Berend A. Verberne

Frictional Properties of Sedimentary Rocks and Natural Fault Gouge from the Longmen Shan Fault Zone, Sichuan, China

In this paper, we report friction experiments performed on samples collected from the region hit by the 2008 Wenchuan earthquake in the Longmen Shan fault zone (LFZ) of Sichuan, southwestern China. The materials tested consisted of simulated gouges prepared from intact clay-rich mudstone and sandstone, a calcite limestone, plus a natural fault gouge from a trenched, surface rupture cutting the mudstone and sandstone. The clay-rich samples, including the natural gouge, were dominated by illite and quartz. In our experiments, we sheared 1-mm-thick gouge layers between saw-cut driver blocks, using a triaxial testing machine at conditions corresponding to ∼2 km depth in the LFZ. Temperature was varied from 25°C to 150°C, and to investigate the velocity dependence of friction, we stepped the shear displacement rate between 1.22 and 0.122 μ m/s. Our results show that the natural gouge was more illite-rich and much weaker than the protolith mudstone and sandstone and showed a steady-state friction coefficient of ∼0.4 compared with ∼0.6 for the latter. The limestone gouge displayed values of 0.6–0.7. All samples, except the limestone, showed stable, velocity-strengthening slip. The limestone showed velocity-strengthening at 25°C–50°C, but quasi-static oscillations at 100°C–150°C along with velocity-weakening behavior at 150°C. We apply our results to discuss the role of the sedimentary rocks studied during events such as the Wenchuan earthquake and argue that the clay-rich sediments of the region may have a damping effect upon ruptures propagating from depth, whereas the limestone may accelerate propagation, producing significant stress drops.

Superplastic nanofibrous slip zones control seismogenic fault friction

Understanding the internal mechanisms controlling fault friction is crucial for understanding seismogenic slip on active faults. Displacement in such fault zones is frequently localized on highly reflective (mirrorlike) slip surfaces, coated with thin films of nanogranular fault rock. We show that mirror-slip surfaces developed in experimentally simulated calcite faults consist of aligned nanogranular chains or fibers that are ductile at room conditions. These microstructures and associated frictional data suggest a fault-slip mechanism resembling classical Ashby-Verrall superplasticity, capable of producing unstable fault slip. Diffusive mass transfer in nanocrystalline calcite gouge is shown to be fast enough for this mechanism to control seismogenesis in limestone terrains. With nanogranular fault surfaces becoming increasingly recognized in crustal faults, the proposed mechanism may be generally relevant to crustal seismogenesis. Nanogranular microstructures found in simulated carbonate faults control the physical sliding mechanism during rupture. Nanofibers involved in fault rupture Changing fault properties during rupture dictates the size and extent of an earthquake. Faulting leads to well-known microstructures that may play a role in how natural faults slip during rupture. Verberne et al. investigated tiny, nanogranular fibers found in microstructures generated on simulated carbonate faults. A microphysical model was able to account for how the small and aligned fiber produced runaway fault slip, similar to that seen in natural faults. These small structures play a role in carbonate faulting and similar microstructures could control fault rupture in other types of rocks. Science, this issue p. 1342

Nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation: Crystal plasticity at sub-seismic slip rates at 18–150 °C

A central aim in fault mechanics is to understand the microphysical mechanisms controlling aseismic-seismic transitions in fault gouges, and to identify microstructural indicators for such transitions. We present new data on the slip stability of calcite fault gouges, and on microstructural development down to the nanometer scale. Our experiments consisted of direct shear tests performed dry at slip rates of 0.1–10 μm/s, at a constant normal stress of 50 MPa, at 18–150 °C. The results show a transition from stable to (potentially) unstable slip above ~80 °C. All samples recovered showed an optical microstructure characterized by narrow, 30–40-μm-wide, Riedel and boundary shear bands marked by extreme grain comminution, and a crystallographic preferred orientation (CPO). Boundary shear bands, sectioned using FIB-SEM (focused ion beam scanning electron microscopy), revealed angular grain fragments decreasing from 10 to 20 μm at the outer margins to ~0.3 μm in the shear band core, where dense aggregates of nanograins also occurred. Transmission electron microscopy, applied to foils extracted from boundary shears using FIB-SEM, combined with the optical CPO, showed that these aggregates consist of calcite nanocrystals, 5–20 nm in size, with the (104)[201] dislocation glide system oriented parallel to the shear plane and direction. Our results suggest that the mechanisms controlling slip include cataclasis and localized crystal plasticity. Because crystal plasticity is strongly thermally activated, we infer that the transition to velocity-weakening slip is likely due to enhanced crystal plasticity at >80 °C. This implies that tectonically active limestone terrains will tend to be particularly prone to shallow-focus seismicity.

Mechanical behavior and microstructure of simulated calcite fault gouge sheared at 20-600°C : Implications for natural faults in limestones

We report ring shear experiments on simulated calcite fault gouges performed at fixed temperatures (T) within the range from 20° to 600 °C. The experiments were performed wet, using pore fluid pressures (Pf) of 10 ≤ Pf ≤ 60 MPa. One series of experiments employed a constant effective normal stress ( σneff) of 50 MPa, while in a second series σneff was sequentially stepped from 30 to 100 MPa. In all experiments, sliding velocity (v) was stepped in the range from 0.03 to 100 µm/s. The results showed stable, velocity strengthening behavior at 20 °C, but velocity weakening at 100° to 550 °C (for all v-steps to <3 µm/s), which was frequently accompanied by stick–slip. At 600 °C, velocity strengthening occurred. Microstructural observations suggest increasing importance of ductile deformation with increasing temperature, as reflected by a localized shear band structure at 20 °C giving way to a pervasive, shear-plane-parallel grain shape fabric at 600 °C. Using existing flow equations for dense calcite polycrystals, we show that dislocation and/ or diffusion creep of 10–30 µm sized bulk gouge grains likely played a role in experiments performed at T ≥ 400 °C. We suggest that the observed velocity weakening behavior can be explained by a slip-mechanism involving dilatant granular flow in competition with creep-controlled compaction. Our results have important implications for the breadth of the seismogenic zone in limestone terrains, and for the interpretation of natural fault rock microstructures. Specifically, while samples sheared at 400-550 °C exhibited essentially brittle/ frictional mechanical behavior (stick–slip), the corresponding microstructures resembled that of a mylonite.

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.

Brittle and semibrittle creep of Tavel limestone deformed at room temperature

Deformation and failure mode of carbonate rocks depend on the confining pressure. In this study, the mechanical behaviour of a limestone with an initial porosity of 14.7 % is investigated at constant stress. At confining pressures below 55 MPa, dilatancy associated with micro-fracturing occurs during constant stress steps, ultimately leading to failure, similar to creep in other brittle media. At confining pressures higher than 55 MPa, depending on applied differential stress, inelastic compaction occurs, accommodated by crystal plasticity and characterized by constant ultrasonic wave velocities, or dilatancy resulting from nucleation and propagation of cracks due to local stress concentrations associated with dislocation pile-ups, ultimately causing failure. Strain rates during secondary creep preceding dilative brittle failure are sensitive to stress while rates during compactive creep exhibit an insensitivity to stress indicative of the operation of crystal plasticity, in agreement with elastic wave velocity evolution and microstructural observations.

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.

Deformation behavior of sandstones from the seismogenic Groningen gas field : Role of inelastic versus elastic mechanisms

Reduction of pore fluid pressure in sandstone oil, gas, or geothermal reservoirs causes elastic and possibly inelastic compaction of the reservoir, which may lead to surface subsidence and induced seismicity. While elastic compaction is well described using poroelasticity, inelastic and especially time‐dependent compactions are poorly constrained, and the underlying microphysical mechanisms are insufficiently understood. To help bridge this gap, we performed conventional triaxial compression experiments on samples recovered from the Slochteren sandstone reservoir in the seismogenic Groningen gas field in the Netherlands. Successive stages of active loading and stress relaxation were employed to study the partitioning between elastic versus time‐independent and time‐dependent inelastic deformations upon simulated pore pressure depletion. The results showed that inelastic strain developed from the onset of compression in all samples tested, revealing a nonlinear strain hardening trend to total axial strains of 0.4 to 1.3%, of which 0.1 to 0.8% were inelastic. Inelastic strains increased with increasing initial porosity (12–25%) and decreasing strain rate (10−5 s−1 to 10−9 s−1). Our results imply a porosity and rate‐dependent yield envelope that expands with increasing inelastic strain from the onset of compression. Microstructural evidence indicates that inelastic compaction was controlled by a combination of intergranular cracking, intergranular slip, and intragranular/transgranular cracking with intragranular/transgranular cracking increasing in importance with increasing porosity. The results imply that during pore pressure reduction in the Groningen field, the assumption of a poroelastic reservoir response leads to underestimation of the change in the effective horizontal stress and overestimation of the energy available for seismicity.