Yangsan Fault Research Articles

Characteristics of the Main Fault Zone Developed Along Yangsan Fault : On the Outcrop of Cheonjeon-ri, Dudong-myeon, Ulju-gun, Ulsan, Korea

Characteristics of the Main Fault Zone Developed Along Yangsan Fault : On the Outcrop of Cheonjeon-ri, Dudong-myeon, Ulju-gun, Ulsan, Korea

Analyzing groundwater level anomalies in a fault zone in Korea caused by local and offshore earthquakes

This study analyzed groundwater level changes in a monitoring well located in Korea’s Yangsan fault zone, induced by local (Gyeongju and Pohang) earthquakes in Korea and offshore earthquakes around the world. Among offshore earthquakes, the MW 7.3 Honshu earthquake changed raw groundwater level of a maximum of 10.3 cm. The groundwater level data were analyzed by using the Hilbert-Huang transform (HHT), modified moving average (MMA), and simple subtraction (SS) methods. In the first stage, earthquake-induced groundwater level changes were identified using the MMA and SS methods because the SS and MMA methods may excellently be used for identifying earthquake-induced groundwater level anomalies as a convenient way to identify abnormal fluctuations. Then, the HHT technique was applied to the 2016–2017 groundwater level data for comparison with the MMA and SS methods. The HHT method has been proven to be the most accurate method for detecting the effects of earthquakes on groundwater levels. The differences of water levels analyzed by the HHT method and the other two methods resulted in between 0.4 and 1.9 cm, showing closer levels between the HHT and SS methods.

Tectonically controlled multiple stages of deformation along the Yangsan Fault Zone, SE Korea, since Late Cretaceous

We describe the structural characteristics and tectonic evolution of the NNE–SSW-striking Yangsan Fault, Korea. The surface trace of the fault extends for over 170 km on land, displaying ∼20–30 km of dextral offset. It transects mainly Mesozoic–Cenozoic sedimentary and igneous rocks. Our field observations suggest that the style of deformation within the fault core is controlled by the ductility of the protoliths. Where sedimentary rocks have been faulted, fault cores tend to be relatively wide, having hosted multiple events. Internal fault core structures are characterized by networks of multiple anastomosing strands of fault gouge and breccia, which enclose lenses of fractured protolith. However, where igneous (crystalline) rocks have been faulted, slip tends to be much more localized within narrow fault cores comprising cataclastic rocks. Combining our new data with previous research results, we infer that four main movement phases have occurred on the fault: (1) Late Cretaceous sinistral slip faulting with a component of extensional deformation, under a regime of NW–SE compression; (2) late Paleogene intense dextral slip faulting, which occurred under a regional NE–SW compressional regime; (3) middle Miocene weak sinistral slip faulting, during NNW–SSE compression; and (4) local Quaternary dextral slip faulting with a reverse component, under ENE–WSW to E-W compression. The most intense phase of deformation appears to have occurred during the late Paleogene due to a major reorganization of plate motion in and around the NW Pacific area. Our findings highlight that the fault has been a preferred zone of weakness for episodic reactivations having significant implications for the changing tectonic environments of East Asia from Late Cretaceous to Quaternary, and that its segments underwent distinguishable deformations at each phase probably due to their geometrical features and fault rock properties.

Temporal and Spatial Variations of the M<sub>L</sub> 5.8 Gyeongju Earthquake on September 12, 2016

An earthquake of ML 5.8 hit the Gyeongju area on September 12, 2016. A sequence of foreshock-mainshockaftershock of 588 events with equal to or greater than magnitude 1.5 occurred for six months in this area. Around ninetynine percentage (98.8%) of the total energy was released intensively within a day, and about 80% of the total events took place within a month after the Gyeongju earthquake. The epicentral distribution of aftershocks of major events (ML 5.1, 5.8, 4.5, and 3.5) were elongated in the direction of N30 o E. They correlate well with the focal mechanism solution. These facts support the inference that the Gyeongju earthquakes occurred on a sub-parallel subsidiary fault of the Yangsan fault zone or on the linking damage zones between Deokcheon and Yangsan fault. During the last six years before the Gyeongju earthquake, there were few events within 10-km radius from the epicenter. This seismic gap area was filled with a sequence of the Gyeongju earthquakes. The b value for aftershock of the Gyeongju earthquakes is 1.09.

Seismicity of the 2016 ML 5.8 Gyeongju earthquake and aftershocks in South Korea

Since the modern seismograph network by the Korea Meteorological Administration (KMA) had been operated from 1978, the largest earthquake (ML 5.8) occurred in Gyeongju, South Korea on 12 September 2016. ML 5.1 and 5.8 earthquakes occurred successively and damaged mostly old building structures around Gyeongju. Aftershocks continued to occur, clustering around the epicenters of the two events. Number of observed aftershocks with ML ≥ 0.1 was 3,376 from 12 September 2016 to 31 March 2017 and these aftershocks were located by manual procedure. Since the origin time of the ML 5.8 earthquake, 339 and 672 aftershocks (ML ≥ 0.1) occurred on 12 and 13 September 2016, respectively. The occurrence rate of aftershocks decreased exponentially with time after 16 September. It is observed that the seismicity of the 2016 Gyeongju earthquake and aftershock sequences is dominant in the Yangsan Fault System. The distribution of aftershocks is NNE-SSW direction along the Yangsan Fault System, which agrees with the fault plane solutions of main shock and foreshock showing the strike-slip faulting. These aftershocks concentrate within the epicentral distance of 5 km from main shock and the depth distribution of aftershocks is mainly around 11–15 km with vertical trends. With regard to the seismicity of the 2016 Gyeongju earthquake’s aftershocks the full catalog with ML ≥ 1.2 based on Mc is suggested in this study. Finally the distribution of aftershock sequence correlated to the focal mechanisms can provide specific seismicity in the Gyeongsang Basin.

양산 단층곡 경주 지역의 단층 지형 분석

양산 단층곡 경주 지역의 단층 지형 분석

Fault Rupture Model of the 2016 Gyeongju, South Korea, Earthquake and Its Implication for the Underground Fault System

AbstractThe 2016 Gyeongju earthquake (ML 5.8) was the largest instrumentally recorded inland event in South Korea. It occurred in the southeast of the Korean Peninsula and was preceded by a large ML 5.1 foreshock. The aftershock seismicity data indicate that these earthquakes occurred on two closely collocated parallel faults that are oblique to the surface trace of the Yangsan fault. We investigate the rupture properties of these earthquakes using finite‐fault slip inversion analyses. The obtained models indicate that the ruptures propagated NNE‐ward and SSW‐ward for the main shock and the large foreshock, respectively. This indicates that these earthquakes occurred on right‐step faults and were initiated around a fault jog. The stress drops were up to 62 and 43 MPa for the main shock and the largest foreshock, respectively. These high stress drops imply high strength excess, which may be overcome by the stress concentration around the fault jog.

On the Latest Tectonic Environment Around Northern Part of the Yangsan Fault, Korea

On the Latest Tectonic Environment Around Northern Part of the Yangsan Fault, Korea

Deep Fault Plane Revealed by High‐Precision Locations of Early Aftershocks Following the 12 September 2016 ML 5.8 Gyeongju, Korea, Earthquake

An M-L 5.8 earthquake, which is large for a stable continental region, occurred in southeastern Korea on 12 September 2016. Ten days of data from a temporary seismic network deployed immediately after the mainshock are combined with data from permanent seismic stations to determine high-precision locations of early aftershocks to reveal the geometry of the causative structure at depth. Well-constrained relative earthquake hypocenters and focal mechanisms are used to define the subsurface fault plane with a strike of similar to N28 degrees E and dip of similar to 78 degrees to the east-southeast. This fault plane extends from 12 to 15 km depth and may have been responsible for most of the early earthquakes in the Gyeongju earthquake sequence. A pre-existing weak zone in a strike-slip duplex that formed from subsidiary Riedel shears beneath the Yangsan fault system may have been reactivated to nucleate the mainshock and aftershocks.

포항 북부지역 양산단층의 재활동 연대

포항 북부지역 양산단층의 재활동 연대

Crustal evolution in the Gyeongsang Arc, southeastern Korea: Geochronological, geochemical and Sr-Nd-Hf isotopic constraints from granitoid rocks

A loosely assembled patchwork of Cretaceous to Paleogene granitoid plutons comprise the platform of the Gyeongsang Arc in southeastern Korea and represent a high-flux pulse of magmatism in Northeast Asia. The present study uses new and published zircon U-Pb ages, whole-rock geochemical and Sr-Nd isotopic data, and zircon Hf isotopic data of the plutons to decipher the crustal evolution in the Gyeongsang Arc and compare it with adjacent accretionary terranes. The ion microprobe zircon U-Pb ages range from 91 to 27 Ma, revealing the selective occurrence of Paleogene plutons in the eastern coastal area, to the east of the NNE-trending Yangsan fault system. Paleoproterozoic to Paleogene xenocrystic cores are occasionally observed in the zircon grains. The plutons are composed of dominantly magnesian, high- to medium-K calc-alkaline granites-granodiorites exhibiting typical geochemical characteristics of subduction zone magma, and less abundant ferroan alkaline granites. Their trace element patterns require residual amphibole and feldspar, but not garnet in the source. The whole-rock Sr-Nd and zircon Hf isotopic compositions of the granitoids (initial 87Sr/86Sr = 0.7043 to 0.7069; eNd(t) = –4.8 to +2.9; eHf(t) = –5.0 to +19.7) are distinctly more primitive than those of inland granitoids distributed outside of the Gyeongsang backarc basin. An important role of relatively young, most likely Paleozoic juvenile crust in the formation of the Late Cretaceous granitoids is suggested by the time-eHf trend of high-eHf zircons that converges toward data points of the Late Permian Yeongdeok adakite composing the arc basement. The asthenospheric mantle input is highlighted by significantly high (>+17) eHf(t) values of some zircons from the early Eocene alkaline plutons. Subsequent reworking of the rejuvenated crust yielded granitoid plutons possessing slightly but recognizably higher eNd(t) and eHf(t) values than the older plutons. These isotopic features demonstrate that the Cretaceous-Paleogene calc-alkaline granitoids in the Gyeongsang Arc are not “juvenile” (sensu stricto) despite their mostly positive epsilon Nd and Hf values, but are basically a product of crustal reworking. The Sr-Nd-Hf isotopic compositions of Late Cretaceous granitoids in the Gyeongsang Arc are comparatively more primitive than those in adjacent accretionary terranes such as Southwest Japan and Fujian province in southeastern China, reflecting differences in the formation age of the basement on which the arc system was built.

단층대 지하수의 수리화학 및 노블가스 동위원소 특성

한반도 동남부에 발달된 양산단층과 감포지역 단층대에서 지하수의 순환에 대한 단층대의 영향을 알아보기 위하여 10개 암반 지하수를 채취하여 화학성분과 영족기체 동위원소 특성을 분석하였다. 분석결과 지하수의 화학적 유형은 <TEX>$Ca-HCO_3$</TEX> 형에서 <TEX>$Ca-SO_4(Cl)$</TEX> 형의 영역을 보이며, 지질과의 뚜렷한 상관성을 보이지 않는다. 영족기체인 <TEX>$^3He/^4He$</TEX>와 <TEX>$4^He/^{20}Ne$</TEX> 동위원소비 상관관계도에서 1점을 제외한 지하수는 대기기원의 <TEX>$^3He$</TEX>가 우세하며, 이는 단층대를 통하여 강수의 지하수로 유입이 활발한 대수층 환경임을 지시한다. 아울러 단층대를 통한 심부의 헬륨공급이 없는 환경임을 시사한다. <TEX>$^4He$</TEX>의 비율이 비교적 높은 지하수는 물-암석반응이 상대적으로 충분한 대수층조건에서 암석기원의 <TEX>$^4He$</TEX>가 일부 기원되었음을 지시한다. The chemical composition and noble gas isotopes of 10 deep groundwater samples were analyzed to know the circulation of groundwaters in the Yangsan fault and the Gampo fault. The chemical types of groundwaters show the <TEX>$Ca-HCO_3$</TEX> type and <TEX>$Ca-SO_4(Cl)$</TEX> type, and show indistinct relationship with geology. Noble gas isotopic data of most groundwaters were plotted along the air-crust mixing line on <TEX>$^3He/^4He$</TEX> vs. <TEX>$4^He/^{20}Ne$</TEX> diagram, and show dominant <TEX>$^3He$</TEX> of air origin except one sample that shows helium mixing of crust origin. This indicates that groundwater actively circulates along fault, and fault could not play an role of upward pathway of a deep-seated helium gas. A comparatively high <TEX>$^4He$</TEX> indicates that groundwater flows in an aquifer assuring relatively enough water-rock interaction.

The 12 September 2016 Gyeongju earthquakes: 1. Observation and remaining questions

Two earthquakes (M L 5.1 and 5.8) ruptured branches of the Yangsan Fault System in Gyeongju, S. Korea on September 12, 2016. After the M L 5.8 earthquake, aftershock earthquakes continued to occur, including two notable earthquakes (M L 4.3 and 4.5) on September 12 and 19, 2016. This paper details the early reports of the Yangsan Fault System in the Gyeongsang Basin from various geological and geophysical/seismological perspectives. Based on a review and an initial seismological analysis of the results of the three earthquakes (M L 5.1, 5.8, and 4.5), we present and discuss the following topics: (1) the tectonic setting and the geophysical/seismic structure of the Yangsan Fault System, (2) historical seismicity and inferred seismic hazard from historical (literature) data, and (3) source mechanisms of the three earthquakes. In the end, we highlight some of the outstanding issues with regard to earthquakes and future research topics.

The 12 September 2016 Gyeongju earthquakes: 2. Temporary seismic network for monitoring aftershocks

The ML 5.8 earthquake in Gyeongju, southeastern Korea, on September 12, 2016 11:32:54 (UTC) was the largest earthquake on the Korean Peninsula since instrumental monitoring began in 1978. It was preceded by an ML 5.1 foreshock and is being followed by numerous aftershocks. Within an hour of the mainshock, the first temporary seismic station to monitor aftershocks was installed at about 1.5 km east of the announced epicenter. The current temporary seismic network consists of 27 stations equipped with broadband sensors covering an area of ∼38 × 32 km in the mainshock region. This is the first high-density aftershock monitoring array in the Korean Peninsula. Initial results, using data from both the regional seismic networks and the aftershock monitoring array, indicate that earthquakes during the first 10 days following the mainshock are related to the Yangsan Fault System. Establishment of an official rapid-response team to monitor aftershocks of major earthquakes is advised.

The offshore Yangsan fault activity in the Quaternary, SE Korea: Analysis of high-resolution seismic profiles

The NNE-trending dextral Yangsan fault is a>190-km-long structure in the Korean Peninsula traced to the southeastern coast. The scarcity of Quaternary deposits onland precludes any detailed investigation of the Quaternary activity and structure of the Yangsan fault using seismic reflection profiling. We acquired offshore high-resolution seismic profiles to investigate the extension of the Yangsan fault and constrain its Quaternary activity using stratigraphic markers. The seismic profiles reveal a NNE-trending fault system consisting of a main fault and an array of subsidiary faults that displaced Quaternary sequences. Stratigraphic analysis of seismic profiles indicates that the offshore faults were activated repeatedly in the Quaternary. The up-to-the-east sense of throw on the main fault and plan-view pattern of the fault system are explained by dextral strike-slip faulting. The main fault, when projected toward the Korean Peninsula along its strike, aligns well with the Yangsan fault. We suggest that the offshore fault system is a continuation of the Yangsan fault and has spatial correlation with weak but ongoing seismicity.

Internal structure and materials of the Yangsan fault, Bogyeongsa area, Pohang, South Korea

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.

제한된 천부자료와 시추코어분석을 통한 심부지질단면도 작성과 단층 인지법: 한반도 남동부 양산단층대 주변에서의 적용

Deep geological cross-sectional data is generally not common nor easy to construct, because it is expensive and requires a great deal of time. As a result, geological interpretations at depth are limited. Many scientists attempt to construct geological cross-sections at depth using geological surface data and geophysical data. In this paper, we suggest a method for constructing cross-sections from limited geological surface data in a target area. The reason for this study is to construct and interpret geological cros-sections at depth to evaluate heat flow anomaly along the Yangsan fault. The Yangsan Fault passes through the south-eastern part of the Korean Peninsula. The cross-section is constructed from Sangbukmyeon to Unchonmyeon passing perpendicularly through the Yangsan Fault System trending NW-SE direction. The geological cross-section is constructed using the following data: (1) Lithologic distributions and main structural elements. (2) Extensity of sedimentary rock and igneous rock, from field mapping. (3) Fault dimension calculated based on geometry of exposed surface rupture, and (4) Seismic and core logging data. The Yangsan Fault System is composed of the Jain fault, Milyang fault, Moryang fault, Yangsan fault, Dongnae fault, and Ingwang fault which strike NNE-SSW. According to field observation, the western section of the Yangsan fault bounded by igneous rocks and in the eastern section sedimentary rocks are dominant. Using surface fault length we infer that the Yangsan Fault System has developed to a depth of kilometers beneath the surface. According to seismic data, sedimentary rocks that are adjacent to the Yangsan fault are thin and getting thicker towards the east of the section. In this study we also suggest a new method to recognize faults using core loggings. This analysis could be used to estimate fault locations at different scales.

상천리 일대 양산단층의 재활동 연대

상천리 일대 양산단층의 재활동 연대

Frictional properties of gouges collected from the Yangsan Fault, SE Korea

한반도 남동부에 발달한 주 단층들 가운데 하나인 양산단층의 마찰특성을 이해하기 위하여 단층비지를 대상으로 전단속도변화실험(velocity stepping test)을 실시하였다. 단층비지 시료들은 경남 양산시 상천리 지역과 경북 포항시 보경사 인근에 위치한 단층 노두의 단층핵(fault core)과 부차 단층(subsidiary fault)으로부터 각각 채취하였다. 실내 전단실험 전에 시료의 전처리, X-선 회절분석 및 입도분석을 통해 단층비지 물질의 특성을 파악하였다. 전처리 과정을 거쳐 250 μm 이하의 크기로 선별한 단층비지 분말을 대상으로 수직응력 5, 10, 20 MPa과 전단속도 1-100 μm/s의 조건 하에서 전단실험을 실시하였다. 상천리 지역 단층비지(SCL 1-1, 1-2, 1-3)와 보경사 지역 단층비지(BGS 3-1, 3-2)의 최대마찰계수는 각각 약 0.3-0.5, 0.2-0.3 사이의 값을 보였다. 시료들 가운데 보경사 지역의 단층핵으로부터 채취한 시료(BGS 3-1)에서 가장 낮은 마찰계수(0.2)가 측정되었다. 이는 다른 시료들에 비해 상대적으로 높은 몬모릴로나이트(montmorillonite) 함량에 기인한 것으로 보인다. 또한 실험 후 회수시료의 미구조 분석결과, 다른 시료에 비해 BGS 3-1에서 전단면 전면에 R1-전단띠 뿐만 아니라 Y-전단띠의 발달이 현저한 것으로 보아 이 같은 구조 발달이 시료에서 측정되는 매우 낮은 마찰계수와 직접적으로 관련된 것으로 해석된다. 실험에 이용된 대부분의 시료와 실험조건에서 양의 마찰율 파라미터(friction rate parameter) 값을 갖는 속도 강화(velocity strengthening)거동이 관찰되었다. 단층 미끌림의 시작 깊이에서 단층암의 특성이 지표 노두에서와 유사하다고 가정하면, 이는 시료 채취 지역의 양산단층 분절에서는 단층이 재활될 경우, 지진을 유발하기보다는 안정한 미끌림(aseismic or stable slip) 거동을 보일 것을 지시한다.

Seismic reflection imaging of Quaternary faulting offshore the southeastern Korean Peninsula

The Yangsan Fault System (YFS) is a dominating tectonic structure in the southeastern part of the Korean Peninsula. The YFS consists of NNE-striking dextral strike-slip faults that are traced to the southeastern coast of the Korean Peninsula. We acquired high-resolution seismic profiles offshore the southeastern Korean Peninsula to investigate how the YFS extends offshore and constrain the age of fault activity using stratigraphic interpretation. The seismic profiles image near-vertical faults trending NE to NNE that constitute a fault zone similar to a duplex structure at a releasing bend of a right-lateral strike-slip fault. The faults are interpreted as an offshore extension of the Ilgwang fault that is a member of the YFS. Stratigraphic interpretation of seismic profiles indicates that the offshore faults were activated repeatedly in the Pliocene and Quaternary. The right-lateral activity of the Ilgwang fault is consistent with the current stress regime in and around the southeastern Korean Peninsula that dictates the P-axis direction in the E-W or ENE-WSW since the Pliocene.