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Abstract Rapid slip, at rates in the order of 1 m/s or more, may induce frictional melting in rocks during earthquakes. The short-lived melting has been thought to be a disequilibrium process, for decades. We conducted frictional melting experiments on acidic, basic, and ultrabasic silicate rocks at a slip rate of 1.3 m/s. The experiments and microstructural observations reveal that all minerals in the rocks are melted at temperatures below their known melting temperatures (Tm); e.g., quartz is melted at ~ 1000–1200 °C, not ~ 1720 °C, while olivine at ~ 1300 °C, rather than ~ 1700 °C. The low-temperature melting is incompatible with the conventional disequilibrium melting, and may be caused predominantly by grain size reduction and phase boundary reactions during the early and later stages of slip, respectively. The newly estimated Tm and the melting mechanisms should be considered for understanding the mechanics of earthquakes, landslides, and caldera collapses.

Fault rocks exhibit structures resulting from different styles of shear deformation (either distributed or localized) during fault displacement. However, how the fault rock structures affect fault slip behavior remains poorly understood. We conducted shear experiments on thick and thin gouge layers of various mineral compositions, which simulate the predominant development of velocity-strengthening distributed deformation zones and velocity-weakening shear localized zones, respectively. Here, we show that deformation zones with contrasting structures and frictional properties can be developed together in a single fault. If the rheological stiffness of shear localization zones in the fault exceeds the elastic stiffness of neighboring wall rocks, stick-slip can intermittently occur along the localization zones while stable slip persists in the distributed deformation zones. These findings suggest that contrasting rheological stiffnesses resulting from fault rock structures may induce the simultaneous operation of multiple slip modes in a fault and explain the occurrence of earthquakes in creeping faults. [ABSTRACT FROM AUTHOR]

Catastrophic caldera collapse following a large volcanic eruption or subsurface magmatic outflow is associated with very large displacements on ring faults. The slip resistance or friction on the ring faults is, thus, a key mechanical property that controls the collapse. However, the dynamic friction on the ring faults during caldera collapse remains unknown. Here, we show that dynamic weakening of ring faults caused by frictional melting can play a critical role in the catastrophic collapse of volcanoes. From direct observations of the ring fault of the ancient Jangsan caldera in southeastern Korea, we identified a layer of solidified frictional melt (or pseudotachylyte) measuring ~0.1 m thick. The dynamic friction (expressed as the ratio of shear to normal stress) estimated from the layer, based on high-velocity shear tests and analyses of microstructures and materials, was lower than 0.1. Given the low resistance, it follows that an extraordinarily large fault slip (>100 m) causing a large earthquake was possible during the formation of the ancient caldera. We conclude that the dynamic weakness of ring faults should be incorporated in mechanical models of caldera collapses. [ABSTRACT FROM AUTHOR]

Faulting that caused surface ruptures in the past, particularly during the Quaternary, is of significant importance for the assessment of potential earthquake hazards. Thus, considerable efforts have been made to infer the timing of last fault movement and the recurrence intervals of age-unknown faults. Previous works have mainly relied on OSL dating of unconsolidated sediments cut by faults. However, this can provide only indirect chronological information on the timing of last faulting. Further, any attempt to directly date faulting event using radiogenic isotopes or ESR signals of constituent minerals in fault rocks has not been successful because of the limitations of each dating scheme. Recently, by several authors, the possibility of applying OSL dating method to fault rocks was investigated. However, these were mostly carried out using simulated fault gouge [1] with a few case studies on natural samples [2]. In the northern segment of the Yangsan fault in the southeastern part of the Korean peninsula, a wellpreserved PSZ (Principal Slip Zone) is observed in a fault core outcrop with the thickness of ~ 1-2 cm. As the PSZ is considered to have been exposed to intense frictional heat and stress during faulting event, we expect that luminescence and ESR signals of quartz or K-feldspar grains in this particular part of the fault rock would have been effectively bleached during fault slip. To test our hypothesis, we collected samples from PSZ, together with those from CZ (Cataclasite Zone) and BZ (Brown gouge Zone), which are located on the west and east sides of the PSZ, respectively [3]. Then, we characterise and compare the optically stimulated luminescence and ESR signals of minerals extracted from each fault core zone, and examine the equivalent dose values obtained using different luminescence signals. Finally, we discuss the potential importance of PSZ for directly dating the timing of last earthquake faulting events using OSL or ESR dating methods. Acknowledgement: This research was supported by a grant (2022-MOIS62-001, National Disaster Risk Analysis and Management Technology in Earthquake) funded by Ministry of Interior and Safety, Korea. [ABSTRACT FROM AUTHOR]

Graphite is a well-known solid lubricant and can be as important as clay minerals in reducing the frictional strength of faults. Some natural fault zones contain carbonaceous material (CM) even where host rocks do not contain it, and seismic fault motion can promote the graphitization of low-grade CM. Thus, the origin of CM in fault zones is an important issue in fault mechanics. Previous high-velocity friction experiments have revealed various chemical reactions in fault zones during seismic fault motion, but most experiments have been conducted in an atmosphere under oxic conditions. Here we report experimental results on Carrara marble (free of CM), conducted under N2 or H2 atmospheres at a slip rate of 1.3 m/s and normal stresses of 2.0-3.1 MPa. A small amount of blackish material formed in generated gouge only under reducing conditions with the H2 atmosphere, and Raman spectroscopic analysis revealed the presence of CM (amorphous carbon) in the material. The CM is attributable to (1) the generation and pyrolitic dissociation of CH4, and/or (2) a reduction reaction of emitted CO2 due to calcite decomposition. We confirmed the formation of CH4 using gas chromatography. The CM produced in experiments resembles CM in the Nojima fault (Japan) gouge in terms of Raman spectra. The granitic host rock of this fault is free of CM, and calcite is precipitated close to the CM; therefore, the CM probably formed through processes similar to those simulated in our experiments. Future research should investigate the amount and origin of CM in natural fault zones. [ABSTRACT FROM AUTHOR]

Pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip. We report from high-velocity friction experiments on siderite-bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte. Experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite, calcite, and quartz, initially at room temperature, under normal stresses of 0.6-1.3 MPa and at seismic slip rates of 1.3-2.0 m/s. In all cases, gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and CO2 gas, as confirmed by CO2 measurement, X-ray diffraction analyses, and transmission electron microscopy. The weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates. Magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude. Those changes can be recognized in natural fault zones, and black gouge in the Chelungpu fault zone in Taiwan is perhaps such an example. Thus our results suggest that thermal decomposition in shallow crustal faults can be an important co-seismic process not only for dynamic fault weakening, but also for leaving seismic slip records. [ABSTRACT FROM AUTHOR]