Large-Scale, Miocene Mud Intrusion Into the Overlying Pleistocene

Home , 2017 Pohang earthquake, Thixotropy

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 accumulation 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 unconsolidated 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

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*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. 2012; Song et al., 2015). The Pohang Basin is composed of alluvial to deep marine sediments, covered unconformably by the Quaternary coastal deposits along the eastern shorelines. (B) A schematic distribution map of marine terraces of the study area (Choi et al., 2009). The three flights of coastal deposits are well developed, and the water-escaped structures occur at interface between the Miocene mudstones and the highest marine terrace (NQT3). of the Pacific Plate and far field stress from the 3. Host Sediments Indian Plate collision (Yoon et al., 2014) (Fig. 1D). The compressional tectonic forces uplifted the SE The water-escaped structures occur over the Korean Peninsula and developed a series of Miocene and Pleistocene sediments in the eastern coastal deposits (Choi et al., 2008; Choi, 2019). part of the Pohang Basin (Figs. 2 and 3). In this area, Most of the coastal deposits formed during MIS 5e both the Miocene and the Pleistocene sedimentary to 5a under a wave-dominated, open coastal setting successions have been excavated for railroad together with negligible tidal influence (< 1 m; construction, exposing a 500 m long (E-W) and Choi et al., 2008). In addition, compressional ca. 10 m high section of strata (Fig. 3A). The host tectonic stress reactivated pre-existing faults, such sediments are composed of the Miocene mudstones as the Yangsan and Ulsan faults, and formed the and overlying the Pleistocene sand and gravel Quaternary fault segments (Kim et al., 2016). units. Their contact is 35 m a.s.l. (above sea-level), Those along the Yangsan Fault (NNE-SSW to NE- corresponding to NQT3 terrace (127±17 Ka) of SW striking) show dextral strike-slip movement Choi et al. (2009) (Figs. 2B and 3A). The mudstones with reverse slip, whereas in those along the Ulsan contain the plant leaves Carpinus kodairae-bracteata, Fault (NNW direction) thrusts are common (Kim C. miofargesiana, C. oblongibracteata (Kim et al., et al., 2016). Recently, the 2017 Pohang Earthquakes 2017) and Cosmopolitodus hastalis and Kentridom were caused by the movement of subsurface teeth (Kim et al., 2018; Lim, 2005). All became intrabasinal faults in the Pohang Basin. extinct in the Miocene, indicating deposition of the mudstone in the Miocene. In the railroad cutting, Large-scale, Miocene Mud Intrusion into the Overlying Pleistocene Coastal Sediment ... 589

Fig. 3. Outcrop photographs along the interface between the Miocene mudstones and the Pleistocene sediments. (A) Two water-escaped structures with reverse funnel shapes and the Pleistocene coastal deposits between the structures. (B) A present-day collapsed slope with continuous discharge of groundwater from the unconformable interface between the Miocene and Pleistocene sediments. (C) Cracks (filled by the overlying Pleistocene sediments) in the topmost part of the mudstones.

Fig. 4. A sedimentologic log and photographs of the host sediments. (A) The stratigraphic log of the host sediments. (B) Massive features and carbonized plant fragments in the Miocene mudstones. (C) Photographs showing sharp boundary between the Pleistocene gravel and sand units. (D) Interbedded layers of sand and mud in the Pleistocene sand unit. (E) Vertical changes in asymmetric (ASR) to symmetric ripple (SR) in the sandy units. (F) A photograph of moderately to well sorted, rounded gravels with crude cross stratifications in the bed of gravel units. (G) Planar stratifications of the lens of sand in the gravel units. 590 Yong Sik Gihm et al. the overlying Pleistocene sediments are friable, centimeters thick, intercalated with thin muddy whereas the mudstones are moderately to semi- layers (Figs. 4A and D). The sand layers commonly consolidated except for their topmost part which is show normal grading with asymmetric ripple still unconsolidated along with continuous discharge (ASR) and symmetric ripple (SR) cross-lamination of groundwater (Figs. 3B and 4A). (Fig. 4A). ASRs consist of seaward-dipping, convex-up, and sigmoidal foresets without sharp 3.1. Miocene mudstones brink points (Fig. 4E). They are commonly The Miocene mudstones (< 5 m thick) are grey overlain by SRs with a gradual decrease in grain in color and composed of homogeneous silt and size. The sandy layers are commonly draped or clay with plant debris (Figs. 4A and B). X-Ray overlain by a few cm thick mud layers (Fig. 4E). Powder Diffraction (XRD) analysis demonstrated This unit is 2.5 m thick in total and overlain by the that the mudstones are composed mostly of quartz gravel unit with a sharp boundary. and montmorillonite (Fig. 5). Locally, vertical The normal grading with vertical transition from cracks (< 1 m long, Fig. 3C) are developed at the ASRs to SRs reflects that the sandy unit was top, which are filled with overlying Pleistocene deposited by decelerating, unidirectional to oscillatory sediment. flows. However, the ASRs have sigmoidal foresets The mudstones are interpreted to have been with smooth brink points, suggesting that the deposited by (hemipelagic) settling of fine-grained unidirectional flows were influenced by oscillatory sediments based on the fine-grained and homo- components during deposition (Dumas et al., geneous nature (Hwang et al., 1995). This 2005). Thus, the sandy unit is interpreted to have interpretation is consistent with the observation of been deposited by combined flows, followed by trace fossils such as Chondrites in the mudstones oscillatory waves as unidirectional components that suggest oxygen depletion (Kim and Paik, waned (Duke et al., 1991). The intercalated mud 2013). layers imply relatively calm conditions between episodes of sand deposition. Thus, the sandy units 3.2. Pleistocene deposits are interpreted to have been deposited by storm The Pleistocene deposits are situated between waves at water depths between storm and fair- two water-escaped structures, exhibiting a bowl-like weather wave base. geometry (Fig. 3A). The deposits are composed of a lower sand and upper gravel unit (Figs. 4A and C). 3.2.2. Gravel unit The overlying gravel unit is composed of well- 3.2.1. Sand unit bedded (0.1–0.4 m thick) pebble to cobble sized The sand unit is composed of moderately to well clasts (Figs. 4C and F). The clasts are well sorted sorted silty sand to fine sand that are cm to tens of and spherical or disc-shaped, but rod-shaped clasts are also common (Fig. 4F). They are clast-supported and voids are filled with well-sorted medium to coarse sand. The gravel beds are massive to low- angle cross-stratified and dip toward the present- day shoreline (Figs. 4A and F). The gravel beds are locally intercalated with thin (< 0.1 m), lenticular planar laminated sand beds or massive muds (Fig. 4G). Although the uppermost part of the gravel unit has been excavated, Choi et al. (2009) reported that the gravel unit was covered by aeolian sand dunes. The well-sorted rounded clasts, low-angle cross stratification and bedded nature indicate deposition Fig. 5. The results of quantitative X-ray powder diffraction of the gravel unit under wave action in upper (XRD) analysis of the Miocene mudstones. foreshore to nearshore environments (Massari and Large-scale, Miocene Mud Intrusion into the Overlying Pleistocene Coastal Sediment ... 591

Parea, 1988). The planar laminated sand was contrasting tectonic settings, although the vertical presumably deposited by swash waves (Hart and facies changes in the study area are similar to those Plint, 1995). The mud lenses suggest temporal of a progradational coastal sequence (Fig. 4A). interruption of wave agitation, probably related The unconsolidated nature of the sediment with episodic sea-level rise during MIS 5. resulted from uplift of the Pohang Basin before significant diagenesis (Son et al., 2015). In addition, 4. Water-escaped Structures hydraulic differences between the Miocene mudstone and the Pleistocene sediments caused the 4.1. Occurrence development of an aquifer at their unconformable The water-escaped structures (5 m high) are interface. This resulted in the dissolution of chemical recognized by the intrusion of the Miocene bonds in the mudstones with the saturation of mudstone into the overlying Pleistocene deposits water in pores at the topmost part (Hansen et al., (Figs. 3A and 6). At the base of the Pleistocene 2007). Thus, although the mudstones (> 10 Ma) are sediments, the structures are 4 m wide and decrease much older than the overlying deposits (127 Ka), in width upward to 0.5 m, showing a reverse these combined effects made the topmost part of funnel shape. Internally, the structures are entirely the mudstone water-saturated and unconsolidated. massive; however, at their margins, bedding and A water seepage from the boundary between the gravel clasts of the Pleistocene sand and gravel Miocene mudstones and the Quaternary sediments units are steeply inclined against the structures also indicates the presence of aquifer at the interface (Fig. 6). Their inclination laterally decreases (Fig. 3B). asymptotically away from the structures. Liquidization caused the transformation of sediment from solid to a semi-liquid state by the 4.1. Interpretation loss of strength (Allen, 1982). In the case of the The upward decrease in width and inclined mudstone, thixotropy or quick clay are possible clasts against them indicate upward injection of deformation mechanisms for liquidization (Owen, unconsolidated mudstones into the overlying 1987). However, the latter is negligible because of Pleistocene strata via liquidization (Owen, 1987). the abundant montmorillonite in the mudstones However, the mudstone was deposited earlier than (Fig. 5). The montmorillonite is a swelling mineral 10 Ma and deformed after c.a. 0.125 Ma. The and increases in liquid limit by a volume increase Miocene and Pleistocene sediments were deposited when wet (e.g., Hansen et al., 2007). Thus, in different depositional environments under thixotropy is only valid deformation mechanism.

Fig. 6. A photograph of contact between water-escaped structures and the Pleistocene host sediments. Inclined bedding planes and clasts (dashed line) are noteworthy. 592 Yong Sik Gihm et al.

The mudstone transformed into semi-liquid state lack of SSDS indicates unsuitable physical conditions via an increase in pore water pressure induced by of the Pleistocene deposits at the time of the the re-arrangement of the water-saturated clay deformation, which may result from insufficient particles. The pressure was strong enough to inject amount of water in pore space. Thus, we interpret the mudstone into the overlying deposits (fluidization; that the deformation occurred after emergence of Owen, 1996) (Fig. 3). the Pleistocene sediments over the groundwater table by regional uplift in the SE Korean Peninsula, 5. Discussion and insufficient volume of water hampered deformation of the Pleistocene deposit. The lack of A causative trigger is necessary for the deformation SSDS in Pleistocene sediments also suggests that mechanism to be activated. The weight of the sediments passively sunk due to compensation for gravel unit is a plausible endogenic trigger as the volume loss of the underlying injected mudstone Quaternary gravel units prograded above the (Fig. 3A). Miocene, semi-consolidated mudstones. In addition, Another possible trigger is seismic shaking wave-influenced sedimentary structures and bedforms (Fig. 7). The water-escaped structures indicate that in the Pleistocene deposits indicate that wave an elevated pressure by thixotropy exceeded the motion, including cyclic loading or breakage of weight of the overlying Pleistocene deposits and waves, may also be a plausible trigger. Hourly intergranular friction of the clast-supported gravels, wave records since 2010 (Pohang Buoy, N 36°21′ resulting in hydraulic fracturing of the Pleistocene 00″, E 126°47′ 00″) show that significant wave deposits (Fig. 4). In addition, their unusually large height is 0.3 to 7.7 m during storm seasons, with size (< 4 m in width and > 5 m high) suggests the highest wave height 12.2 m (KMA, 2019). The severe liquidization of the topmost part of the maximum wave height is compatible to the optimum mudstone together with significant volume loss by conditions for wave-induced liquidization (Alfaro injection, causing subsidence of the Pleistocene et al., 2002). Constituent silty sand to fine sand in sediments. Obermeier (1996) and Obermeier et al. the sand unit is optimum grain size for liquefaction (2005) pointed out that development of hydro- when these endogenic triggers occurred (Fig. 4). In fracturing and associated water-escaped structures addition, the sand and gravel layers in the needs significant strength and/or long duration of Pleistocene deposits are commonly intercalated deformation mechanism. They suggested the usage with muddy sediments. If deformation mechanism of resultant clastic dikes and sills as paleoseismic by the triggers acted on the Pleistocene sediments, indicators, because ground shaking during moderate well-developed, small-scale load and flame structures to strong earthquakes (> Mw 5.0) meet these physical would be expected at the boundary because of the conditions. reverse density gradient (Owen, 2003). Thus, the The Quaternary geological context also indicates

Fig. 7. A schematic model for the development of the water-escaped structures in the study area. Large-scale, Miocene Mud Intrusion into the Overlying Pleistocene Coastal Sediment ... 593 active ground motion of the study area. Studies of the marine terraces in SE Korean Peninsula show that the elevation of marine terrace MIS 5e is 33 to 50 m a.s.l. was caused by E-W to ENE-WSW tectonic compression (Choi et al., 2008; 2009). The rates of uplift are 0.2 to 0.3 m/kyr since the late Pleistocene (Choi et al., 2008). Although the Korean Peninsula is located in an intraplate setting, this uplift rate is comparable to that of the present- day convergent margins (Choi et al., 2008). This active condition is also reflected by numerous historical records of moderate to hazardous historical earthquakes in SE Korea (Mw 5-7, A.D. 2 to A.D 1904) and moderate earthquakes (> Mw 5.0) recorded on seismographs since 1978 (Fig. 8). In addition, compressional tectonic stresses have stimulated pre-existing faults in SE Korea, such as the Yangsan and Ulsan faults, some of which has cut the Quaternary sediments, resulting in the development of Quaternary faults segments. In case Fig. 8. Historical and instrumental earthquake records in of the Pohang Basin, more than five Quaternary the SE Korea. Note frequent moderate to strong fault outcrops have been reported near the western earthquakes since AD. 2, exceeding threshold for boarder fault (Choi et al., 2012) (Fig. 2A). Based seismogenic liquidization (After Lee and Yang, 2006). on paleo-seismological research, the latest move- ments of these faults were estimated to have Kim and Jin, 2006.). Furthermore, during the post- occurred after 125 ka, and one was evaluated to seismic investigation of the 2017 Pohang Earth- have generated strong earthquakes (Mw 6.5-7.5; quakes, a Quaternary fault was identified 400 m

Fig. 9. (A) Location and (B) photographs of the Quaternary fault at the nearby study area. 594 Yong Sik Gihm et al. away from the study area (Fig. 9), which cut coastal (ca. 400 m from the study area). This study showed deposits of NQT2 or NQT3 terraces (Choi et al., that the eastern part of the Pohang Bain is 2019). This implies the occurrence of moderate to paleosismologically active. Thus, the fault that strong paleo-earthquakes near the study area, generated the 2017 Pohang Earthquakes may have because near-surface faults commonly involve been at a near-critical state, and an increased pore earthquakes exceeding Mw 5.5 (Bonilla, 1988; pressure by the water injection at the Pohang Wells and Coppersmith, 1994). These lines of the Enhanced Geothermal System was enough to evidence strongly indicate that earthquake(s) stimulated the fault. occurred in and around the study area after MIS 5, and their magnitudes exceeded the minimum Acknowledgement threshold for liquidization (> Mw 5.0). The water-escaped structures are geological This study was supported by KIGAM’s projects evidence of paleo-seismic activity around the Pohang “Research in active tectonics and development of Enhanced Geothermal System site. However, it fault segment model for intraplate regions” and does not directly indicate that the fault that caused “Geological survey in the Korean Peninsula and the 2017 Pohang Earthquakes caused the develop- publication of the geological maps” funded by ment of water-escaped structures. Seismically Ministry of Science and ICT. This work was also induced SSDS form anywhere if sufficient cyclic supported by the National Research Foundation of shear stress propagates through susceptible sedi- Korea (NRF) grant funded by the Korea government ments. In addition, the limited areal distribution of (MSIT) (No. 2020R1F1A1070752). This manuscript Quaternary sediments and ground modification by has been improved by constructive comments industrial development near the study area has from two reviewers. hampered detection of the epicenter and magnitude of the causative earthquakes. Nevertheless, the References water-escaped structures plus the nearby Quaternary faults show that the eastern part of the Pohang Alfaro, P., Delgado, J., Estévez, A., Molina, J.M., Moretti, Basin is a paleosismologically active area under M. and Soria, J.M. (2002) Liquefaction and fluidization structures in Messinian storm deposits (Bajo Segura the influence of Quaternary tectonic stress fields, Basin, Betic Cordillera, southern Spain). International and pre-existing faults and weaknesses would be Journal of Earth Sciences, v.91, p.505-513. susceptible to rupture by small changes in local Allen, J.R.L. (1982) Sedimentary structures: their character stress fields, such as fluid injection at the Pohang and physical basis. Vol. II, Elsevier, Amsterdam, 663p. Bahk, J.J. and Chough, S.K. (1996) An interplay of syn- Enhanced Geothermal System. and intereruption depositional processes: the lower part of the Jangki Group (Miocene), SE Korea. Sedimentology, 6. Conclusions v.43, p.421-438. Bonilla, M.G. (1988) Minimum earthquake magnitude associated with coseismic surface faulting: Bulletin of During the post-seismic field investigation of the the Association of Engineering Geology, v.25, p.17-29. 2017 Pohang Earthquakes, large-scale, water- Cheon, Y., Son, M., Song, C.W., Kim, J.-S. and Sohn, Y.K. escaped structures were identified 4 km away from (2012) Geometry and kinematics of the Ocheon Fault System along the boundary between the Miocene the epicenter. The structures were developed in the Pohang and Janggi basins, SE Korea, and its tectonic eastern part of the Pohang Basin, crosscut Pleistocene implications. Geosciences Journal, v.16, p.253-273. coastal deposits and were sourced from the Choi, S.J. (2019) Review on Marine Terraces of the East underlying Miocene Mudstones. The structures Sea Coast, South Korea : Gangreung – Busan. Economic and Environmental Geology, v.52, p.409-25. were formed by elevated pore water pressure in Choi, S.J., Merritts, D.J. and Ota, Y. (2008) Elevations and the mudstone by thixotropy as the result of ages of marine terraces and late Quaternary rock uplift tectonic uplift before significant diagenesis and the in southeastern Korea. Journal of Geophysical Research: development of an aquifer at their unconformable Solid Earth, v.113, p.B10403. Choi, J.H., Kim, J.W., Murray, A.S., Hong, D.G., Chang, upper interface. The increased pressure was H.W. and Cheong, C.S. (2009) OSL dating of marine triggered by strong ground motions, probably terrace sediments on the southern coast of Korea with induced by activity of the nearby Quaternary fault implications for Quaternary tectonics. Quaternary Large-scale, Miocene Mud Intrusion into the Overlying Pleistocene Coastal Sediment ... 595

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