During seismic slip, the elastic strain energy released by the wall rocks drives grain fragmentation and flash heating in the slipping zone, resulting in formation of (nano)powders and melt droplets, which lower the fault resistance. With progressive seismic slip, the frictional melt covers the slip surface and behaves as a lubricant reducing the coseismic fault strength. However, the processes associated to the transition from grain fragmentation to bulk frictional melting remain poorly understood. Here we discuss in situ microanalytical investigations performed on experimentally produced solidified frictional melts from the transition regime between grain fragmentation and frictional melting. The experiments were performed on granitic gneiss at seismic slip rates (1.3 and 5 m/s), normal stresses ranging from 3 to 30 MPa. At normal stresses <12 MPa, the apparent friction coefficient μ app (shear stress versus normal stress) evolves in a complex manner with slip: μ app decreases because of flash weakening, increases up to a peak value μ p1~0 .6-1.0, slightly decreases and increases again to a second peak value μ p2~0 .44-0.83, and eventually decreases with displacement to a steady-state value μ ss~0 .3-0.45. In situ synchrotron observations of the solidified frictional melt show abundance of ultrafine quartz grains before μ p2 and enrichment in SiO 2 at μ p2 . Because partial melting occurs on the ultrafine quartz grains and, as a consequence, it suggested that the second re-strengthening (μ p2 ) is induced by the higher viscosity of the melt due to its enrichment in Si from melting of the ultrafine quartz grains derived from grain fragmentation. Key Points:• Granitic rocks sheared at seismic slip velocities and low normal stress show double-strengthening friction evolution before final weakening • Particle size distribution constrained by in situ synchrotron analysis shows the presence of additive ultrafine quartz grains • The re-strengthening was due to viscous frictional melts by quasi-equilibrium melting and the Gibbs-Thomson effect on quartz grains

The 2008 Mw 7.9 Wenchuan earthquake generated ∼270 and ∼80 km long surface ruptures along the Longmenshan fault belt, namely the Yingxiu‐Beichuan fault (YBF) and the Guanxian‐Anxian faults (GAF), respectively. So far, most of the frictional investigations were performed on the YBF gouge materials. Here, we present the results of rotary shear friction experiments performed on the GAF gouges recovered from the depth of ∼1.25 km of the Wenchuan Earthquake Fault Scientific Drilling project‐3 along the GAF. The fault gouges, mainly composed of quartz, illite, chlorite, and kaolinite, were sheared at slip velocities V ranging from 10−5 to 2 m/s and normal stresses from 8.5 to 10 MPa under both room humidity and wet conditions. At any imposed slip velocity, the wet gouges have an apparent friction coefficient lower than the room humidity one. In addition, enhanced velocity‐strengthening behavior at intermediate velocities (10−2 m/s < V ≤ 10−1 m/s) was recognized. We characterized the products using field‐emission scanning electron microscopy combined with synchrotron X‐ray diffraction analysis. These microanalytical investigations evidence the formation of size‐reduced particles (without mineral phase changes) and R‐ and Y‐shears in the principal slip zone (PSZ). Regardless of the ambient conditions, the width of PSZ was proportional to the input frictional work density (the product of shear stress times displacement). Our results support the hypothesis that the GAF preferentially ruptures through wet fault gouges; however, the enhanced velocity‐strengthening regime at intermediate velocities may act as a barrier to slip acceleration during fault rupture propagation.

&lt;p&gt;Understanding strain localization and development of shear fabrics within brittle fault zones at subseismic slip rates is crucial as they have critical implications for the mechanical strength and stability of faults and for earthquake physics. We performed direct shear experiments on ~1 mm thick layers of simulated quartz-rich fault gouge at an effective normal stress of 40 MPa, pore fluid pressure of 15 MPa, and temperature of 100&amp;#176;C. Microstructures were analyzed from strain hardening state (~1.3 mm displacement) to strain softening (~3.3 mm displacement) to steady-state (~5.6 mm) at different imposed shearing velocities of 1 &amp;#181;m/s, 30 &amp;#181;m/s, and 1 mm/s. We performed X-ray Computed Tomography (XCT) on sheared samples with a strain marker to analyze slip partitioning. To analyze and quantify localization from few hundreds to thousands of cross-section images, we used machine learning and developed an automatic boundary detection method to identify the type of shear fabrics and quantify the amount of them. Our results reveal that R&lt;sub&gt;1&lt;/sub&gt; and Y (or boundary) shears are the two major localization features that developed in a repeatable manner. Slip on R&lt;sub&gt;1&lt;/sub&gt; shears shows little dependency on both shear displacement and slip velocity and amounts to ~5 to ~30% of slip through the entire frictional sliding. On the other hand, Y and boundary shears show a strong correlation with displacement and velocity where more than 40% of strain was accommodated at steady-state for all velocities. However, Y and boundary shears become less prominent with increasing velocity, suggesting that velocity-weakening and the associated nucleation of unstable sliding are less likely to occur at higher slip rates as the overall friction behavior would be controlled by a thicker gouge layer. In other words, this suggests that Y shear development by grain size reduction is less efficient at high slip velocities which has important implications for the amount of heat generated during accelerating slip.&lt;/p&gt;

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&lt;p&gt;We used three-dimensional numerical simulations of the discrete element method (DEM) to investigate slip localization in sheared granular faults under seismic velocities. An aggregate of non-destructive spherical particles with assigned contact properties is subjected to direct shear with periodic boundary in horizontal directions. To investigate whether particle size distribution (PSD) influences slip accommodation, three distinct PSDs, namely Gaussian, log-normal, and power-law with fractal dimension D ranging from 0.8 to 2.6, are employed. In additional simulations, we impose a thin layer of particles with smaller grain size along the boundary as well as in the middle of the granular assemblages to simulate boundary and Y shears occurring in both natural and laboratory fault gouges. Transient microscopic properties, such as particle motion and contact forces, as well as macroscopic properties, such as friction, of the granular layer, are continuously monitored during numerical shearing. Results show that no visible slip localization is observed for all different PSDs based on the current particle motion analysis. On the other hand, we find that much more strain (i.e., displacement) is accommodated in the finer-grained layer even with a small contrast in grain size. Up to 90 % of the displacement is localized in a finer-grained layer when the contrast ratio of the grain size is 50 %. Since more frictional heat will be generated in the localized slip zone, the results provide crucial information on the heat generation and associated slip accommodation in sheared gouge zones. A possible mechanism of slip localization in the simulations is the transfer of the momentum across the granular system. We conclude that the occurrence of a weaker, fine-grained layer within a dense fault zone is likely to result in self-enhanced weakening of the fault planes.&amp;#160; Ongoing work includes (1) varying the thickness, grain size, and internal friction of the thinner layer; (2) applying triangulation methods to further analyze the microscale stress and strain tensor between particles; (3) changing the rolling friction of particles.&lt;/p&gt;