Search

Natural fault zones are complex, spatially heterogeneous systems. Rock deformation experimental studies simplify the complexity of natural fault zones either as a surface discontinuity between intact rocks (bare‐rock surfaces) or as a few mm‐thick gouge layer. However, depending on the simplified fault type and its slip history, the response to applied deformation can vary. In this work, we conduct laboratory experiments for investigating the evolution of mechanical parameters of simulated faults made of calcite gouge subjected to multiple (four) identical seismic slip‐rate pulses. We observed that, as the number of applied slip‐rate pulses increased, (a) initial friction and steady‐state friction remained approximatively constant, (b) peak friction and normalized strength excess increased and, (c) the slip distances to achieve peak and steady‐state friction, Da and Dc, decreased. The greatest changes occurred between the first and the second slip‐rate pulse. From this pulse onward, the dissipated energy of the calcite gouge fault was similar to those obtained in bare‐rock surfaces experiments. Microstructural analysis showed that, strain is localized in up to two (recrystallized) principal slip zones (PSZ) with sub‐micrometric grain size, surrounded by low porosity sintered and non‐sintered comminuted gouge domains. We conclude that previous seismic slip episodes impact on both the structure and the strain localization processes within a fault, contributing to its shear fabric evolution. We highlight that the strain localization process identifies the PSZ, dissipating the least amount of energy within the entire experimental fault zone. Plain Language Summary: Earthquakes are caused by the propagation of seismic ruptures and sliding of rocks along geological structures called faults. Within the fault, seismic ruptures propagate in mm‐cm thick slip zones that cut cm‐ to meters‐thick fault cores. Both slip zones and fault cores typically exhibit microstructural assemblages different from those of nearby rocks. Laboratory experiments are used to investigate the processes that result in the decrease of fault strength with slip and slip‐rate and govern seismic rupture propagation. However, experiments are performed on simplified fault cores consisting of either a surface discontinuity between intact rocks (i.e., slip zone = "bare‐rock surfaces") or a mm‐thick layer of powdered rocks (i.e., slip zone in a "fault gouge"). In this work, we investigate how slip zones form and evolve in an experimental fault (gouge) core depending on seismic slip history. We apply repeated pulses of seismic slip and show that (a) in the first pulse a slip zone with similar microstructure and friction properties to bare‐rock is formed, (b) in subsequent pulses these slip zones are abandoned and new slip zones are formed in the fault core. Notably, these structural changes, occur to minimize the mechanical energy dissipated in the FC. Key Points: Friction experiments applying up to four consecutive seismic slip‐rate pulses (1 m/s), in fault gouge and bare‐rock surfacesThe greatest changes in dissipated energy occurred between the first and the second slip‐rate pulseStrain localization in the principal slip zones minimizes the energy dissipated within the whole experimental fault zone [ABSTRACT FROM AUTHOR]

Typical rocks at shallow depths of seismogenic faults are fluid-rich gouges. During earthquakes, on-fault frictional heating may trigger thermal pressurization and dynamic fault weakening. We show that frictional melting, rather than thermal pressurization, occurred at shallow depths during the 2008 MW 7.9 Wenchuan earthquake, China. One year after the Wenchuan earthquake, we found an ~2-mm-thick, glass-bearing pseudotachylyte (solidified frictional melt) in the fault gouges retrieved at 732.6 m depth from the first borehole of the Wenchuan Earthquake Fault Scientific Drilling Project. The matrix of pseudotachylyte is enriched in barium and cut by barite-bearing veins, which provide evidence of co- and postseismic fluid percolation. Because pseudotachylyte can be rapidly altered in the presence of percolating fluids, its preservation suggests that gouge melting occurred in a recent large earthquake, possibly the Wenchuan earthquake. Rock friction experiments on fluid-rich fault gouges deformed at conditions expected for seismic slip at borehole depths showed the generation of pseudotachylytes. This result, along with the presence of a second slip zone attributed to the Wenchuan earthquake at 589.2 m depth, implies that during large earthquakes, frictional melting can occur at shallow depths and that seismic slip can be accommodated by multiple faults. This conclusion is consistent with the evidence from surface faulting that multiple ruptures propagated during the Wenchuan earthquake. [ABSTRACT FROM AUTHOR]

Numerous field and laboratory studies have been conducted to investigate the relationship between radon variation and seismic events, as well as the complex link between radon emission and rock deformation mechanisms. However, a clear understanding of this correspondence and systematic observations of these phenomena are still lacking, and recent experimental studies have yet to yield conclusive results. In this study, we investigate the possible relationships between radon migration dynamics and rock deformation at the micro-scale through laboratory experiments using the SHIVA apparatus under shear stress-controlled conditions and simultaneous high-resolution radon measurements. We studied the behaviour of three different lithologies to show that radon emission varies in response to rock deformation and this variation is highly dependent on the mineralogy and microstructure. This study represents the first attempt to define radon gas as an indicator of transient and rapid rock deformation at the micro-scale., (© 2023. Springer Nature Limited.)

The advent of novel technologies revealed that other geophysical signals than those directly related to fault motion could be used to probe the state of deformation of the Earth's crust. Electromagnetic signals belonging to this category have been increasingly investigated in the last decade in association to natural earthquakes and laboratory rock fractures. These studies are hampered by the lack of continuous recordings and a systematic mathematical processing of large data sets. Indeed, electromagnetic signals exhibit characteristic patterns on a specific frequency band (the very low frequency, VLF) that correlate uniquely with the paroxistic rupture of rocks specimens under uniaxial laboratory tests and were also detected in the atmosphere, in association to moderate magnitude earthquakes. The similarity of laboratory and atmospheric VLF offers an unique opportunity to study the relation between VLF and rock deformation on at least two different scales and to enlarge the dataset by combining laboratory and atmospheric data. In this paper we show that the enlarged VLF dataset can be successfully used, with a neural network approach based on LSTM neural networks to investigate the potential of the VLF spectrum in classifying rock rupture precursors both in nature and in the laboratory. The proposed approach lays foundation to the automatic detection of interesting VLF patterns for monitoring deformations in the seismically active Earth's crust., (© 2022. The Author(s).)