Original Article

Econ. Environ. Geol. 2020; 53(5): 585-596

Published online October 31, 2020

https://doi.org/10.9719/EEG.2020.53.5.585

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

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

Correspondence to : naress@knu.ac.kr

Received: September 3, 2020; Revised: October 15, 2020; Accepted: October 16, 2020

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.

Abstract

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.

Keywords thixotropy; liquidization; fluidization; earthquakes; Pohang Basin

Article

Original Article

Econ. Environ. Geol. 2020; 53(5): 585-596

Published online October 31, 2020 https://doi.org/10.9719/EEG.2020.53.5.585

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

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

Correspondence to:naress@knu.ac.kr

Received: September 3, 2020; Revised: October 15, 2020; Accepted: October 16, 2020

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.

Abstract

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.

Keywords thixotropy; liquidization; fluidization; earthquakes; Pohang Basin

    KSEEG
    Jun 30, 2024 Vol.57 No.3, pp. 281~352

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