pISSN 1225-7281 자원환경지질, 제53권, 제5호, 585-596, 2020 eISSN 2288-7962 Econ. Environ. Geol., 53(5), 585-596, 2020 http://dx.doi.org/10.9719/EEG.2020.53.5.585 Large-scale, Miocene Mud Intrusion into the Overlying Pleistocene Coastal Sediment, Pohang City, SE Korea: Deformation Mechanism, Trigger, and Paleo-seismological Implication for the 2017 Pohang Earthquakes Yong Sik Gihm1*, Kyoungtae Ko2, Jin-Hyuk Choi2 and Sung-ja Choi2 1Department of Geology, School of Earth System Science, Kyungpook National University, Daegu 41566, Republic of Korea 2Geology Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea (Received: 3 September 2020 / Revised: 15 October 2020 / Accepted: 16 October 2020) 포항지진은 포항지열발전소의 수리자극에 의한 촉발지진으로 조사되었으며, 수리자극을 위해 주입된 유체가 임계상태 에 도달한 지하단층을 재활성시킨것으로 알려져 있다. 하지만 포항지열발전소의 건설 이전, 포항지진 진앙지 인근에서 단층운동에 의한 제4기층 변형연구는 보고되지 않았다. 포항지진 이후 지표지질조사를 통해 진앙지로부터 약 4km 떨어 진 지점에서 대규모 물빠짐구조를 확인하였다. 마이오세 이암에에서 발생한 이 물빠짐 구조는 MIS 5에 형성된 상부 해 안퇴적층을 관입하고 있다. 이는 마이오세 퇴적층과 해안퇴적층의 부정합면을 따라 존재하는 지하수면과 마이오세 퇴적 층이 속성작용 완료되기 전에 융기된 영향으로 인해, 마이오세 퇴적층이 충분히 고화되지 않아 연질퇴적변형구조를 형 성할 수 있었음을 지시한다. 이 물빠짐구조는 미고화된 이암의 공극수압이 상부지층의 하중을 초과하여 발생한 구조로 서 지진에 의해 발생한 것으로 해석된다. 이러한 해석은 물빠짐구조로부터 약 400m 떨어진 지점에서 확인된 제4기 단 층의 존재, 한반도 남동부의 빠른 융기율, 포항인근 양산단층을 따라 보고된 제4기 단층과 역사지진 기록과도 잘 부합한 다. 따라서, 포항지진의 진앙지 일원은 제4기 동안 지구조운동과 이와 관련된 지표변형이 발생한 지점으로서 포항지진을 일으킨 단층 또한 지진발생 이전에 임계상태에 도달했을 것으로 추정된다. 주요어 : 요변성, 액체화, 유체화, 지진, 포항분지 The 2017 Pohang Earthquakes occurred near a drill site in the Pohang Enhanced Geothermal System. Water injected for well stimulation was believed to have reactivated the buried near-critically stressed Miocene faults by the accu- mulation of the Quaternary tectonic strain. However, surface expressions of the Quaternary tectonic activity had not been reported near the epicenter of the earthquakes before the site construction. Unusual, large-scale water-escaped structures were identified 4 km away from the epicenter during a post-seismic investigation. The water-escaped structures comprise Miocene mudstones injected into overlying Pleistocene coastal sediments that formed during Marine Isotope Stage 5. This indicates the vulnerable state of the mudstones long after deposition, resulted from the combined effects of rapid tectonic uplift (before significant diagenesis) and the development of an aquifer at their unconformable interface of the mudstone. Based on the detailed field analysis and consideration of all possible endogenic triggers, we interpreted the structures to have been formed by elevated pore pressures in the mudstones (thixotropy), triggered by cyclic ground motion during the earthquakes. This interpretation is strengthened by the presence of faults 400 m from the study area, which cut uncon- solidated coastal sediment deposited after Marine Isotope Stage 5. Geological context, including high rates of tectonic uplift in SE Korea, paleo-seismological research on Quaternary faults near the study area, and historical records of paleo- earthquakes in SE Korea, also support the interpretation. Thus, epicenter and surrounding areas of the 2017 Pohang Earth- quake are considered as a paleoseismologically active area, and the causative fault of the 2017 Pohang Earthquakes was expected to be nearly critical state. Key words : thixotropy; liquidization; fluidization; earthquakes; Pohang Basin This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited. *Corresponding author: [email protected] 585 586 Yong Sik Gihm et al. 1. Introduction During post-seismic field investigations, large- scale, water-escaped structures were identified ca. Earthquake-induced soft sediment deformation 4 km from the epicenter. The water-escaped structures structures (SSDS) are near-surface deformation are sourced from underlying Miocene mudstones structures that have resulted from cyclic ground that have intruded into Pleistocene costal sediments motions (> Mw 5.0) (van Loon, 2009; Owen et al., deposited in Marine Isotope Stage 5 (MIS 5). Based 2011). These motions give rise to an increase in on sedimentological analysis and consideration of pore water pressure in water-saturated, semi- to the Quaternary geological context, this structure is unconsolidated sediments, which temporarily interpreted to have been triggered by earthquakes. supports the weight of sediment grains by pressure This indicates the occurrence of moderate to (liquefaction and thixotropy) and/or associated strong ground motions around the epicenter after upward moving fluid (fluidization). These phenomena the MIS 5. The objectives of this study are to transform sediments from a solid to semi-liquid describe the water-escaped structures and their state with a decrease in sediment strength, resulting host sediments, examine reasons why the Miocene in the formation of SSDS, as driving forces act on mudstones were susceptible to deformation long the weakened sediments (Allen, 1982; Maltman after deposition, and investigate the causative trigger and Bolton, 2003; Owen, 2003; van Loon, 2009). of the water-escaped structures in consideration of Earthquake-induced SSDS can develop over possible alternatives and the Quaternary geological extensive areas as a result of the influence of wide- context. ranging ground shaking (Galli, 2000). Also, underground formation of the SSDS protects them 2. Geological Backgrounds from surface modifications, and they have a high preservation potential (Gihm et al., 2018). Thus, During the Late Oligocene, the SE Korean research on earthquake-induced SSDS can provide Peninsula began to undergo dextral, strike-slip tectonic and/or paleo-seismic information, and deformation as a consequence of the opening of many studies have been conducted with a tectonic East Sea (Sea of Japan) (Jolivet et al., 1992; Yoon and paleo-seismological focus (Leeder, 1987; et al., 2014; Son et al., 2015) (Fig. 1). At ca. 22 Ma Rossetti, 1999; Tuttle et al., 2002; Törő and Pratt, (Early Miocene), a series of NNW-SSW trending 2016). strike-slip fault systems produced nonmarine pull- On November 15, 2017, a cluster of moderate apart basin (Figs. 1A and B). The Early Miocene ground motions (≤ Mw 5.5, the 2017 Pohang basin fill comprises fluvial, fluvio-lacustrine, and Earthquakes) shook Pohang city, SE Korea where lacustrine sediments which are commonly intercalated 0.5 million residents live with a dense industrial with dacitic to basaltic volcanic and volcaniclastic complex. Because of the proximity (< 1 km) rocks (Bahk et al., 1996; Jeong et al., 2008; Sohn between the epicenter and a drilling site for the et al., 2013; Son et al., 2015). Pohang Enhanced Geothermal System, the cause As deformation continued, the NNW-SSE striking of the 2017 Pohang Earthquake is a controversial master fault (Yeonil Tectonic Line: YTL) propagated issue (Grigoli et al., 2018; Kim et al., 2018). The northwestward and connected with NNE-SSW Geological Society of Korea assessed the cause of trending fault systems, forming zigzag pattern, the 2017 Pohang Earthquakes and determined that NNE-SSW and NNW-SSE trending, fault segments the earthquakes were triggered by the injection of (Son et al., 2015) (Fig. 1A). The NNE-SSW trending water for hydrofracturing, which stimulated a segments acted as normal faults due to NW-SE buried fault that had reached a near-critical state extension induced by regional dextral shear stress, by the accumulation of tectonic strain during the and opened the Pohang Basin ca. 17 Ma (Middle Quaternary (Lee et al., 2019). However, Quaternary Miocene). Normal faulting propagated basinward, faults and associated ground expressions have not forming intrabasinal faults. To the west, the basin been reported near the epicenter before the site is bordered by the NNE-SSW trending fault system construction. and the eastern boundary faults are submerged Large-scale, Miocene Mud Intrusion into the Overlying Pleistocene Coastal Sediment ... 587 Fig. 1. (A) The distribution of the Miocene sediment basins in SE Korean Peninsula (After Son et al., 2015). (B-D) Temporal changes in the tectonic circumstance around the Korean Peninsula since the Miocene (B: early Miocene, C: middle to late Miocene, D: present) (After Yoon et al., 2014; Kim et al., 2016). (Figs. 1A and 2A). In contrast to the Early Miocene and Paik, 2013). basin fill, the Pohang Basin fill consists of alluvial At ca. 15 Ma, collision of the Izu-Bonin Arc and to marine sediments and lacks volcaniclastic sediment. Japanese Island by northward migration of the A high sediment supply sourced through transfer Philippine Sea Plate gave rise to counter-clockwise zone drainage systems caused the development of rotation of the Honshu Block (Yoon et al., 2014). fan-delta systems along the western border fault, The resultant NNW-SSE trending stress regime with the deposition of coarse-grained sediments caused tectonic inversion in the SE Korean (Hwang et al., 1995). Rapid block subsidence by Peninsula from NW-SE extension to compression normal faulting sufficiently accommodated the (Fig. 1C). This progressively uplifted the southeastern supply of coarse-grained sediment, and grey to part of the Korean Peninsula, and sedimentation in dark-grey mudstones are the main lithofacies in the Pohang Basin ceased by ca. 10 Ma (Hwang et central and eastern parts of the Pohang Basin (Sohn al., 1995; Cheon et al., 2012; Son et al., 2015). and Son, 2004) (Fig. 2A). Based on microfossil Since ca. 5 Ma, the Korean Peninsula has been studies, these mudstones were deposited in upper under influence of E-W to ENE-WSW stress fields to lower bathyal environments (Kim, 1990; Kim by the combined effects of the shallow subduction 588 Yong Sik Gihm et al. Fig. 2. (A) The geological map of the Miocene sediment fills in the Pohang Basin and outcrops and trench sites of the Quaternary fault (combined figure after Choi et al.