In crustal faults dominated by granitoid gouges, the frictional-viscous transition marks a significant change in strength constraining the lower depth limit of the seismogenic zone. Dissolution-precipitation creep (DPC) may play an important role in initiating this transition, especially within polymineralic materials. Yet, it remains unclear to what extent DPC contributes to the weakening of granitoid gouge materials at the transition. Here we conducted sliding experiments on wet granitoid gouges to large displacement (15 mm), at an effective normal stress and pore fluid pressure of 100 MPa, at temperatures of 20-650°C, and at sliding velocities of 0.1-100 μm/s, which are relevant for earthquake nucleation. Gouge shear strengths were generally ~75 MPa even at temperatures up to 650°C and at velocities > 1 mm/s. At velocities ≤ 1 mm/s, strengths decreased at temperatures ≥ 450°C, reaching a minimum of 37 MPa at the highest temperature and lowest velocity condition. Microstructural observations showed that, as the gouges weakened, the strain localized into thin, dense, and ultrafine-grained (≤ 1 μm) principal slip zones, where nanopores were located along grain contacts and contained minute biotite-quartz-feldspar precipitates. Though poorly constrained, the stress sensitivity exponent n decreased from ≥17 at 20°C to ~2 at 650°C at the lowest velocities. These findings suggest that high temperature, slow velocity and/or small grain sizes promote DPC-accommodated granular flow over cataclastic frictional granular flow, leading to the observed weakening and strain localization. Field observations together with extrapolation suggest that DPC-induced weakening occurs at depths of 7-20 km depending on geothermal gradient.

In recent laboratory experiments, varying nucleation locations of accelerating slip with changing nucleation lengths were observed. Spatial variations in effective normal stress, due to the controlling influence on fault strength and fracture energy, play an important role. We quantitatively explain how spatially heterogeneous effective normal stresses affect earthquake nucleation and slip behavior. We simulate a meter-scale laboratory experiment in a numerical earthquake sequence model with stochastically variable normal stresses. We identify five regimes of earthquake nucleation and slip behaviors, controlled by the ratio of the heterogeneity wavelength (\(\lambda\)) to the nucleation length (\(L_c\)). When \(\lambda\) is much smaller than \(L_c\), full ruptures are observed. Slip rates and recurrence intervals are similar to those on homogeneous faults with comparable averaged normal stress. When \(\lambda\) is much larger than \(L_c\), slow slip events and partial ruptures occur frequently and the nucleation length of each earthquake depends on the local stress level. We find locations of nucleation and arrest in both low and high normal stress regions (LSR and HSR, respectively) when \(\lambda\) and \(L_c\) are of the same magnitude. When \(\lambda\) is larger than \(L_c\), earthquakes nucleate in LSRs, and arrest in HSRs. However, HSRs and LSRs switch these roles when \(\lambda\) is smaller than \(L_c\). Interestingly, we observe that nucleation migrates from an LSR to its neighboring HSR in one earthquake, when \(\lambda\) is between the minimum and maximum local nucleation lengths. We observe a large amount of aseismic slip and associated stress drop in the initial LSR, which might be linked to the migration of foreshocks as documented in natural and laboratory observations. This improved understanding of earthquake nucleation is important in estimating the seismic potential of different fault patches for natural and induced seismicity.

Nucleation of earthquake slip at the plate boundary fault (décollement) in subduction zones has been widely linked to the frictional properties of subducting sedimentary facies. However, recent seismological and geological observations suggest that the décollement develops in the subducting oceanic crust in the depth range of the seismogenic zone, at least in some cases. To understand the frictional properties of oceanic crustal material and their influence on seismogenesis, we performed hydrothermal friction experiments on simulated fault gouges of altered basalt, at temperatures of 100-550 ℃. The friction coefficient (μ) lies around 0.6 at most temperature conditions but a low μ down to 0.3 was observed at the highest temperature and lowest velocity condition. The velocity dependence of μ, a−b, changes with increasing temperature from positive to negative at 100-200 ºC and from negative to positive at 450-500 ºC. Compared to gouges derived from sedimentary facies, the altered basalt gouge showed potentially unstable velocity weakening over a wider temperature range. Microstructural observations and microphysical interpretation infer that competition between dilatant granular flow and viscous compaction through pressure-solution creep of albite contributed to the observed transition in a−b. Alteration of oceanic crust during subduction produces fine grains of albite and chlorite through interactions with interstitial water, leading to reduction in its frictional strength and an increase in its seismogenic potential. Therefore, shear deformation possibly localizes within the altered oceanic crust leading to a larger potential for the nucleation of a megathrust earthquake in the depth range of the seismogenic zone.

We use densely spaced campaign GPS observations and laboratory friction experiments on fault rocks from one of the world’s most rapidly slipping low-angle normal faults, the Mai’iu fault in Papua New Guinea, to investigate the nature of interseismic deformation on active low-angle normal faults. GPS velocities reveal 8.3±1.2 mm/yr of horizontal extension across the Mai’iu fault, and are fit well by dislocation models with shallow fault locking (above 2 km depth), or by deeper locking (from ~5-16 km depth) together with shallower creep. Laboratory friction experiments show that gouges from the shallowest portion of the fault zone are predominantly weak and velocity-strengthening, while fault rocks deformed at greater depths are stronger and velocity-weakening. Evaluating the geodetic and friction results together with geophysical and microstructural evidence for mixed-mode seismic and aseismic slip at depth, we find that the Mai’iu fault is most likely strongly locked at depths of ~5-16 km and creeping updip and downdip of this region. Our results suggest that the Mai’iu fault and other active low-angle normal faults can slip in large (M > 7) earthquakes despite near-surface interseismic creep on frictionally stable clay-rich gouges.

Probabilistic seismic hazard analysis (PSHA) is the standard method used for designing earthquake-resistant infrastructure. In recent years, several unexpected and destructive earthquakes have sparked criticism of the PSHA methodology. The seismological part of the problem is the true frequency-magnitude distribution of regional seismicity. Two major models exist, the Gutenberg-Richter (G-R) and the Characteristic Earthquake (CE) model, but it is difficult to choose between them. That is because the instrumental, historical, and paleoseimological data available are limited in many regions of interest. Here we demonstrate how a friction experiment on aggregates of glass beads can produce both regular (CE equivalent) and irregular (G-R equivalent) stick-slip. Using a new rotary shear apparatus we produced and analysed large catalogs of acoustic emission (AE) events related to stick-slip. The distributions of AE sizes, interevent times and interevent distances were found to be sensitive to particle size and the applied normal stress, and, to a lesser degree, the stiffness of the loading apparatus. More importantly, the system spontaneously switched behavior for short periods of time. In the context of PSHA, if faults are able to switch behavior as our experimental system does, then justifying the choice of either the CE or the G-R model is impossible based on existing observations.

Laboratory studies suggest that seismogenic rupture on faults in carbonate terrains can be explained by a transition from high friction, at low sliding velocities (V), to low friction due to rapid dynamic weakening as seismic slip velocities are approached. However, consensus on the controlling physical processes is lacking. We previously proposed a microphysically-based model (the ‘Chen-Niemeijer-Spiers’ model) that accounts for the (rate-and-state) frictional behavior of carbonate fault gouges seen at low velocities characteristic of rupture nucleation. In the present study, we extend the CNS model to high velocities (1mm/s≤ V ≤10m/s) by introducing multiple grain-scale deformation mechanisms activated by frictional heating. As velocity and hence temperature increase, the model predicts a continuous transition in dominant deformation mechanisms, from frictional granular flow with partial accommodation by plasticity at low velocities and temperatures, to grain boundary sliding with increasing accommodation by solid-state diffusion at high velocities and temperatures. Assuming that slip occurs in a localized shear band, within which grain size decreases with increasing velocity, the model results capture the main mechanical trends seen in high-velocity friction experiments on room-dry calcite-rich rocks, including steady-state and transient aspects, with reasonable quantitative agreement and without the need to invoke thermal decomposition or fluid pressurization effects. The extended CNS model covers the full spectrum of slip velocities from earthquake nucleation to seismic slip rates. Since it is based on realistic fault structure, measurable microstructural state variables and established deformation mechanisms, it offers an improved basis for extrapolating lab-derived friction data to natural fault conditions.

A (micro)physical understanding of the transition from frictional sliding to plastic or viscous flow has long been a challenge for earthquake cycle modelling. We have conducted ring-shear deformation experiments on layers of simulated calcite fault gouge under conditions close to the frictional-to-viscous transition previously established in this material. Constant velocity (v) and v-stepping tests were performed, at 550 ˚C, employing slip rates covering almost six orders of magnitude (0.001 - 300 μm/s). Steady-state sliding transitioned from (strong) -strengthening, flow-like behavior to -weakening, frictional behavior, at an apparent ‘critical’ velocity () of ~0.1 μm/s. Velocity-stepping tests using < showed ‘semi-brittle’ flow behavior, characterized by high stress-sensitivity (‘-value’) and a transient response resembling classical frictional deformation. For ≥ , gouge deformation is localized in a boundary shear band, while for < , the gouge is well-compacted, displaying a progressively homogeneous structure as the slip rate decreases. Using mechanical data and post-mortem microstructural observations as a basis, we deduced the controlling shear deformation mechanisms, and quantitatively reproduced the steady-state shear strength-velocity profile using an existing micromechanical model. The same model also reproduces the observed transient responses to -steps within both the flow-like and frictional deformation regimes. We suggest that the flow-to-friction transition strongly relies on fault (micro-)structure and constitutes a net opening of transient micro-porosity with increasing shear strain rate at < , under normal-stress-dependent or ‘semi-brittle’ flow conditions. Our findings shed new insights into the microphysics of earthquake rupture nucleation and dynamic propagation in the brittle-to-ductile transition zone.