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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.

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.