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Gihm, Y.S.,Hwang, I.G. Elsevier Scientific Pub. Co 2014 Journal of volcanology and geothermal research Vol.273 No.-
The Cretaceous Beolkeum Member was deposited in a lacustrine environment affected by explosive volcanism, and a number of syneruptive lithofacies are intercalated with lacustrine mudstones. The syneruptive lithofacies were formed by primary volcanic processes and resedimented depositional processes during and after eruptions and exhibit unique depositional features, which can be grouped into four syneruptive lithofacies assemblages (SLA-1-to SLA-4, from bottom to top). Primary syneruptive lithofacies are represented by welded massive lapilli tuff in SLA-1 and a normally graded lapilli tuff in SLA-2. In SLA-1, sustained pyroclastic density currents were able to displace lake water and rarely interacted with lake water, resulting in the formation of a welded texture and columnar joints. In SLA-2, unsteady pyroclastic density currents rapidly disintegrated, mixed with ambient water, and transformed into high-density turbidity currents, depositing thick normally graded lapilli tuff. Resedimented syneruptive lithofacies were primarily controlled by volcaniclastic sediment supply after eruptions. When volcaniclastic sediments were continuously supplied into the lake by debris flows, a volcaniclastic fan can be formed (SLA-1), resulting in a coarsening-upward trend and progradational geometry. In case of relatively small amounts of volcaniclastic sediment supply, turbidity currents would be a main depositional process, depositing a series of normally graded tuff on the primary syneruptive lithofacies (SLA-2), showing a fining-upward trend. In SLA-3 and SLA-4, there are only a few cm thick, normally graded tuff, reflecting minor volcaniclastic sediment supply. However, overlying normally graded sandstones in SLA-3, showing a coarsening-upward trend, and a thick normally graded sandstone in SLA-4 suggest favorable conditions for the generation of the sediment gravity flows, probably due to an increase in volcaniclastic sediment supply after eruptions.
Gihm, Yong Sik,Ko, Kyoungtae,Choi, Jin-Hyuk,Choi, Sung-ja The Korean Society of Economic and Environmental G 2020 자원환경지질 Vol.53 No.5
포항지진은 포항지열발전소의 수리자극에 의한 촉발지진으로 조사되었으며, 수리자극을 위해 주입된 유체가 임계상태에 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 떨어진 지점에서 대규모 물빠짐구조를 확인하였다. 마이오세 이암에에서 발생한 이 물빠짐 구조는 MIS 5에 형성된 상부 해안퇴적층을 관입하고 있다. 이는 마이오세 퇴적층과 해안퇴적층의 부정합면을 따라 존재하는 지하수면과 마이오세 퇴적층이 속성작용 완료되기 전에 융기된 영향으로 인해, 마이오세 퇴적층이 충분히 고화되지 않아 연질퇴적변형구조를 형성할 수 있었음을 지시한다. 이 물빠짐구조는 미고화된 이암의 공극수압이 상부지층의 하중을 초과하여 발생한 구조로서 지진에 의해 발생한 것으로 해석된다. 이러한 해석은 물빠짐구조로부터 약 400m 떨어진 지점에서 확인된 제4기 단층의 존재, 한반도 남동부의 빠른 융기율, 포항인근 양산단층을 따라 보고된 제4기 단층과 역사지진 기록과도 잘 부합한다. 따라서, 포항지진의 진앙지 일원은 제4기 동안 지구조운동과 이와 관련된 지표변형이 발생한 지점으로서 포항지진을 일으킨 단층 또한 지진발생 이전에 임계상태에 도달했을 것으로 추정된다. 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 paleoearthquakes in SE Korea, also support the interpretation. Thus, epicenter and surrounding areas of the 2017 Pohang Earthquake are considered as a paleoseismologically active area, and the causative fault of the 2017 Pohang Earthquakes was expected to be nearly critical state.
Yong Sik Gihm,In Gul Hwang 한국지질과학협의회 2016 Geosciences Journal Vol.20 No.2
The Cretaceous Beolkeum Member was formed in a lacustrine environment affected by explosive volcanic eruptions, and hyperpycnal flow deposits are well developed following the eruptions. Beds of hyperpycnal flow deposit are generally less than 25 cm thick and consist of an inversely graded and planar laminated lower part, a poorly-sorted and massive middle part, and a normally graded, planar laminated upper part. An internal erosional surface is common between the lower and middle parts. After explosive volcanic eruptions, subaerial drainage systems would be highly disturbed by deposition of fine-grained and loose pyroclasts. The pyroclasts can easily be remobilized by surface runoff, and the surface runoff evolved into sedimentladen floods with excess density to plunge into the lake, providing favorable conditions for the occurrence of the hyperpycnal flows. Compared with classic models of hyperpycnal flow deposits, predominant planar laminations in the lower and upper parts suggest high fallout rates of suspended sediments from the hyperpycnal flows during initial and late stages of deposition. This implies that the hyperpycnal flows were driven from relatively highly concentrated subaerial floods owing to erodible subaerial conditions following the eruptions. Relatively thinly bedded hyperpycnal flow deposits (<25 cm thick) in comparison with the classic models (1 to 4 m thick) can be attributed to short-lived hyperpycnal flows, arising from the disturbed subaerial conditions following eruptions together with relatively small-scale drainage basins around the lake.
호수 내 화쇄밀도류의 퇴적과정: 밀양시 단장면 일원 백악기 정각산층의 예
김용식,박승익,Gihm, Yong Sik,Park, Seung-Ik 대한자원환경지질학회 2022 자원환경지질 Vol.55 No.3
We studied the Cretaceous Jeonggaksan Formation to determine depositional processes of pyroclastic density currents entering into the lacustrine environments. This formation is composed largely of sandstone-mudstone couplets and (tuffaceous) normally graded sandstones deposited in lacustrine environments, interbedded with two pyroclastic beds: welded massive lapilli tuff and normally graded lapilli tuff. The welded massive lapilli tuff (10 m thick) is composed of poorly sorted, structureless lapilli supported by a welded ash matrix. The normally graded lapilli tuff (4 m thick) is characterized by moderately to well sorted natures and multiple normally graded divisions in the lower part of the bed with internal boundaries. The contrasting depositional features between these lapilli tuff are suggestive of different physical characteristics and depositional processes of pyroclastic density currents in the lake. Overall poorly sorted and massive natures of the thick, welded massive lapilli tuff are interpreted to have been formed by rapid settling of pyroclastic sediments from highly concentrated and sustained pyroclastic density currents. In this case, the pyroclastic density currents were able to displace lake water from shoreline and the pyrolclastic density currents preserved their own heat except for frontal parts of the currents. As a result, welded textures can be formed despite entrance of pyroclastic density currents into the lake. The internal boundaries of the normally graded lapilli tuff reflect unsteady natures of the pyroclastic density currents at the time of the deposition and the pyroclastic density currents can not provide sufficient pressure to displace lake water. As a consequence, the pyroclastic density currents transformed into water-saturated turbidity currents, forming relatively well sorted, normally graded lapilli tuff.