Fig 1 - uploaded by Ohsang Gwon
Content may be subject to copyright.
(a) Tectonic map around the Korean Peninsula (modified from Schellart and Rawlinson, 2010). (b) Simplified geological map of the Gyeongsang Basin, SE Korea and major faults (modified from Chough and Shon, 2010; Son et al., 2013). Epicenters for the Gyeongju earthquake (Mw 5.5, 12 th September 2016) and the Pohang earthquake (Mw 5.4, 15 th November 2017). (c) Topographic overview of the study area.
Source publication
Based on historical earthquakes and paleoseismical data, the Ulsan fault zone has a higher potential of large earthquakes in Korea. Thus, it is very important to understand the paleoseismic characteristics of the Ulsan fault in order to reduce future earthquake hazards. The NNW-trending Ulsan fault has been known as a reverse dominant fault. Howeve...
Similar publications
During earthquakes, fault rupture can involve multiple segments in synchronous or cascade mechanisms, leading to an increasing magnitude of the mainshock or rate of aftershocks. Since the seismogenic portions of faults are inaccessible, studying the geometrical and mechanical interaction between exhumed fault segments can contribute to the understa...
Citations
... The identification of the Seokgye fault and understanding of its paleoseismic activity is crucial for SE Korea, as it is one of the most active areas in the Korean Peninsula. While numerous Quaternary fault outcrops have been identified and reported along the southeastern Korean peninsula, existing studies have primarily focused on large faults with clear morphologies (Choi et al., 2015;Kim et al., 2020a, b;Gwon et al., 2021;Naik et al., 2022;Park et al., 2022;Kim et al., 2023a, b). However, small-scale faults within major fault systems in the intraplate region are equally crucial for comprehending the overall tectonic process and seismic hazards. ...
... Our interpretation aligns with reported evidence of Quaternary thrust faults along the SE Korean Peninsula. Previous studies (Okada et al., 2001;Ryu et al., 2002;Ree et al., 2003;Kim et al., 2004;Choi et al., 2014;Gwon et al., 2021;Park et al., 2022;Naik et al., 2022). suggest that many Quaternary faults in the region may have experienced reactivation under changing stress conditions, either at a local or regional scale, along pre-existing normal faults (Ree et al., 2003;Kim et al., 2004Kim et al., , 2011Kim et al., , 2023aSon et al., 2015;Choi et al., 2015). ...
... Therefore, it has long been considered a tectonically stable region for earthquake hazards compared with neighboring countries such as China and Japan. The largest instrumentally recorded earthquakes (the 2016 Gyeongju earthquake of M w 5.5 and the 2017 Pohang earthquake of M w 5.4, Fig. 1a) occurred in the southeastern part of the Korean Peninsula, and here there are also many records of paleoseismic activity along the predominant mature faults such as the Yangsan and Ulsan faults (Fig. 1a, e g; Cheon et al., 2020Cheon et al., , 2023aCheon et al., , 2023bGwon et al., 2021;Ha et al., 2022;Ko et al., 2022;Lee et al., 2022;Naik et al., 2022;Park et al., 2022;Kim et al., , 2023. For these reasons, most of the structural and paleoseismic studies of active or Quaternary faults have been focused on this region of Korea. ...
... Chang, 2002;Chwae et al., 1998;Cheon et al., 2020;Choi, 2003;Choi, 2005;Choi et al., 2012;Gwon et al., 2021;Ha et al., 2022;Im et al., 2003;Jin et al., 2013;Kang and Ryoo, 2009;Kee et al., 2007;Kim and Jin, 2006;Kim et al., 2004Kim et al., , 2011Kim et al., 2016;Kim et al., 2021;Ko et al., 2022;Kyung, 1997Kyung, , 2003Kyung et al., 1999aKyung et al., , 1999bKyung and Chang, 2001;Lee et al., 1999;Lee, 2003;Lee et al., 2015;Lee et al., 2017;Lee et al., 2022;Lim et al., 2021;Naik et al., 2022;Okada et al., 1994;Ryoo et al., 1996Ryoo et al., , 1999Ryoo et al., , 2001Ryoo et al., , 2002Ryoo et al., , 2006Ryoo et al., , 2021Song et al., 2020), 대규모 단층들을 따른 제4기 단층의 연장, 시· 공간적 고지진 이력 등에 대한 자료들이 축적됨에 따라 한반도 남동부에 대한 고지진 연구가 발전하고 있다. 이번 연구는 경상남도 양산시 동면 금산리 일원의 가산일반산업단지 조성 공사현장 사면에서 발견된 미 고결 거력층을 절단하는 단층들에 대하여 소개하고자 한다 (Fig. 1b, Fig. 2 (Cha, 1976;Lee, 1980;Lee, 1982;Lee et al., 1999;Son et al., 1978;Yun et al., 2005) (Chang et al., 1990;Cheon et al., 2019;Choi et al., 1980;Hwang et al., 2004Hwang et al., , 2007aHwang et al., , 2007bReedman and Um, 1975;Um et al., 1983). ...
... 단층 상반과 하반에서 관찰되는 화강암과 미고결 퇴적층 사이의 부정합면은 최대 약 1 m의 낙차를 보이지만, 단층으 로 직접 절단된 부정합면은 약 20 cm의 수직 변위를 보인다 (Fig. 5b). Choi et al. (2017) 는 여러 가지 변수(평균변위, 최대변위, 지표파열길 이, 파열면적 등)들 중 굴착단면에서 관찰되는 최대 변위를 주로 사용하며 (Kanamori, 1977;Slemmons, 1982;Bonilla et al., 1984;Khromovskikh, 1989;Wells and Coppersmith, 1994), 대부분의 지구조 환 경에서 적용 가능한 Wells and Coppersmith (1994) 가 제안한 최대변위-모멘트규모 경험식을 활용하여 규모를 산정하는 경우가 많다 (Kyung, 2010;Kim and Jin, 2006;Jin et al., 2013;Lee et al., 2017;Song et al., 2020;Lim et al., 2021;Gwon et al., 2021;Lee et al., 2022 ...
We analyzed the structural characteristics of faults that cut unconsolidated conglomerates on three construction slopes in Geumsan-ri, Yangsan-si. This outcrop is located approximately 0.6 km north of the Gasan site, where Quaternary faulting has been previously reported. The six faults observed in Geumsan-ri have N14o-32oE strikes; the dip of three faults is 53o-62oSE, while that of other faults is 77o-87oNW. The unconsolidated conglomerates cut by the faults are the fanglomerates derived from Mt. Geumjeong and are mainly composed of granitic or volcanic boulders with a diameter of more than 0.5 m. Slickenlines observed on the fault surfaces indicate a dextral strike-slip sense with a small reverse-slip component under ENE-WSW maximum horizontal stress field, which corresponds with the current stress environment in the Korean Peninsula. The vertical separations of the faults on the east and west sides, calculated from the offset unconformity between the unconsolidated sediments and granite, were estimated to be 15 m and 1 m, respectively.
... Muryongsan (450 m), from north to south. Paleoseismic and structural investigations along the western front of the mountain range, which is composed mostly of alluvial fans, show records of reverse faulting during the late Quaternary, characterized by several NNW-SSE-to N-S-striking fault strands and top-to-the-west kinematics ( Figure 1C; e.g., Song and Kyung, 1996;Okada et al., 1998;Chang, 2001;Ryoo et al., 2002;Ryoo, 2009;Choi et al., 2014Choi et al., , 2015Kim et al., 2021c;Gwon et al., 2021;Naik et al., 2022;Park et al., 2022). The geomorphic feature of the western valley has been interpreted as an incised fault valley ( Figure 1C; Kim, 1973;Kim et al., 1976;Kang, 1979a and b), although no clear field evidence has been presented for faults within the valley itself. ...
... Along the eastern margin of the valley (i.e., the western front of the mountain range), a number of fault outcrops have been identified, some of which show that the fault cuts young unconsolidated sediments ( Figure 1C; Table 1). In outcrops, trench walls, and drilling surveys, the faults show typical reverse-slip features, such as east-side-up apparent offset, drag pattern, and flat-ramp geometry (e.g., Ryoo et al., 2001Ryoo et al., , 2002Ryoo et al., , 2004Choi et al., 2002Choi et al., , 2003Park et al., 2020;Kim et al., 2021c;Gwon et al., 2021;Naik et al., 2022;Park et al., 2022). The fault strands are featured by slightly curved multiple strands (generally NNW-SSE to N-S), which anastomose and link up with each other. ...
... Fault strands on the two sides of the Ulsan Fault Zone show variation in their predominant geometry, that is, their dip angle and kinematics. Faults along the eastern margin of the valley generally exhibit moderate-to low-angle dips (20°-65°) and mainly reverseslip sense (Table 1; e.g., Kim et al., 2020b;Kim et al., 2021c;Gwon et al., 2021;Naik et al., 2022). Here, we discuss why the eastern edge of the Ulsan Fault Zone shows different features from the western edge and the potential relationship of the fault zone with the YTL. ...
Integration of geological and geophysical data is essential to elucidate the configuration and geometry of surface and subsurface structures, as well as their long-term evolution. The NNW–SSE-striking incised valley and parallel mountain range in the southeastern margin of the Korean Peninsula, extending 50 km from Gyeongju to Ulsan cities, are together regarded as one of the most prominent geographical features in South Korea. This paper presents an investigation into the structural architecture and deformation history of the valley and mountain range during the late Cenozoic based on combined data from field observations and gravity and electrical resistivity surveys. Our results based on integrated and reconciled geological, structural, and geophysical data are as follows. First, the incised fault valley can be divided into 1) the northern part, which comprises several distributed buried or exposed fault strands; and 2) the southern part, which comprises a concentrated deformation zone along the eastern margin of the valley. Different deformation features between the two parts are controlled by the lithology of host rocks and by the location and geometry of the neighboring major structures, that is, the Yeonil Tectonic Line (YTL) and the Yangsan Fault. Second, we defined the Ulsan Fault Zone as a NNW–SSE-to N–S-striking fault within the incised valley and along the eastern margin of the valley. In particular, the constituent strands located along the eastern margin of the valley have acted mainly as an imbricate thrust zone, characterized by an east-side-up geometry with moderate to low dip angles and reverse-dominant kinematics in the shallow subsurface during the Quaternary. Third, reactivated strands within the Ulsan Fault Zone during the Quaternary are interpreted as shortcut faults developed in the footwall of Miocene subvertical structures, predominantly the YTL. In addition, movements on the Ulsan Fault Zone and the YTL during the Miocene to Quaternary were arrested by the NNE–SSW-striking Yangsan Fault, which was a prominent and mature pre-existing structure. Our results highlight the spatiotemporal structural variation in SE Korea and emphasize the strong control of the configuration and geometry of pre-existing structures on the distribution and characteristics (i.e., geometry and kinematics) of the subsequent deformation under changing tectonic environments through the late Cenozoic.
... 이번 Kanamori, 1977;Slemmons, 1982;Bonilla et al., 1984;Khromovskikh, 1989;Mason, 1992;Wells and Coppersmith, 1994). 이중 Wells and Coppersmith (1994)의 경험식은 전 세계적으로 가 장 많이 인용되며 모든 지구조 환경과 모든 단층 종 류에 적용할 수 있고, 판 내부에 속하는 한반도 환경 에서도 널리 활용되고 있다 (Kyung, 2010;Kim and Jin, 2006;Jin et al., 2013;Lee et al., 2017;Song et al., 2020;Gwon et al., 2021). 이번 연구에서는 Wells and Coppersmith (1994) ...
In this study, we conducted a geomorphologic analysis, fieldwork, geophysical survey, and trench survey to trace the Quaternary faulting along the northern Yangsan fault. We also complement the study with the Electron Spin Resonance and luminescence datings of the fault rocks and Quaternary sediments, respectively, to
constrain the timing of the Quaternary faulting. As a result of geomorphological analysis, two lineaments with lengths 6.8 km and 0.35 km are recognized by the linear distribution pattern of fault saddles and deflected streams. The amount of deflection of deflected streams is 2.05-2.82 km, 1.72 and 0.74 km, and 0.43 km in Seojeongcheon, Cheonghacheon, and Gwangcheon stream, respectively, which decreases gradually toward the north. Faults cutting the unconsolidated sediments were found at an outcrop and trench site, and both the sites have the following similar features: slickenlines on the fault surface in contact with the Quaternary sediments, indicating a dextral strike-slip including a reverse component, and a mixed zone of Quaternary sediment and fault gouge between the fault core and the Quaternary sediment. Their cross-cutting relationship indicates at least two faulting events during the Quaternary. Luminescence ages obtained from the sediment layers indicate that the latest faulting event occurred between 154±13 ka and 0.9±0.1 ka. The slip rate, calculated by dividing the net displacement by the age gap, is 0.007 to 0.008 mm/yr. Also, the conversion of the vertical displacement of the Yugye site reported in the previous study into the net displacement gives rise to an estimated slip rate of about 0.08 mm/yr. Combining the results, the slip rate of the northern Yangsan fault in the study area is estimated to be in the range of 0.007 to 0.08 mm/yr. The moment magnitude (Mw) obtained using the net displacement in the empirical equation is about 6.7, and the magnitude of about 6.0 is derived using the lineament length as the surface rupture length.
... The southern segment of the Ulsan fault is ~20 km long. Despite of its ~20 km long length, only one reported fault outcrop suggests the faulting activity prior to 55 ka along the southern Ulsan fault (Son et al., 2013;Gwon et al., 2021). The exact location of the fault traces is still not available accurately. ...
... (a) Photomosaic of Seongjeong-3 trench wall, (b) sketch of the Seongjeong-3 trench log showing evidences of Quaternary reverse faulting, (c) slickenline at the base of the Unit-b indicate the fault moved after deposition of Unit-b (modified fromGwon et al., 2021). ...
In seismic hazard assessment, subsurface geophysical surveying has gained popularity in recent years towards fault mapping and determining seismic deformation parameters such as fault offset, recurrence, and depth of fault, locating proper trench sites based on the subsurface information. In the present study, electrical resistivity tomography (ERT) was used to (1) locate the trace of the southern segment of the Ulsan fault, (2) to test the applicability of ERT techniques for active fault mapping in a close to the highly urbanized and complex geological environment with a slow tectonic activity. We have applied the ERT technique at five sites. At one place, we used the Wenner array, Schlumberger array, Dipole-Dipole array, and Seismic-profiling techniques to know which method provides a better result in complex geological conditions like Korea. Out of these methods, the Dipole-Dipole array provided high-resolution results and was used for the other two sites. The ERT result shows high and low resistivity zones interpreted as bedrock (mainly Tertiary and Cretaceous formations) and coarse fluvial sediment layer, respectively. The maximum vertical displacement recorded along the fault varies from 6 m to 12 m. Based on the ERT results, two trenches were excavated to directly investigate the deformation pattern associated with the southern segment of the Ulsan fault. The ERT and trench survey results support that the southern Ulsan fault has slipped multiple times since Quaternary. Using this multi-approach, ~5 km long active fault map was prepared for the southern Ulsan Fault. It is found from this study that the integrated approach is highly beneficial where contrasting sub-lithological units exist in terms of their physical properties, even though human activity or the ongoing urbanization process has modified the surface morphology. This study argues for judicious use of ERT techniques to delineate the shallow subsurface geology across various active faults in the Korean peninsula and similar tectonic settings.
Transpression occurs in response to oblique convergence across a deformation zone in intraplate regions and plate boundaries. The Korean Peninsula is located at an intraplate region of the eastern Eurasian Plate and has been deformed under the ENE–WSW maximum horizontal compression since the late Pliocene. In this study, we analyzed short-term instrumental seismic (focal mechanism) and long-term paleoseismic (Quaternary fault outcrop) data to decipher the neotectonic crustal deformation pattern in the southeastern Korean Peninsula. Available (paleo-)seismic data acquired from an NNE–SSW trending deformation zone between the Yangsan and Ulleung fault zones indicate spatial partitioning of crustal deformation by NNW–SSE to NNE–SSW striking reverse faults and NNE–SSW striking strike-slip faults, supporting a strike-slip partitioned transpression model. The instantaneous and finite neotectonic strains, estimated from the focal mechanism and Quaternary outcrop data, respectively, show discrepancies in their axes, which can be attributed to the switching between extensional and intermediate axes of finite strain during the accumulation of wrench-dominated transpression. Notably, some major faults, including the Yangsan and Ulsan fault zones, are relatively misoriented to slip under the current stress condition but, paradoxically, have more (paleo-)seismic records indicating their role in accommodating the neotectonic transpressional strain. We propose that fluids, heat flow, and lithospheric structure are potential factors affecting the reactivation of the relatively misoriented major faults. Our findings provide insights into the accommodation pattern of strain associated with the neotectonic crustal extrusion in an intraplate region of the eastern Eurasian Plate in response to the collision of the Indian Plate and the subduction of the Pacific/Philippine Sea Plates.
