Effect of particle shape on the shear strength of fault gouge

김광염,Hyoung Suk Suh,윤태섭,Seong-Woo Moon,Yong-Seok Seo 한국지질과학협의회 2016 Geosciences Journal Vol.20 No.3

The shear strength of fault gouge plays an important role in the dynamic behavior of faults, ranging from small-scale displacements to severe earthquakes. The characteristics and interactions of constituent materials in fault gouge are the main determinants of shear strength. Assessing the shear strength of fault gouge by means of experiments, however, requires time-consuming procedures, including sampling, shear testing, and reliability checking; consequently, simple and indirect methods to assess shear strength in terms of the characteristics of fault gouge fragments have been investigated. This study focuses on the influence of the shape of fault gouge particles on the shear strength of gouge. We introduce a novel technique to obtain shape parameters of particles using Xray computed tomography (CT), and then show the effects of particle shape on the friction angle of the fault gouge. Samples collected from fault zones developed in various parent rock materials were tested in laboratory experiments to characterize their shear strengths. After shear testing, the particles in the fault gouge were collected, scanned by X-ray CT, and then analyzed for shape characterization. We successfully determined the shape parameters (sphericity, elongation, flatness, and slenderness) of the fault gouge fragments, and found that the parameters are well correlated with the friction angle of the gouge.

Structural records and mechanical characteristics of seismic slip along an active fault crosscutting unconsolidated Quaternary sediments: Suryum fault, SE Korea

한래희,김창민,우상우,정기영,Takehiro Hirose 한국지질과학협의회 2020 Geosciences Journal Vol.24 No.4

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.

논문 : 부산 금정산일대에 분포하는 단층비지의 전단특성에 관한 실험적 고찰

우익 ( Ik Woo ) 대한지질공학회 2012 지질공학 Vol.22 No.1

본 연구에서는 부산시 금정구 금정산 일대에서 채취한 단충비지대의 공학적 특성을 여러 형태의 절리모형실험을 통하여 파악하였다. 이 지역의 양산단충과 동래단충과 같은 주요 대규모 단층과 부수적인 소규모 단충이 다수 분포하고 있어, 이 지 역의 시추공중 이러한 단충을 관통한 시추공에서 단충비지 및 흑운모 화강암 절리 시료를 추출하여 실내 실험에 사용하였다. 우전 단층비지대의 광물학적 특정을 파악하기 위한 XRD 및 SEM 분석에서는 단층비지대가 수분 함유에 따른 급격한 전단강도 감소를 예측할 수 있는 판상구조의 점토광물인 kaolinite, montmorillonite, illite, sericite으로 이루어져 있는 것으로 파악되었다. 다층비지에 대한 전단강도 특성은 비지 자체의 전단강도 측정과 단층비지를 충전한 여러 형태의 자연 및 인공 절리면에 대한 절리면의 전단강도 측정으로 구하였다. 실험 결과, 단층비지가 충전된 절리면은 충전물질이 없는 절리면보다 급격한 전단강도의 감소가 발생하였으며, 단충비지의 내부 마찰각보다 절리면 사이에 충전된 단층비지의 마찰각이 더욱 낮게 측정되었다, 이러한 실험 결과는 암반과 접하고 있는 단층비지의 전단 특성은 단충비지 자체가 가친 전단강도보다 더욱 작은 값을 가지기 때문에 암반 구조물의 안정성을 저해하는 요소로 작용할 수 있다는 것을 보여주고 있다. The mechanical characteristics of a fault gouge from near Mt. Kumjung in Kumjung-Gu, Busan, were estimated from laboratory tests on different joint models. Fault gouge samples and joint samples in biotite granite were obtained from boreholes in the study area that had penetrated small faults associated with the Dongnae and Yang-san faults. XRD and SEM analyses revealed that for the fault gouge consists of several clay minerals with tabular structure (kaolinite, montmorillonite, illite, sericite), which could cause the considerable reduction of shear strength when wet. The shear strength of the fault gouge was obtained from direct shear tests of the fault gouge itself and from direct shear tests of several natural/artificial joint surfaces coated with fault gouge. The results indicate that the reduction of shear strength is more abrupt for the joint surfaces coated with fault gouge compared with uncoated joint surfaces, and that the friction angle of the fault gouge between joint surfaces is much lower than the internal friction angle of the fault gouge itself Fault gouges in contact with rock, therefore, could have a stronger negative effect on the stability of structures in rock masses than the fault gouge itself.

단층비지대의 성장: 경주시 양북면 부근의 사례

장태우 ( Tae Woo Chang ),장윤득 ( Yun Deuk Jang ) 대한지질공학회 2008 지질공학 Vol.18 No.2

단층의 내부 구조가 잘 구분되는 하나의 단층핵이 경주시 양북면 용당리 부근 작은 계곡에 인접하여 노출되어 있다. 단층핵의 중앙에는 약 25 cm두께의 단층비지대가 놓여 있으며 색깔에 의해 청회색 비지대와 녹황색 비지대로 구분된다. 현미경하에서 비지들은 옛 비지편, 석영, 장석, 철광물 등의 반상쇄편으로 구성되며 기질은 점토광물이 풍부하고 점토광물은 선택정렬하며 P엽리를 잘 발달시킨다. 청회색 비지와 녹황색 비지의 차이점은 녹황색 비지가 청회색 비지에 비해 점토광물이 훨씬 풍부하고, 반상쇄편의 수와 크기가 적으며, 철광물 또한 매우 풍부하고 철광물의 종류는 청회색 비지대에서 대부분 황철석인데 반해 주로 적철석으로 이루어진다. 이 적철석은 청회색 비지 형성 단층작용 시에 유입된 열수가 청회색비지대에 황철석을 침전시킬 동안 녹황색 비지대에는 기존의 황철석을 변질시켜 적철석을 형성한 것으로 생각된다. 단층핵 내 청회색 비지대와 녹황색 비지대는 점진적인 단층대 발달 단계를 거쳐 형성된 것으로 생각된다. 첫째 단계에서는 이른 단층작용이 시작되고 이에 수반되어 손상대가 생성되었다. 두 번째 단계는 손상대에서 반복적인 단열작용으로 각력대가 형성되었다. 세 번째 단계에서는 입자 마모와 연마작용이 활발하게 일어나며 녹황색 비지대가 발달하였다. 네 번째 단계에서 변형작용은 단층핵의 중심에서 옛 비지대(녹황색 비지대)가 변형경화로 비활동적으로 남을 때 단층핵의 연변을 향하여 새로운 비지대인 청회색 비지대가 생성되어 녹황색 비지대에 인접하여 부가되었으며 이로 인해 단층핵 내 비지대는 더 큰폭으로 성장하였다. 비지대 내에서 청회색 비지대 생성 후 변형작용은 반상쇄편의 내부 파쇄작용과 혼합면열구조의 발달로 비지대 내 큰 변형이 수용되었다. A fault gouge zone which is about 25cm thick crops out along a small valley in Yangbuk-myeon, Gyeongju city. It is divided into greenish brown gouge and bluish gray gouge by color. Under the microscope, the gouges have a lot of porphyroclasts composed of old gouge fragments, quartz, feldspar and iron minerals. Clay minerals are abundant in matrix, defining strikingly P foliation by preferred orientation. Microstructural differences between bluish gray gouge and greenish brown gouge are as follows: greenish brown gouge compared to bluish gray gouge is (1) rich in clay minerals, (2) small in size and number of porphyroclasts, and (3) plentiful in iron minerals which are mostly hematites, while chiefly pyrites in bluish gray gouge. Hematites are considered to be altered from pyrites in the early-formed greenish brown gouge under the influence of hydrothermal fluids accompanied during the formation of bluish gray gouge that also precipitated pyrites. It is believed that the fault core including bluish gray gouge zone and greenish brown gouge zone was formed by progressive cataclastic flow. In the first stage the fault core initiates from damage zone of early faulting. In the second stage damage zone actively transforms into breccia zone by repeated fracturing. The third stage includes greenish brown (old) gouge formation in the center of the fault core mainly by particle grinding. In the third stage further deformation leads to the formation of new (bluish gray) gouge zone while old gouge zone undergoes strain hardening. Consequently, the whole gouge zone in the core widens.

당진 지역 제4기 진관단층의 운동 특성과 단층비지의 ESR 연령

최범영,황재하,배한경,이희권,경재복 한국지구과학회 2015 한국지구과학회지 Vol.36 No.1

In order to outline the kinematics and movement history of a new Quaternary fault, Jingwan Fault in Dangjin, West Korea, we analyzed the geometry of the fault zone composed of a few gouge zones, and made ESR dating for fault gouge materials. The N55°E striking Jingwan Fault is a normal fault and exhibits a gradual change in dip (gentle in the lower part, steep in the upper part), indicating a listric fault. As for the fault gouge zone, its thickness varies and reaches 2~3cm in the lower part or between basement rocks, and 20~30cm in the middle-upper part or between the basement and Quaternary deposit. It is observed in the latter case that more than three gouge zones develop with different colors, and branch out and re-merge, or they are partly superimposed, indicating different movement episodes. The cumulative displacement is estimated to be about 10m using the geological cross-sections, from which it is inferred that the total length of fault may be about 2.5km on the basis of the empirical relation between cumulative displacement and fault length. Therefore, a more study would be needed to verify the entire fault length. The results of ESR dating for three gouge samples at different spots along the fault yields ages of 651±47ka, 649±96ka and 436±66ka, indicating at least two movement episodes. Slickenlines observed on the fault planes indicate a pure dip slip (normal faulting), which suggests that the ENE-WSW trending Jingwan Fault was presumably moved under a NNW-SSE extensional environment. 한반도 서부인 당진 지역에서 새로이 발견된 제4기 진관단층의 운동 특성과 단층활동 이력에 대한 윤곽을 잡기 위해 몇 개의 단층비지대로 구성된 진관단층의 기하 분석과 단층비지 물질에 대한 ESR 연령측정을 하였다. 진관단층은 정단층으로, N55°E 주향의 이 단층은 낮은 곳에서는 경사가 57°NW, 상부로 갈수록 경사가 수직에 가깝게 변화를 보여주며 리스트릭 단층임을 보여준다. 단층비지대의 두께는 노두 하부의 기반암과 기반암 사이에서 2~3cm, 중상부의 기반암과 제4기층 사이 경계에서는 20~30cm 폭으로 보다 두껍다. 후자의 경우에 있어서 3개 이상의 단층비지대가 분리되거나 합쳐지기도 하며, 부분적으로는 서로 중첩되기도 하는데 이는 서로 다른 단층운동 사건들이 있었음을 지시한다. 지질단면도로부터 산출된 단층의 누적 총변위는 약 10m이며 누적 총변위와 단층의 길이 사이 경험식에 따르면 약 2.5km의 단층 길이에 해당된다고 할 수 있다. 그러므로 단층의 길이를 제대로 알기 위해 보다 연구가 요구된다. 단층대를 따라 채취한 세 단층암 샘플에 대한 ESR 연령측정 결과, 651±47ka, 649±96ka, 436±66ka 등으로 나타났으며 최소 두 번의 단층운동이 있었음을 지시한다. 단층면에서 관찰되는 단층조선은 순전한 경사이동만을 지시하며(정단층 운동), 동북동-서남서 주향의 진관단층은 북북서-동남동 방향의 수평 최소 압축응력 환경에서 단층운동이 있었음을 제시한다.

Microstructural Features and K-Ar Ages of Fault Gouges from Quaternary Faults along the Northern Yangsan Fault, SE Korea

추창오,장태우,남경훈,김종태,이창주,정교철 대한지질공학회 2023 지질공학 Vol.33 No.1

Microstructural characterization, identification of mineral assemblages, and K-Ar age dating of fault gouges from five Quaternary fault sites segmented along the northern Yangsan Fault, SE Korea were performed to understand formation condition and multiple activity of faults. The mean and median sizes of particles of bulk gouges vary among the studied faults: 1.75 μm and 1.43 μm for the Danguri Fault, 1.94 μm and 1.79 μm for the Yukjae Fault, 5.57 μm and 4.16 μm for the Yugye Fault, and 5.55 μm and 2.31 μm for the Bogyeongsa Fault. Fault gouges contain abundant secondary minerals, including smectite, chlorite, illite, kaolinite, laumontite, and mordenite, which are found in association with quartz and feldspar. K-Ar dating of the fault gouges (both bulk samples and separate size fractions) yields ages ranging from 59.1 to 18.8 Ma, with bulk ages of 47.6 Ma for the Yukjae Fault, 59.1 Ma for the Ansim Fault, 39.4 Ma for the Yugye Fault, and 22.6 Ma for the Bogyeongsa Fault. The finer fractions generally have younger K-Ar ages compared with the coarser fractions, and the finest fraction (<0.2 μm) is the youngest for each fault. Hydrothermal alteration of the gouges is considered to have occurred under low-temperature (100~200°C) conditions during faulting. Microstructural features and clay mineral assemblages of fault gouges and brecciated rocks should be considered when interpreting fault events and reactivation, in addition to age dating of faulting.

Formation of talc fault gouge analog using high-energy ball mill

김현나,김진우,소병달,김영석,이범한,김진철 한국지질과학협의회 2022 Geosciences Journal Vol.26 No.6

Fault gouge, located in highly deformed fault cores, shows substantial particle size reduction and loss of crystallinity, which could affect the physicochemical properties and thus control the slip behavior and earthquake stability of a fault. To systematically investigate the effect of deformed fault gouge on fault slip behavior in laboratory-scale experiments, a fault gouge analog with variable particle size and crystallinity is required. However, the systematic study for formation of a fault gouge analog (i.e., controlling the particle size and crystallinity) was not performed well. In this study, we investigated the effect of the rotation speed of a high-energy ball mill on particle size reduction rate and degree of amorphization. As the rotation speed increased from 500 to 2000 rpm, the comminution rate linearly increased, and reached a reduced particle size limit. The degree of amorphization and its rate also increased with increasing rotation speed. Upon grinding, the X-ray diffraction (XRD) peaks markedly decreased and reached critical amorphization points within 120 min, except for the case of grinding with a rotation speed of 500 rpm. Talc ground with a rotation speed of 500 rpm did not reach steady-state in mechanical amorphization after 360 min of grinding, thus necessitating prolonged grinding for further amorphization. A comparison of talc powders having similar specific surface areas, but ground at different rotation speeds, shows that grinding with a higher rotation speed for a shorter duration preserves the crystalline structure relatively well compared with grinding at a lower rotation speed for a longer duration. These results indicate that optimizing the grinding rotation speed facilitates the formation of talc fault gouge analogs with systematically varying particle size and crystallinity. The specific surface area of talc increased from 6.1 to 365 m2/g, and the degree of crystallinity decreased from approximately 75% to 11%. Our results indicate that an artificial talc fault gouge can be prepared by varying the particle size and crystallinity using a high-energy ball mill.

Occurrence of Dolomite in Yangsan Fault, Yeongdeok, Gyeongbuk, Korea, and Implications for Possible Fault Activity Related to Seawater

( Chang-oh Choo ),( Suhyeon Jo ),( Gyo-cheol Jeong ) 대한지질공학회 2019 대한지질공학회 학술발표회논문집 Vol.2019 No.2

This study focused on characterization of dolomite occurring in Yangsan Fault, Namsan-ri, Yeongdeok, Korea and provided implications for possible fault activity related to seawater. Dolomite was commonly identified as veinlets or irregular lenticular pocket in volcanic rocks, suggesting specific origin so called hydrothermal dolomite (HTD). The fault is characterized by a fault core with 1.2 m thickness and the fault planes have N40~50E/74~80SE. Brecciated fragments and veinlets develop in the fault zone where fault gouge with 10 cm thickness is observed, though negligible in some places. We analyzed fractured materials collected from the fault core using photomicroscope on thin sections, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Main mineralogy of fractured materials consists of quartz, illite, dolomite, pyrite, and iron oxides. Quartz as residual grains in gouge indicates that it experienced size reduction with grain rotation during faulting. It was assumed from field and textural observations that dolomite formed right after main fault event, possibly due to seismic pump effect or injection of seawater associated with highly brine deep water. HTD has implications for fault activity related to seawater in northern part of the Yangsan Fault, quite different from that found in other sites of the fault. The research was supported by Earthquake disaster prevention specialist training funded by Administration safety, Korean government.

영덕군 병곡면의 단층 노두 특성에 대한 연구

신원정 ( Won Jeong Shin ),김종연 ( Jong Yeon Kim ) 한국지형학회 2021 한국지형학회지 Vol.28 No.3

In the southeastern part of the Korean Peninsula, the Yangsan Fault, an active fault zone, has developed. Many earthquakes occur around these faults, and the possibility of earthquakes occurring along the branch faults is being discussed. On the other hand, the Yeongdeok Fault is reported in Yeongdeok-gun, which is the northern part of the Yangsan fault. In this study, goemorphic characteristics of a set faults found on the outcrop of the gentle slope of the coast of Byeonggok-myeon were analyzed and granulometric and geochemical characteristics of sediments and other materials, including fault gouges were analyzed. The outcrop of Byeonggok-myeon is the part of the fault core and can be divided into two parts. Theses fault are formed on the upper part of the Mesozoic bedrock and the tertiary sedimentary layer of red sand-supported clasts are covered in several sedimentary units. The faults were normal fault sets, and a number of vertical cracks were developed, and glossy surfaces were observed in the fault area. It appears that these faults have occurred after alluvial deposition had been formed. In the case of samples from fault gouges, there were differences in particle size and geochemical characteristics from the surrounding area.

울산단층 동부지역 제4기단층 비지대의 체적변화와 유체-암석비에 대한 고찰

장태우 ( Tae Woo Chang ),채연준 ( Yeon Zoon Chae ),추창오 ( Chang Oh Choo ) 대한지질공학회 2005 지질공학 Vol.15 No.3

울산단층 동부 지역에는 제4기로 추정되는 미고결 퇴적층을 절단하며 파쇄대와 단층비지를 수반하는 제4기 단층이 여러 곳에서 발견되고 있다. 본 연구에서는 제4기 단층비지대의 성분변화, 체적변화, 실리카의 손실 그리고 유체-암석 비를 XRF, XRD, EPMA를 이용하여 계산하였다. 단층비지의 성분변화는 모암에 비해 단층비지대에서 SiO2, K2O, Na2O는 감소하고, Al2O3, MgO, P2O5, Fe2O3, MnO, CaO, LOI는 증가한다. 단층별 체적감소는 개곡 제1단층 56%, 개곡 제2단층 22%, 신계단층 34%, 마동단층 8%, 원원사단층 2%, 진현단층 53% 이다. 체적감소, 실리카 손실, 유체-암석비가 낮게 나타난 마동단층과 원원사단층 비지대는 유체활동에 대하여 닫힌계이고, 체적감소, 실리카 손실, 유체-암석 비가 높은 개곡 제1단층과 진현단층의 비지대는 유체 활동의 통로로 작용한 열린계였을 것으로 볼 수 있다. 단층의 유체-암석 체적비는 모든 단층에서 102~104로 나타나지만 기질의 함량이 각각 88%와 77%로 높고 체적감소가 큰 개곡 제1단층과 진현단층에서 가장 높게 나타난다. 따라서 이들 단층에서 유체의 활동이 가장 활발하였다. Many Quaternary faults are recognized as thin gouge and narrow cataclastic zone juxtaposing the Bulguksa granite and Quaternary deposit bed in the eastern block of the Ulsan Fault, Korea: Gaegok 1, Gaegok 2, Singye, Madong, Wonwonsa and Jinhyeon faults. This study was performed to calculate chemical change, volume change, silica loss and fluid-rock ratio taken place in gouge zones of these Quaternary faults using XRF, XRD, EPMA. The chemical compositions of fault rocks reveal that the fault gouges are depleted in SiO2, Na2O and K2O and enriched in Al2O3, Fe2O3, P2O5, MgO, MnO CaO and LOI(H2O+CO2) relative to protoliths. The fact that there is enrichment of relatively immobile elements and depletion of the more soluble elements in the fault gouges relative to protoliths can be explained by fluid-assisted volume loss of 56% for Gaegok 1 fault, 22% for Gaegok 2 fault, 34% for Singye fault, 8% for Madong fault, 2% for the Wonwonsa fault and 53% for the Jinhyeon fault. Madong fault and Wonwonsa fault where ratios of the volume change, silica loss and fluid-rock are low might have acted as a closed system for fluid activity, whereas Gaegok 1 fault and Jinhyeon fault with high ratios in those factors be an open system. The volumetric fluid-rock ratios range 102~104 for all faults, being highest in Gaegok 1 fault and Jinhyeon fault whose fluid activity was most significant.