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Abundant heterogeneity has been documented on faults in nature across a wide range of length scales, including structural, mineralogical, and roughness variations. The role of complex heterogeneity on fault mechanics and frictional stability is not well established, and experiments investigating heterogeneity have typically incorporated a single source of heterogeneity. Here, we conduct rock friction experiments on rough, bimaterial faults that are creeping, or steadily sliding, to explore the role of lithological heterogeneity on fault mechanics and stability. When strong asperities juxtapose weak gouge, stable sliding occurs with a low friction coefficient, µ. Encounters of strong diabase asperities on talc gouge lined faults initiate dramatic increases in µ and transitions to unstable sliding characterized by frequent stick-slip events (StSE). Seismic moments and stress drops of StSE decrease with increasing asperity abundance. Stress is concentrated at asperities during encounters, increasing with decreasing asperity abundance and leading to extensive mechanical damage. Interactions between strong, velocity weakening asperities provide a model to explain the nucleation of seismic and aseismic slip events on nominally stable, creeping faults. 

We develop a linear viscous constitutive relationship for pressure solution constrained by models of deformed metasedimentary rocks and observations of exposed rocks from ancient subduction zones. We include pressure and temperature dependence on the solubility of silica in fluid by parameterizing a practical van’t Hoff relationship. This general flow law is well suited for making predictions about interseismic behavior of subduction zones. We apply the flow law to Cascadia, where thermal structure, geometry, relative plate velocity, and Global Positioning System velocity field are well constrained. Results are consistent with the temperature conditions at which resolvable ductile strain is recorded in subducted mudstones (at depths near the updip limit of the seismogenic zone) and with relative plate motion accommodated completely by viscous deformation (at depths near the downdip limit of the seismogenic zone). The flow law also predicts the observed forearc tapering of slip rate deficit with depth. 

Key Points Changes in hydraulic diffusivity and pressurization factor during thermal pressurization (TP) balance each other in low permeability and low porosity fault rocks Hydraulic diffusional length scales as time 0.7 when considering TP parameters that depend on temperature and pressure The constant case model should be considered with ambient initial conditions and not time‐averaged ones 

Type II and IV twins with irrational twin boundaries are studied by high-resolution transmission electron microscopy in two plagioclase crystals. The twin boundaries in these and in NiTi are found to relax to form rational facets separated by disconnections. The topological model (TM), amending the classical model, is required for a precise theoretical prediction of the orientation of the Type II/IV twin plane. Theoretical predictions also are presented for types I, III, V, and VI twins. The relaxation process that forms a faceted structure entails a separate prediction from the TM. Hence, faceting provides a difficult test for the TM. Analysis of the faceting by the TM is in excellent agreement with the observations. 

The asthenosphere plays a fundamental role in present-day plate tectonics as its low viscosity controls how convection in the mantle below it is expressed at the Earth’s surface above. The origin of the asthenosphere, including the role of partial melting in reducing its viscosity and facilitating deformation, remains unclear. Here we analysed receiver-function data from globally distributed seismic stations to image the lower reaches of the asthenospheric low-seismic-velocity zone. We present globally widespread evidence for a positive seismic-velocity gradient at depths of ~150 km, which represents the base of a particularly low-velocity zone within the asthenosphere. This boundary is most commonly detected in regions with elevated upper-mantle temperatures and is best modelled as the base of a partially molten layer. The presence of the boundary showed no correlation with radial seismic anisotropy, which represents accumulated mantle strain, indicating that the inferred partial melt has no substantial effect on the large-scale viscosity of the asthenosphere. These results imply the presence of a globally extensive, partially molten zone embedded within the asthenosphere, but that low asthenospheric viscosity is controlled primarily by gradual pressure and temperature variations with depth. 

Abstract Slow slip is part of the earthquake cycle, but the processes controlling this phenomenon in space and time are poorly constrained. Hematite, common in continental fault zones, exhibits unique textures and (U-Th)/He thermochronometry data patterns reflecting different slip rates. We investigated networks of small hematite-coated slip surfaces in basement fault damage of exhumed strike-slip faults that connect to the southern San Andreas fault in a flower structure in the Mecca Hills, California, USA. Scanning electron microscopy shows these millimeter-thick surfaces exhibit basal hematite injection veins and layered veinlets comprising nanoscale, high-aspect-ratio hematite plates akin to phyllosilicates. Combined microstructural and hematite (U-Th)/He data (n = 64 new, 24 published individual analyses) record hematite mineralization events ca. 0.8 Ma to 0.4 Ma at <1.5 km depth. We suggest these hematite faults formed via fluid overpressure, and then hematite localized repeated subseismic slip, creating zones of shallow off-fault damage as far as 4 km orthogonal to the trace of the southern San Andreas fault. Distributed hematite slip surfaces develop by, and then accommodate, transient slow slip, potentially dampening or distributing earthquake energy in shallow continental faults. 

Abstract Understanding the generation of damaging, high‐frequency ground motions during earthquakes is essential both for fundamental science and for effective hazard preparation. Various theories exist regarding the origin of high‐frequency ground motions, including the standard paradigm linked to slip heterogeneity on the rupture plane, and alternative perspectives associated with fault complexity. To assess these competing hypotheses, we measure ground motion amplitudes in different frequency bands for 3 ≤ M ≤ 5.8 earthquakes in Southern California and compare them to empirical ground motion models. We utilize a Bayesian inference technique called the Integrated Nested Laplace Approximation (INLA) to identify earthquake source regions that produce higher or lower ground motions than expected. Our analysis reveals a strong correlation between fault complexity measurements and the high‐frequency ground motion event terms identified by INLA. These findings suggest that earthquakes on complex faults (or fault networks) lead to stronger‐than‐expected ground motions at high frequencies. 

Abstract Novel fluid medium pressure cells were used to deform antigorite under constant stress creep conditions at low temperature, low strain rate (10⁻⁹ − 10⁻⁴1/s), and high pressure (1 GPa) in a Griggs‐type apparatus. Antigorite cores were deformed at constant temperatures between 75°C and 550°C, by applying 8–12 stress‐strain steps per temperature. The microstructures of deformed samples share features documented in previous work (e.g., shear microcracks), and highlight the importance of basal shear and kinks to antigorite plasticity. Rheological data were fit with a low temperature plasticity law, consistent with a deformation mechanism involving large lattice resistance. When applied at geologic stresses and strain rates, the extrapolated viscosity agrees well with predictions based on subduction zone thermal models. 

Abstract. Viscous flow in ice is often described by the Glen flow law – anon-Newtonian, power-law relationship between stress and strain rate with astress exponent n ∼ 3. The Glen law is attributed tograin-size-insensitive dislocation creep; however, laboratory and fieldstudies demonstrate that deformation in ice can be strongly dependent ongrain size. This has led to the hypothesis that at sufficiently lowstresses, ice flow is controlled by grain boundary sliding, which explicitly incorporates the grain size dependence of ice rheology. Experimental studiesfind that neither dislocation creep (n ∼ 4) nor grain boundarysliding (n ∼ 1.8) have stress exponents that match the value ofn ∼ 3 in the Glen law. Thus, although the Glen law provides anapproximate description of ice flow in glaciers and ice sheets, itsfunctional form is not explained by a single deformation mechanism. Here weseek to understand the origin of the n ∼ 3 dependence of theGlen law by using the “wattmeter” to model grain size evolution in ice.The wattmeter posits that grain size is controlled by a balance between themechanical work required for grain growth and dynamic grain size reduction.Using the wattmeter, we calculate grain size evolution in two end-membercases: (1) a 1-D shear zone and (2) as a function of depth within anice sheet. Calculated grain sizes match both laboratory data and ice coreobservations for the interior of ice sheets. Finally, we show thatvariations in grain size with deformation conditions result in an effectivestress exponent intermediate between grain boundary sliding and dislocationcreep, which is consistent with a value of n = 3 ± 0.5 over the rangeof strain rates found in most natural systems.