rocks appears to be dominated by paramagnetic clay minerals. Magnetite in trace amounts is identified as the predominant ferrimagnetic fraction in all samples, decreasing from the host rock, via fault breccia to (proto-)cataclasite. Significant mass-losses (10.7–45.6%) are determined for the latter two with the “isocon” method. Volatile contents and alteration products (i.e., chlorite) are enriched toward the fault core relative to the host rocks. These observations suggest that magnetite depletion occurred in these fault rocks—exhumed from the shallow crust—plumbed by fluid-assisted processes. Chlorite, interpreted to result from hydrothermal activity, occurs throughout almost the entire fault core and shows high coefficients of determination (R2 > 0.6) with both low and high-field magnetic susceptibility. Close relationships, with R2 > 0.70, are also observed between both low and high-field magnetic susceptibility and the immobile elements (e.g., TiO2, P2O5, MnO), H2O+, and the calculated mass-losses of fault rocks. Hence, magnetic properties of fault rocks can serve as proxy indicators of fluid infiltration within shallow fault zones.

Microscale cavitation as a mechanism for nucleating earthquakes at the base of the seismogenic zone

Major earthquakes frequently nucleate near the base of the seismogenic zone, close to the brittle-ductile transition. Fault zone rupture at greater depths is inhibited by ductile flow of rock. However, the microphysical mechanisms responsible for the transition from ductile flow to seismogenic brittle/frictional behaviour at shallower depths remain unclear. Here we show that the flow-to-friction transition in experimentally simulated calcite faults is characterized by a transition from dislocation and diffusion creep to dilatant deformation, involving incompletely accommodated grain boundary sliding. With increasing shear rate or decreasing temperature, dislocation and diffusion creep become too slow to accommodate the imposed shear strain rate, leading to intergranular cavitation, weakening, strain localization, and a switch from stable flow to runaway fault rupture. The observed shear instability, triggered by the onset of microscale cavitation, provides a key mechanism for bringing about the brittle-ductile transition and for nucleating earthquakes at the base of the seismogenic zone.Earthquakes frequently occur in the brittle-ductile transition near the base of the seismogenic zone. Using shear experiments on calcite faults, here the authors show that microscale cavitation plays a role in controlling the brittle-ductile transition, and in nucleating earthquakes at the base of the seismogenic zone.

Seismogenic Potential of a Gouge‐filled Fault and the Criterion for its Slip Stability: Constraints from a Microphysical Model

Physical constraints for the parameters of the rate-and-state friction (RSF) laws have been mostly lacking. We presented such constraints based on a microphysical model and demonstrated the general applicability to granular fault gouges deforming under hydrothermal conditions in a companion paper. In this paper, we examine the transition velocities for contrasting frictional behavior (i.e. strengthening to weakening and vice versa) and the slip stability of the model. The model predicts a steady-state friction coefficient that increases with slip rate at very low and high slip rates, and decreases in between. This allows the transition velocities to be theoretically obtained and the unstable slip regime (Vs→w< V <Vw→s) to be defined. In a spring-slider configuration, linear perturbation analysis provides analytical expressions of the critical stiffness (Kc) below which unstable slip occurs, and of the critical recurrence wavelength (Wc) and static stress drop (Δμs) associated with self-sustained oscillations or stick-slips. Numerical implementation of the model predicts frictional behavior that exhibits consecutive transitions from stable sliding, via periodic oscillations, to unstable stick-slips with decreasing elastic stiffness or loading rate, and gives Kc, Wc, Δμs, Vs→w and Vw→s values that are consistent with the analytical predictions. General scaling relations of these parameters given by the model are consistent with previous interpretations in the context of RSF laws, and agree well with previous experiments, testifying to high validity. From these physics-based expressions which allow a more reliable extrapolation to natural conditions, we discuss the seismological implications for natural faults and present topics for future work.

Thermal Alteration of Pyrite to Pyrrhotite During Earthquakes: New Evidence of Seismic Slip in the Rock Record

Seismic slip zones convey important information on earthquake energy dissipation and rupture processes. However, geological records of earthquakes along exhumed faults remain scarce. They can be traced with a variety of methods that establish the frictional heating of seismic slip, although each has certain assets and disadvantages. Here we describe a mineral magnetic method to identify seismic slip along with its peak temperature through examination of magnetic mineral assemblages within a fault zone in deep‐sea sediments cored from the Japan Trench—one of the seismically most active regions around Japan—during the Integrated Ocean Drilling Program Expedition 343, the Japan Trench Fast Drilling Project. Fault zone sediments and adjacent host sediments were analyzed mineral magnetically, supplemented by scanning electron microscope observations with associated energy dispersive X‐ray spectroscopy analyses. The presence of the magnetic mineral pyrrhotite appears to be restricted to three fault zones occurring at ~697, ~720, and ~801 m below sea floor in the frontal prism sediments, while it is absent in the adjacent host sediments. Elevated temperatures and coseismic hot fluids as a consequence of frictional heating during earthquake rupture induced partial reaction of preexisting pyrite to pyrrhotite. The presence of pyrrhotite in combination with pyrite‐to‐pyrrhotite reaction kinetics constrains the peak temperature to between 640 and 800°C. The integrated mineral‐magnetic, microscopic, and kinetic approach adopted here is a useful tool to identify seismic slip along faults without frictional melt and establish the associated maximum temperature.