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Earthquake fault slip accompanied by surface ruptures may occur not only along main fault cores but also along subsidiary faults in damage zones of major (or mature) faults. Nevertheless, most previous studies of fault and earthquake geology have focused on geological observations of main core zones rather than subsidiary faults. We conducted microstructural and mineralogical analyses of fault rock materials from two subsidiary faults (F1 and F2) of the NNE-SSW-striking Yangsan Fault, which is a major strike-slip fault in southeastern Korea (F1 at Pohang Bogyeongsa and F2 at Ulsan Eonyang-Bangok), to understand their possible slip zone processes and slip behaviors. The fault cores of the subsidiary faults are up to 20 cm thick and are composed of clay-rich gouge bands measuring a few millimeters in thickness and enclosed fractured lenses. Microscopic observations reveal that linear, and narrow micro-scale principal slip zones (micro-PSZs; < 20 µm thick), which are characterized by strong preferred orientation of clay minerals, occur not only at the boundaries between the gouge band and adjacent fault rocks but also in the central part of the gouge band. Along the micro-PSZs, microstructures such as clasts truncated by rapid slip localization and gouge injections by thermal pressurization of wet gouge materials during rapid slip are observed. Thus, the structures together may indicate the occurrence of seismic slip on the subsidiary faults. Mineralogical analyses reveal that the total clay fractions (consisting mainly of illite, chlorite, and kaolin) of the gouge materials of F1 and F2 are 60.1 and 59.7 wt%, respectively. The gouge band of F2 is enriched with kaolin (59.7 wt%), which is regarded as a gouge material that can trigger dynamic weakening by dehydration-induced thermal pressurization during seismic slip. Therefore, these results imply that the kaolin-rich gouge band in F2 may be dynamically weakened when seismic reactivation occurs along F2. This study shows that a comprehensive investigation of slip behaviors of subsidiary faults as well as main fault cores is necessary to improve our understanding of the seismic faulting mechanisms of major tectonic fault zones.

취성변형에 의해 비다공질 암석에서는 단열이, 다공질 암석에서는 변형띠가 주로 발달하며, 이들 중 일부는 단층으로 진화할 수 있다. 본 연구에서는 다공질 암석 내 변형띠의 산상과 구조적, 물질적 특성을 기반으로 변형띠의 진화를 이해하기 위하여 경주 어일분지 반고결 퇴적층 내 변형띠를 대상으로 구조관찰 및 물질분석을 실시하였다. 노두규모 관찰 결과, 변형띠는 운동학적 분류로 볼 때 전단띠이다. 식별되는 대부분의 전단띠는 수 ㎜ 폭과 10 ㎝ 미만의 겉보기 전단변위를 보인다. 대표적인 전단띠의 미구조 관찰결과, 파쇄로 인한 입자크기의 감소는 있으나 암편이 기질보다 많은 초기단계 파쇄변형 조직이 확인된다. 한편, 노두관찰에서 겉보기 변위량이 증가함에 따라 전단띠의 폭이 증가하는 경향과 전단띠 중 소수는 단층활면의 발달과 함께 최대 ~3 ㎝의 넓은 폭을 갖는 단층성 변형띠(또는 단층)로 진화하였음이 확인되었다. 단층성 변형띠 내부에는 전단변형이 집중된 영역인 수십 μm 폭의 주미끌림대가 발달하는데, 특징적으로 석영, 장석의 초미세입자들과 점토광물(특히, 스멕타이트)들로 채워져 있다. 이러한 관찰은, 작은 변위의 전단띠에서는 초기단계 파쇄변형 조직의 발달이 응력변형강화 거동과 그에 따른 전단띠 폭의 증가를 야기할 수 있는 반면, 초파쇄변형에 의한 초미세입자들의 형성이나 낮은 마찰강도의 점토광물들의 생성은 주미끌림대에서의 응력변형약화 거동과 전단변형 국지화(또는 전단띠의 단층으로의 진화)에 기여할 수 있음을 지시한다.

The Suryum fault is an active fault in southeast Korea crosscutting the Quaternary sediment deposit. We conducted outcrop-scale to microscale observations of the fault slip zone and performed shear experiments on the fault gouge at subseismic (3–30 µm/s) and seismic slip rates (0.53 m/s). The gouge is rich in clay minerals (> 58%), particularly in expandable clay minerals (40%). In the gouge zone, a very narrow (20–150 µm thick) principal slip zone (PSZ), identified by the strong alignment of clay minerals, was developed. At several places along the fault, the gouge is observed to have been injected into the Quaternary wallrock sediments. The development of the narrow PSZ is incompatible with the velocity-strengthening behavior of the Suryum fault gouge observed in the shear tests, and it may indicate that seismic rupture could be propagated along the PSZ in the clay-rich gouge. Slip localization into the PSZ was presumably possible because of significant dynamic fault weakening due to thermal pressurization (or buildup of pore pressure caused by frictional heating) in the low-permeability clay gouge during fast slip. The gouge injections along the Suryum fault may be the geological record of gouge fluidization caused by thermal pressurization. This idea is supported by observations of very low friction of the Suryum fault gouge when sheared experimentally at the seismic slip rate and of the gouge injections in the sheared gouge. The dynamic weakness of the Suryum fault implies that large displacement and low-frequency ground motion at near-surface depths would be possible when it is reactivated.

Caldera fault zones, identified in many modern and ancient volcanoes, have been the subject of geological and geophysical observations and of analog and numerical modeling. However, the physicochemical processes in fault zones during a caldera collapse are still poorly understood. Here, we present field observations from a caldera fault zone in the Cretaceous Jangsan Caldera, SE Korea. The fault zone is ∼30 m wide and juxtaposes an intracaldera rhyolitic volcanic complex against older dacitic rocks; it consists of a minor fault, a main fault, and a series of fault-related intrusions (rhyolite and tuffisite). The main fault dips 90°–88°NW (vertically to steeply inward) and strikes N30°–40°E. A layer of pseudotachylyte (less than ∼10 cm thick) with some injection veins occurs along the main fault. The pseudotachylyte contains rounded to sub-rounded clasts, commonly displaying fuzzy or embayed boundaries, as well as euhedral feldspar microlites (

Fabric transition by a switch in the dominant slip system of minerals in the plastic regime can be induced by changes in temperature, strain rate, or water content. We propose here this fabric transition by frictional heating in seismogenic fault zones in the brittle regime. The Garam Thrust in the Taebaeksan Basin of South Korea has a hanging wall of Cambrian dolostone juxtaposed against a footwall of Ordovician limestone and records a minimum displacement of ~ 120 m. In a 10 cm thick plastically deformed layer adjacent to the principal slip layer of the fault zone, the lattice preferred orientation of calcite grains suggests that the dominant slip system changes, approaching the principal slip layer, from r 〈02–21〉 and e-twinning, through r 〈02–21〉 and basal 〈a〉, to basal 〈a〉. This fabric transition requires a high temperature-gradient of 40 °C/cm, which we infer to result from frictional heating of the seismic fault zone. We suggest that fabric transition within a thin plastically deformed layer adjacent to the principal slip layer of a fault zone indicates an unusually steep temperature gradient and provides strong evidence of seismic slip.

Quartz minerals in Earth’s crust are thought to melt at high temperatures. Laboratory friction experiments, however, show that metastable melting of quartz on a fault surface can occur at lower temperatures, and could lead to large earthquakes.

The internal structure and materials of a fault are crucial to understanding its mechanical and fluid flow behaviors because they are interrelated. The Yangsan fault is a major strikeslip fault in SE Korea, and its possible reactivation has been an issue of great interest and debate. However, the internal structure of the fault has not been extensively studied; one reason for this is the scarcity of the fault outcrop. Based on field observations and materials analysis, here we report the characteristics of the internal structure and the materials of the fault in the Bogyeongsa area, where a new outcrop showing the entire section of the fault is found. The fault is composed of a core zone (9−10 m in thickness) and surrounding damage zones (>200 m in thickness). The core zone has two subzones: one is an ~8–9-m thick alternating blue and purple gouge zone (BPZ) and the other is an ~0.5-m thick brown gouge zone (BZ). The former exhibits large fractured lenses and anastomosing gouges, and the latter has a strong network of foliation. Given the smaller content and size of clasts in the BZ than in the BPZ, it follows that larger fault displacement was accommodated in the BZ than in the BPZ. The damage zones around the core zone are characterized by dense fractures and veins and subsidiary faults of multiple generations. According to an analysis of the materials, the core zone gouges are clay-rich (>50 wt%). The BPZ is enriched in illite and chlorite, whereas the BZ is enriched in smectite, which has a notably low frictional strength when wet. Given the occurrence of the Quaternary slip faults in the damage zone of the Yangsan fault in other areas, further studies of the materials and mechanical properties on both the main core zone and the subsidiary faults in the damage zones are necessary to determine which is likely to be reactivated seismogenically under the current stress field.

Monitoring of CO2 release through the ground surface is essential to confirm the safety of carbon storage projects. We conducted a feasibility study of the multi-channel surface-soil CO2-concentration monitoring (SCM) system as a soil CO2 monitoring tool with a small scale injection test. The background concentrations showed the distinct diurnal variation. The negative relation of CO2 with temperature and the low CO2 concentrations during the day imply that surface-soil CO2 depends on photosynthesis and respiration. After 4.2 kg of CO2 injection (1 m depth for 29 minutes), surface-soil CO2 concentrations increased in the all five chambers, which were located less than 2.8 m of distance from each other. The CO2 concentrations seem to be recovered to the background around 4 hours after the injection ended. To determine the leakage, the data from Chamber 2 and 5 with low increase rates were used for statistical analyses. Coefficient of variation for 30 minutes (CV30min.) is efficient to determine a leakage signal, with reflecting the fast change in CO2 concentrations. Consequently, SCM and CV30min could be applied for an efficient monitoring tool to detect CO2 release through the ground surface. Also, this study provides ideas for establishing action steps after leakage detection.

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.

Experimental fault zones developed in Carrara marble that were deformed at seismic slip rates (1.18–1.30 m s −1 ) using a high-velocity-rotary-shear apparatus exhibit very low friction (friction coefficient as low as 0.06) at steady state due to nanoparticle lubrication of the decomposition product (lime). The fault zones show a layered structure; a central slip-localization layer (5–60 μm thick) of lime nanograins mantled by gouge layers (5–150 μm thick) and a plastically deformed layer (45–500 μm thick) between the wall rock and gouge layer in the marginal portion of cylindrical specimens. Calcite grains of the wall rock adjacent to the slip zone deform by dislocation glide when subjected to frictional heating and a lower strain rate than that of the principal slip zone. The very fine (2–5 μm) calcite grains in the gouge layer show a foam structure with relatively straight grain boundaries and 120° triple junctions. This foam structure is presumed to develop by welding at high temperature and low strain once slip is localized along the central layer. We suggest that a seismic event can be inferred from deformed marbles, given: (i) the presence of welded gouge with foam structure in a fault zone where wall rocks show no evidence of thermal metamorphism and (ii) a thin plastically deformed layer immediately adjacent to the principal slip zone of a cataclastic fault zone.

Clayey gouges are common in fault slip zones at shallow depths. Thus, the fault zone processes and frictional behaviors of the gouges are critical to understanding seismic slip at these depths. We conducted rotary shear tests on clayey gouge (~41 wt % clay minerals) at a seismic slip rate of 1.3 m/s. Here we report that the gouge was melted at 5 MPa of normal stress and room humidity conditions. The initial local melting was followed by melt layer formation. Clay minerals (e.g., smectite and illite) and plagioclase were melted and quenched to glass with numerous vesicles. Both flash heating and bulk temperature increases appear to be responsible for the melting. This observation of clayey gouge melting is comparable to that of natural faults (e.g., Chelungpu fault, Taiwan). Due to heterogeneous fault zone properties (e.g., permeability), frictional melting may be one of the important processes in clayey slip zones at shallow depths.

The Permo–Triassic collision of the North and South China blocks caused the development of the Dabie–Sulu Orogen in China and Songrim Orogen in the Korean Peninsula. Extension after this collision is known from the Dabie–Sulu Orogen, but post-orogenic extension is not well defined in the Korean Peninsula. Extensional deformation along the southern boundary of the Gyeonggi Massif in Korea is characterized by top–down-to-the-south ductile shearing and subsequent brittle normal faulting, and was predated by regional metamorphism and north-vergent contractional deformation. Extension occurred between ~220 and 185 Ma based on the ages of pre-extensional regional metamorphism and post-extensional pluton emplacement. 40Ar/39Ar dating of syn-extensional muscovite in quartz–mica mylonite yields an age of 187.8 ± 5.6 (2σ) Ma, in agreement with constraints from structural relationships. Together with the extensional deformation identified along the northern boundary of the Gyeonggi Massif (~226 Ma), the extension along the southern boundary is probably related to the exhumation of the massif during late-orogenic or post-orogenic extension associated with the Songrim Orogeny of the Korean Peninsula and forms an important event in the Phanerozoic crustal evolution of East Asia.

[1] Slide-hold-slide tests were conducted on gray blackish gouge (GBG) and yellowish gouge (YG) from Pingxi fault zone to see how rapidly the strength of Longmenshan fault recovers after the 2008 Wenchuan earthquake. Twenty dry runs were made at a normal stress of 0.8 MPa, at a seismic slip rate of 1.4 m/s, and with hold time th ranging 0.3–105 s. Results exhibit very rapid healing by more than 0.4 in friction coefficient μ in less than 5–10 s, followed by gradual healing in proportion to log(th). Healing rates, (increase in μ)/(log (th)), during rapid and slow healing are 0.188 and 0.015 for GBG and 0.154 and 0.016 for YG, respectively. The average temperature in the outer half of a 5 µm thick slip zone decreases from 260–300°C to 110–170°C in 5–10 s, and hence, temperature drop appears to be correlated with the rapid healing. Previously reported rapid healing at subseismic slip rates (85–90 mm/s) begins to occur in 10–300 s after the stop of sliding, and this cannot be explained by the cooling of gouge. The difference in healing between subseismic slip rates (delayed and rapid healing) and seismic slip rates (immediate and rapid healing) suggests that the dominant weakening mechanism shifts from tribochemical processes at subseismic slip rates to frictional heating at seismic slip rates. Slip-zone structures are too complex and variable from run to run to reveal microscopic mechanisms for the strength recovery. Rapid healing following seismic slip can be a cause for reduced aftershocks along major coseismic faults.

The evolution of friction as a function of slip rate is important in understanding earthquake nucleation and propagation. Many laboratory experiments investigating friction of fault rocks are either conducted in the low velocity regime (10−810−8–View the MathML source10−4 ms−1) or in the high velocity regime (0.01–1 m s−1). Here, we report on the evolution of friction and corresponding operating deformation mechanisms in analog gouges deformed from low to high slip rates, bridging the gap between these low and high velocity regimes. We used halite and halite–muscovite gouges to simulate processes, governing friction, active in upper crustal quartzitic fault rocks, at conditions accessible in the laboratory. The gouges were deformed over a 7 orders of magnitude range of slip rate (10−710−7–1 m s−1) using a low-to-high velocity rotary shear apparatus, using a normal stress of 5 MPa and room-dry humidity. Microstructural analysis was conducted to study the deformation mechanisms. Four frictional regimes as a function of slip rate could be recognized from the mechanical data, showing a transitional regime and stable sliding (10−710−7–10−6 m s−1), unstable sliding and weakening (10−610−6–10−3 m s−1), hardening (10−210−2–10−1 m s−1) and strong weakening (10−110−1–1 m s−1). Each of the four regimes can be associated with a distinct microstructure, reflecting a transition from mainly brittle deformation accompanied by pressure solution healing to temperature activated deformation mechanisms. Additionally, the frictional response of a sliding gouge to a sudden acceleration of slip rate to seismic velocities was investigated. These showed an initial strengthening, the amount of which depended on the friction level at which the step was made, followed by strong slip weakening.

A common problem encountered in studies of gouge-bearing natural faults is the difficulty of ascertaining whether the observed gouge was sheared seismically or aseismically; this problem arises because of the scarcity of indicators of fault slip rates for gouge. Recently, clay–clast aggregates (CCAs; a CCA comprises a clastic core mantled by a rim of ultrafine particles) were proposed as a possible indicator of seismic slip in gouge, on the basis of shear experiments on gouge at seismic slip rates. To examine the processes and conditions of CCA formation, we conducted rotary shear experiments on quartz and quartz–bentonite gouges under normal stresses (0.3–3.0 MPa) and slip rates (0.0005–1.3 m s −1 ), and in both room-humidity (room-dry) and water-saturated (wet) conditions. We found that CCAs could be produced in room-dry gouges even at the lowest slip rates, which are considerably slower than actual seismic slip rates. This finding demonstrates that thermal pressurization and fluidization at elevated temperature during seismic slip are not necessarily needed for the formation of CCAs, contrary to previous views. Given the occurrence of CCAs over a wide range of slip rates, we suggest that the presence of CCAs is not an unequivocal indicator of fault slip at seismic slip rates.

The Songrim and Daebo orogenies represent two major Phanerozoic tectonic events that are well-preserved across much of the present-day Korean Peninsula. The Songrim orogeny corresponds to the Late Permian–Triassic collision of the North and South China cratons whereas the Jurassic Daebo tectonic event represents a thin-skinned contractional deformation in a continental arc setting. It is well-established that the Songrim orogeny left a strong imprint on the geological record preserved in the middle and northern parts of the Korean Peninsula (e.g., the Pyeongnam Basin and the Imjingang belt) with only a minor impact on the geology of southern Korea (e.g., the Okcheon and Taebaeksan basins). It was the Daebo tectonic event, however, which generated most of the deformational structures observed within the Okcheon and Taebaeksan basins. The Deokpori thrust in the Taebaeksan Basin is a significant fault structure that formed during the Daebo tectonic event; no other regional structures related to the Songrim orogeny have been found in the Taebaeksan Basin. In the vicinity of the Deokpori thrust, we have identified in this study a previously undocumented reverse-slip shear zone, which is named the Geounri shear zone. Microfabrics observed in phyllonite and marble mylonite samples suggest that this shear zone was developed in a plastic deformation regime at ∼400 °C, in contrast with the nearby Deokpori thrust, which formed in a brittle deformation regime. A geochronological analysis of muscovite isolated from phyllonite samples of the Geounri shear zone, yielded two 40 Ar/ 39 Ar age spectra with a combined weighted mean age of 209 ± 5 (2 σ ) Ma. Thermal modeling of the Ar data, combined with structural interpretations, suggests that this muscovite 40 Ar/ 39 Ar age represents a robust lower age limit for the timing of the Geounri shearing event, therefore linking formation of the shear zone with deformational events caused by the Songrim orogeny. These results imply that structural features caused by ‘Songrim deformation’ were developed at higher temperatures than structures caused by ‘Daebo deformation’ as found within the Taebaeksan Basin.

Nanoparticles are known to form in narrow slip zones in natural and experimental fault zones, but their possible role in dynamic weakening of faults during seismic slip remains almost unexplored. We conducted friction experiments on periclase (MgO) nanoparticles (50 nm in average size) at rates as high as 1.3 m s −1 , a typical speed of seismic slip. The nanoparticles were used as the initial gouge to avoid complexities arising from comminution and fluid release. The dynamic friction decreased with increasing slip rate and the steady-state frictional coefficient reduced to as low as 0.1 at a slip rate of 1.3 m s −1 . Flash heating is not effective for nanoparticles, and we propose that development of slickensides and dominant operation of nanoparticle rolling cause such a weakening. Nanoparticle lubrication appears to be as effective as melt lubrication and thermal pressurization, and the formation of nanoparticles in slip zones may be an important fault lubrication process.

Anomalously low heat flow around active faults has been a recurrent subject of debate over past decades. We present a series of high-velocity friction experiments on gypsum rock cylinders showing that the temperature of the simulated fault plane is efficiently buffered due to large-scale endothermic dehydration reaction. The tests were performed at 1 MPa normal stress and a velocity of 1.3 m s −1 , while measuring the temperature close to the sliding surface and the relative humidity around the sample. The temperature close to the sliding surface is remarkably stable at ∼100 °C during the dehydration reaction of gypsum. Microstructural and X-ray diffraction investigations show that dehydration occurs at the very beginning of the test, and progresses into the bulk as slip increases. In the hottest parts of the sample, anhydrite crystal growth is observed. The half-thickness of the dehydrated layer ranges from 160 μm at 2 m slip to 5 mm at 68 m slip. Thermodynamic estimates of the energy needed for the dehydration to occur yield values ranging from 10% to 50% of the total mechanical work input. The temperature plateau is thus well explained by the energy sink due to the dehydration reaction and the phase change from liquid water into steam. We suggest that similar endothermic reactions can efficiently buffer the temperature of fault zones during an earthquake. This is a way to explain the low heat flow around active faults and the apparent scarcity of frictional melts in nature.

The Gongsuwon Thrust in the Taebaeksan Basin is one of the important regional structures for the Phanerozoic tectonic evolution of the Korean Peninsula. The late Early Jurassic Bansong Group (Daedong Supergroup) is structurally overlain by the early Paleozoic Joseon Supergroup along the thrust in the Yeongwol area. We present four lines of evidence indicating that the siliciclastics of the Bansong Group suffered a prelithified deformation within the fault zone of the Gongsuwon Thrust; (1) strainfree quartz clasts both in the hinge and limb areas of isoclinal folds of the sandstone layer, indicating granular flow, (2) intact gravels in the conglomerate with pressure shadows filled by sandstone or shale matrix, (3) injection of mud into sandy layers, and (4) lensoidal marble blocks (Joseon Supergroup) with plastically deformed calcite grains within siliciclastics of the Bansong Group. We suggest that the deposition of the Bansong Group and the thrusting were coeval during the middle Mesozoic Daebo tectonic event.

High-velocity weakening of faults may drive fault motion during large earthquakes. Experiments on simulated faults in Carrara marble at slip rates up to 1.3 meters per second demonstrate that thermal decomposition of calcite due to frictional heating induces pronounced fault weakening with steady-state friction coefficients as low as 0.06. Decomposition produces particles of tens of nanometers in size, and the ultralow friction appears to be associated with the flash heating on an ultrafine decomposition product. Thus, thermal decomposition may be an important process for the dynamic weakening of faults.

The Bansong Group (Daedong Supergroup) in the Korean peninsula has long been considered to be an important time marker for two well-known orogenies, in that it was deposited after the Songnim orogeny (Permian–Triassic collision of the North and South China blocks) but was deformed during the Early to Middle Jurassic Daebo tectonic event. Here we present a new interpretation on the origin of the Bansong Group and associated faults on the basis of structural and geochronological data. SHRIMP (Sensitive High-Resolution Ion MicroProbe) U–Pb zircon age determination of two felsic pyroclastic rocks from the Bansong Group formed in the foreland basin of the Gongsuweon thrust in the Taebaeksan Basin yielded ages of 186.3 ± 1.5 and 187.2 ± 1.5 Ma, respectively, indicating the deposition of the Bansong Group during the late Early Jurassic. Inherited zircon component indicates ca. 1.9 Ga source material for the volcanic rocks, agreeing with known basement ages. The Bansong Group represents syntectonic sedimentation during the late Early Jurassic in a compressional regime. During the Daebo tectonic event, the northeast-trending regional folds and thrusts including the Deokpori (Gakdong) and Gongsuweon thrusts with a southeast vergence developed in the Taebaeksan Basin. This is ascribed to deformation in a continental-arc setting due to the northwesterly orthogonal convergence of the Izanagi plate on the Asiatic margin, which occurred immediately after the juxtaposition of the Taebaeksan Basin against the Okcheon Basin in the late stage of the Songnim orogeny. Thus, the Deokpori thrust is not a continental transform fault between the North and South China blocks, but an “intracontinental” thrust that developed after their juxtaposition.

Feldspar grain-size reduction occurred due to the fracturing of plagioclase and K-feldspar, myrmekite formation and neocrystallization of albitic plagioclase along shear fractures of K-feldspar porphyroclasts in the leucocratic granitic rocks from the Yecheon shear zone of South Korea that was deformed under a middle greenschist-facies condition. The neocrystallization of albitic plagioclase was induced by strain energy adjacent to the shear fractures and by chemical free energy due to the compositional disequilibrium between infiltrating Na-rich fluid and host K-feldspar. With increasing deformation from protomylonite to mylonite, alternating layers of feldspar, quartz and muscovite developed. The fine-grained feldspar-rich layers were deformed dominantly by granular flow, while quartz ribbons were deformed by dislocation creep. With layer development and a more distributed strain in the mylonite, lower stresses in the quartz-rich layers resulted in a larger size of dynamically recrystallized quartz grains than that of the protomylonite.

[1] Typhoon Morakot struck southern Taiwan in the summer of 2009, causing the most severe flooding since the 1950s. In the early morning of August 9, rainfall triggered the Hsiaolin landslide, and the resulting debris avalanche covered the township of Hsiaolin Village, Kaohsiung. Around five hundred people were buried alive. Reconstruction of the runout of the debris avalanche would increase understanding of the large-scale avalanches for future hazard mitigation purposes. Simulation of the debris avalanche runout can provide valuable information for this purpose. A new continuum shallow-water model is applied to flow over general topography. The Coulomb friction law is adopted; the friction coefficient is initially determined by high pressure rotary-shearing tests and subsequently fine-tuned by an iterative procedure to minimize the difference between the simulation and the measurement. The friction coefficients measured by laboratory tests are found to be in reasonable agreement with the best-fit result of the simulation. In addition, Voellmy rheology is applied, but it is found that the role of the fluid viscous drag is insignificant. The simulation result in the village area is further corroborated by near-surface magnetic surveys. These indicate that the northern part of the village is dislocated, while the artifact structures of the southern part are buried near their original locations. By comparing the landslide front and the flow direction of the simulation, we are able to confirm, as also described by survivors, that the landslide swept the northern part of the village into the Cishan River, while the southern part was flooded subsequently by the debris from a dam breach about 20 min after the landslide.

[1] The Deokpori Thrust, juxtaposing the early Paleozoic Yeongwol-type Joseon Supergroup (Yeongwol Group) against the Duwibong-type Joseon Supergroup (Taebaek Group) in the Okcheon belt, is an important regional structure in the Phanerozoic tectonic evolution of Korea. Although the initiation of the thrust has recently been conjectured to occur during the Middle Jurassic Daebo tectonic event in a continental magmatic arc setting, its movement history has never been studied. We report timing of brittle reactivation of the thrust obtained from K-Ar dating of fault gouges in the core of the Deokpori Thrust zone. K-Ar illite age data of fault gouges suggest reactivation of the thrust at ca. 72 Ma after a contact metamorphism around 94 Ma. Our result is the first documentation of the reactivation of a major Jurassic ‘Daebo’ structure and the reactivation may represent a significant contractional tectonic event during the Late Cretaceous in the Korean Peninsula.

A review of about 300 published and unpublished rock friction experiments that reproduce seismic slip conditions suggests that a significant decrease in friction occurs at high slip rate. Extrapolating the experimental data to conditions that are typical of earthquake nucleation depths, the authors conclude that faults are lubricated during earthquakes, irrespective of the fault rock composition or specific weakening mechanism involved. This study reviews a large set of fault friction experiments and finds that a significant decrease in friction occurs at high slip rate. Extrapolating the experimental data to conditions typical of earthquake nucleation depths, it is concluded that faults are lubricated during earthquakes, irrespective of the fault rock composition or specific weakening mechanism involved. The determination of rock friction at seismic slip rates (about 1 m s−1) is of paramount importance in earthquake mechanics, as fault friction controls the stress drop, the mechanical work and the frictional heat generated during slip1. Given the difficulty in determining friction by seismological methods1, elucidating constraints are derived from experimental studies2,3,4,5,6,7,8,9. Here we review a large set of published and unpublished experiments (∼300) performed in rotary shear apparatus at slip rates of 0.1–2.6 m s−1. The experiments indicate a significant decrease in friction (of up to one order of magnitude), which we term fault lubrication, both for cohesive (silicate-built4,5,6, quartz-built3 and carbonate-built7,8) rocks and non-cohesive rocks (clay-rich9, anhydrite, gypsum and dolomite10 gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger11,12 a number of physicochemical processes (gelification, decarbonation and dehydration reactions, melting and so on) whose products are responsible for fault lubrication. The similarity between (1) experimental and natural fault products and (2) mechanical work measures resulting from these laboratory experiments and seismological estimates13,14 suggests that it is reasonable to extrapolate experimental data to conditions typical of earthquake nucleation depths (7–15 km). It seems that faults are lubricated during earthquakes, irrespective of the fault rock composition and of the specific weakening mechanism involved.

The final slip of about 450 m at about 30 m/s of the 1963 Vaiont landslide (Italy) was preceded by >3 year long creeping phase which was localized in centimeter-thick clay-rich layers (60–70% smectites, 20–30% calcite and quartz). Here we investigate the frictional properties of the clay-rich layers under similar deformation conditions as during the landslide: 1–5 MPa normal stress, 2 × 10^(−7) to 1.31 m/s slip rate and displacements up to 34 m. Experiments were performed at room humidity and wet conditions with biaxial, torsion and rotary shear apparatus. The clay-rich gouge was velocity-independent to velocity-weakening in both room humidity and wet conditions. In room humidity experiments, the coefficient of friction decreased from 0.47 at v < 5.0 × 10^(−5) m/s to 0.12 at 1.31 m/s. Microstructural and mineralogical analyses of the gouge after experiments indicate that the dramatic weakening results from thermo-chemical pressurization of pore fluids (smectite decomposition to illite-type clays) and powder lubrication. In wet experiments, the coefficient of friction decreased from 0.17 at v < 1.0 × 10^(−4) m/s to 0.0 at v > 0.70 m/s: full lubrication results from the formation of a continuous water film in the gouge. The Vaiont landslide occurred under wet to saturated conditions. The unstable behavior of the landslide is explained by the velocity-weakening behavior of the Vaiont clay-rich gouges. The formation of a continuous film of liquid water in the slipping zone reduced the coefficient of friction to almost zero, even without invoking the activation of thermal pressurization. This explains the extraordinary high velocity achieved by the slide during the final collapse.

[1] High-velocity friction tests were conducted on solid and hollow cylinders of Carrara (calcite) marble, dolomite marble, silicate-bearing calcite marble, and calcite gouge to investigate the strength of carbonate faults during seismic slip. The experiments, performed at normal stresses of 0.6–14.7 MPa, slip rates of 0.03–1.60 m/s, and room temperature in a rotary-shear friction testing machine, yielded an extraordinarily low steady state friction coefficient (

[1] The evolution of the frictional strength along a fault at seismic slip rates (about 1 m/s) is a key factor controlling earthquake mechanics. At mantle depths, friction-induced melting and melt lubrication may influence earthquake slip and seismological data. We report on laboratory experiments designed to investigate dynamic fault strength and frictional melting processes in mantle rocks. We performed 20 experiments with Balmuccia peridotite in a high-velocity rotary shear apparatus and cylindrical samples (21.8 mm in diameter) over a wide range of normal stresses (5.4–16.1 MPa), slip rates (0.23–1.14 m/s), and displacements (1.5–71 m). During the experiments, shear stress evolved with cumulative displacement in five main stages (stages 1–5). In stage 1 (first strengthening), the coefficient of friction μ increased up to 0.4–0.7 (first peak in friction). In stage 2 (abrupt first weakening), μ decreased to about 0.25–0.40. In stage 3 (gradual second strengthening), shear stress increased toward a second peak in friction (μ = 0.30–0.40). In stage 4 (gradual second weakening), the shear stress decreased toward a steady state value (stage 5) with μ = 0.15. Stages 1 and 2 are of too short duration to be investigated in detail with the current experimental configuration. By interrupting the experiments during stages 3, 4, and 5, microstructural (Field Emission Scanning Electron Microscope) and geochemical (Electron Probe Micro-Analyzer and Energy Dispersive X-Ray Spectroscopy) analysis of the slipping zone suggest that second strengthening (stage 3) is associated with the production of a grain-supported melt-poor layer, while second weakening (stage 4) and steady state (stage 5) are associated with the formation of a continuous melt-rich layer with an estimated temperature up to 1780°C. Microstructures formed during the experiments were very similar to those found in natural ultramafic pseudotachylytes. By performing experiments at different normal stresses and slip rates, (1) the “thermal” (as it includes the thermally activated first and second weakening) slip distance to achieve steady state from the first peak in strength decreased with increasing normal stress and slip rate and (2) the steady state shear stress slightly increased with increasing normal stress and, for a given normal stress, decreased with increasing slip rate. The ratio of shear stress versus normal stress was about 0.15, well below the typical friction coefficient of rocks (0.6–0.8). The dependence of steady state shear stress with normal stress was described by means of a constitutive equation for melt lubrication. The presence of microstructures similar to those found in natural pseudotachylytes and the determination of a constitutive equation that describes the experimental data allows extrapolation of the experimental observations to natural conditions and to the study of rupture dynamics in mantle rocks.

International audience; To determine the processes responsible for slip-weakening in clayey gouge zones, rotary-shear experiments were conducted at seismic slip rates (equivalent to 0.9 and 1.3 m/s) at 0.6 MPa normal stress on a natural clayey gouge for saturated and non-saturated initial conditions. The mechanical behavior of the simulated faults shows a reproducible slip-weakening behavior, whatever initial moisture conditions. Examination of gouge obtained at the residual friction stage in saturated and non-saturated initial conditions allows the definition of two types of microstructures: a foliated type reflecting strain localization, and a non-foliated type composed of spherical aggregates. Friction experiments demonstrate that liquid-vapor transition of water within gouge due to frictional heating has a high capacity to explain the formation of spherical aggregates in the first meters of displacement. This result suggests that the occurrence of spherical aggregates in natural clayey fault gouges can constitute a new textural evidence for shallow depth pore water phase transition at seismic slip velocity and consequently for past seismic fault sliding.

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 CO 2 gas, as confirmed by CO 2 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.