Importance of precise identification and characterization of active faults in seismic hazard assessment

Sambit Prasanajit Naik,Javed N. Malik,Young-Seog Kim 대한지질학회 2017 대한지질학회 학술대회 Vol.2017 No.10

Evidence of Late Pleistocene earthquake from the Central Segment of Ulsan Fault

Naik Sambit Prasanajit,Ohsang Gwon,Kiwoong Park,Hyeonjo Shin,Young-Seog Kim 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10

Due to lack of surface-rupturing earthquakes during the instrumental earthquake recording time, the Korean peninsula has not been considered as seismically active country. Although there are several historic records of seismic damage, especially along with the southeastern part of the Korean Peninsula, the locations and sizes of most of these earthquakes are not well constrained. The historical record suggests the southeastern part of the Korean peninsula have experienced several damaging earthquakes including the recent 2016 Gyeongju and 2017 Pohang earthquakes. Since Pliocene, this area is under the compressional tectonic regime, which resulted in the formation of Yangsan-Ulsan Fault System. The Yangsan Fault has relatively clear geometry and well preserved morphological feature, and more studies have done along this fault than the Ulsan Fault with more complex structural characteristics. Although the central part of the Ulsan fault is one of most well studied area in terms of paleoseismology, most of the studies are confined to outcrop scale. There is ambiguity still remains towards the trace of the fault lines and recurrence interval of surface rupturing earthquakes in historical past. This study is an attempt to trace the Ulsan Fault line and understand the recurrence interval of earthquakes along the central part of the Ulsan fault. Considering this aim, detailed geomorphic mapping and paleoseismic study has been conducted at 2 sites near Hwalsung-ri, close to Gyeongju city. The Trenches were named as Hwalsung-ri North and Hwalsung-ri South. The present paleoseismic study reveals evidences of multiple earthquakes history along the central segment of the Ulsan fault near Hwalsung-ri. The crosscutting relationship of the exposed stratigraphic units at Hwalsung-ri North suggests the fault has been reactivated more than four times with evidences of four earthquake events whereas the Hwalsung-ri South site shows the evidences of three faulting events. Due to unavailability of suitable material and presence of pebbles we were not able to obtain ages for all the lithounits exposed in the trench. With this constraints, the Hwalsung-ri north site shows evidences of the most recent event occur after 103±2 ka whereas the Hwalsung-ri south trench suggests the most recent event occurred after 61±3 ka. This will help us to understand the past seismicity along the central segment of the Ulsan Fault and to trace the exact location of Ulsan fault which is one of the most potentially dangerous faults in Korea.

Kinematic analysis and fault-dependence of building-wall fracture patterns during moderate earthquakes

김영석,Sambit Prasanajit Naik,최진혁,진광민,Gong-Ruei Ho,김태형,이진현 한국지질과학협의회 2023 Geosciences Journal Vol.27 No.6

Two recent moderate earthquakes in South Korea, the 2016 MW 5.5 Gyeongju earthquake and 2017 MW 5.4 Pohang earthquake, caused damages to modern residential buildings. These events occurred with almost the same magnitude and duration in the same seismotectonic environment but exhibited remarkably different focal depths, faulting types, surface deformation, and especially structural damage features, but the reasons for these contrasts remain unknown. Furthermore, the building damage patterns are different from the natural damages, which have typical patterns depending on the fault types. It is important to understand the key reasons of these different phenomena to prevent destructive hazards from future earthquakes, particularly in densely populated intraplate regions. Here, we reveal the relationships between the geological-seismic parameters and earthquake damage features based on the patterns of building damage associated with these two events. During post-event urgent field surveys, we systematically observed en-echelon (or Riedel-type) sub-horizontal fractures in building walls associated with strike-slip motion and high-angle conjugate X-shaped fractures in building walls associated with predominantly reverse oblique-slip motion. We attribute the different patterns of earthquake damage to variations in faulting types and associated ground motions; strike-slip faulting resulting in horizontal shear and oblique-slip faulting yielding vertical ground motion. We argue that these interesting characteristics of building damage are mainly caused by stress conditions depending on the environmental change from the underground crust to the ground surface of free face. Our study highlights the importance of post-event investigations of earthquake damage to improve the level of seismic hazard assessment. Our findings from this study could serve as a reference for establishing proper anti-earthquake design and reinforcement for seismic protection.

고지진학적 자료를 이용한 울산단층대 중부 말방지역에서의 단층운동 특성 해석

박기웅,NAIK SAMBIT PRASANAJIT,권오상,신현조,김영석 한국지구과학회 2022 한국지구과학회지 Vol.43 No.1

According to the records of historical and instrumental earthquakes, the southeastern part of the Korean Peninsula is considered the highest seismic activity area. Owing to recent reports of numerous Quaternary faults along the Yangsan and Ulsan fault zones, paleoseismological studies are being actively conducted in these areas. The study area is located in the central part of the Ulsan fault zone, where the largest number of active faults have been reported. Based on lineament and geomorphic analysis using LiDAR images and aerial photographs, fault-related landforms showing topographic relief were observed and a trench survey was conducted. The trench length 20 m, width 5 m, depth 5 m is located approximately 300 m away to the northeast from the previously reported Malbang fault. From the trench section, we interpreted the geometric and kinematic characteristics of the fault based on the deformed features of the Quaternary sedimentary layers. The attitude of the reverse fault, N26o W/33o NE, is similar to those of the reported faults distributed along the Ulsan fault zone. Although a single apparent displacement of approximately 40 cm has been observed, the true displacement could not be calculated due to the absence of the slickenline on the fault plane. Based on the geochronological results of the cryogenic structure proposed in a previous study, the most recent faulting event has been estimated as being earlier than the late Würm glaciation. We interpreted the thrust fault system of the study area as an imbrication structure based on the previous studies and the fault geometry obtained in this additional trench. Although several previous investigations including many trench surveys have been conducted, they found limited success in obtaining the information on fault parameters, which could be due to complex characteristics of the reverse fault system. Additional paleoseismic studies will contribute to solving the mentioned problems and the comprehensive fault evolution 역사지진과 계기지진 기록에 따르면 한반도 남동부는 우리나라에서 지진활성도가 가장 높게 평가되는 곳으로, 최근에 양산단층대와 울산단층대를 따라 제4기 단층이 다수 보고되어 고지진학적 연구가 활발하게 이루어지고 있다. 특히 울산단층대의 중부지역에 해당하는 경북 경주시 외동읍 말방리 일원은 울산단층대 내에서 가장 많은 활성단층이 보고된 지역이다. 따라서 이 지역에 대한 고지진학적 특성을 이해하기 위하여 먼저 LiDAR 영상 및 항공사진을 이용한지형 및 선형구조 분석을 실시하여 단층에 의한 기복으로 추정되는 지형인자를 확인하고, 야외답사와 물리탐사를 통해단층을 추적하여 기 보고된 말방단층 지점에서 약 300 m 북서쪽에 위치한 곳에서 길이 20 m, 너비 5 m, 깊이 5m의굴착조사를 실시하였다. 굴착단면을 통해 분석된 제4기 퇴적층의 특징을 바탕으로 단층의 기하학적·운동학적 특성을해석하여 고지진학적 특성을 규명하고자 하였다. 이번 굴착단면에서 확인된 역단층의 기하를 보이는 단층의 자세는N26o W/33o NE로 울산단층대를 따라 분포하는 기 보고된 단층들과 유사하다. 약 40 cm의 단일 겉보기 변위가 인지되었으나 단층조선의 부재로 실변위는 산출할 수 없었다. 선행연구에서 제안된 극저온구조층의 연대결과 값을 토대로 단층의 최후기 운동시기는 후기 뷔름빙기 이전으로 추정하였다. 기 보고된 연구결과와 본 굴착단면에서 획득한 단층기하를종합하여 이 지역에 발달하는 단층계를 인편상구조로 해석하였고, 단층특성을 반영한 모델을 제시하였다. 말방리 일원에서 수 회의 굴착조사를 비롯한 다수의 선행연구가 수행되었음에도 불구하고 구체적인 단층변수에 대한 정보가 미진하고 각 지점들 간의 상관성이 명확하게 규명되지 않은 것은 역단층의 복잡한 운동학적 특성에 기인한 것으로 판단된다. 추후 고지진학적 연구가 추가적으로 수행된다면 상기의 문제점들을 해결하여 종합적인 단층의 형태와 운동사가 규명될 수 있을 것으로 판단된다.

울산단층대를 따라 발달하는 활성단층들의 고지진학적 특성

박기웅,( Sambit Prasanajit Naik ),권오상,신현조,김영석 대한지질공학회 2022 대한지질공학회 학술발표회논문집 Vol.2022 No.2

Seismic Hazard Assessment in a Volcano-tectonic Environment: Macroseismic Intensity of 2021 Gisenyi Earthquake

( Francois Hategekimana ),( Sambit Naik Prasanajit ),( Young-seog Kim ) 대한지질공학회 2023 대한지질공학회 학술발표회논문집 Vol.2023 No.1

First hand observations from the April 28, 2021 Sonitpur (MW 6.4) earthquake, Assam, India: possible mechanism involved in the occurrence of widespread ground effects

Mayank Joshi,Sambit Prasanajit Naik,Asmita Mohanty,Arun Bhadran,Drishya Girishbai,Swakangkha Ghosh 한국지질과학협의회 2023 Geosciences Journal Vol.27 No.2

On April 28 2021, an earthquake of MW 6.4 occurred near Sonitpur, Assam, India. The epicenter was 43 km away from Tezpur, Assam with a focal depth of 34 km. The National Center for Seismology (NCS), Delhi reported the maximum intensity of MMI–V whereas the United States Geological Survey (USGS) reported the maximum intensity of MMI–VII. Preliminary reports suggest a reverse slip component for the earthquake, which occurred close to the previously reported Kopili fault. This fault is the cause of two damaging earthquakes in the past i.e., 1869 Cachar earthquake and the 1943 earthquake. The April 28 2021, Sonitpur earthquake caused widespread liquefaction, building damage, and lateral spreading as far as 90 km from the epicenter. The present study reports preliminary damages and ground effects observed soon after the earthquake along with a comparative analysis with previously reported damages around the area during the historical earthquakes. The farthest reported liquefaction occurred during the earthquake was plotted with other case studies which were well in agreement with the previous literature. The possible reason for the extensive liquefaction and ground cracks is inferred to be due to site amplification within a sedimentary basin. Therefore, the occurrence of the 2021Sonitpur, Assam earthquake emphasizes the seismic hazard scenario for the Brahmaputra Basin and Bengal Basin, which further requires more detailed study in terms of paleoseismology, liquefaction hazard zonation and seismic hazard assessment. In addition, the damages to the buildings highlight the implementation of proper building codes considering the liquefaction hazard zonation map for the study area.

울산단층과 연일구조선의 신기지구조운동 및 단층분절화 예비연구

박기웅(Kiwoong Park),Sambit Prasanajit Naik,권오상(Ohsang Gwon),신현조(Hyeonjo Shin),김영석(Young-Seog Kim) 대한지질학회 2022 대한지질학회 학술대회 Vol.2022 No.10

울산단층 남부지역의 제4기 운동 특성: 차일지점과 호계지점을 중심으로

권오상(Ohsang Gwon),Naik Sambit Prasanajit,박기웅(Kiwoong Park),신현조(Hyeon-Cho Shin),김영석(Young-Seog Kim) 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10

한반도 남동부에 발달해 있는 제4기 단층들의 단층변수(운동시기, 재발주기, 변위 등) 도출을 위해 다양한 연구가 진행되고 있다. 미래에 활성단층으로부터 야기될 수 있는 지진 규모를 결정하는 것은 극도의 안정성이 요구되는 국가중요시설물의 안정성 평가에 중요하다. 본 연구에서는 울산단층대 남부지역에서 확인된 차일지점과 호계지점을 상세히 기재하고, 운동시기, 변위 등의 다양한 단층변수를 보고하고자 한다. 차일지점에서는 굴착단면의 절단된 퇴적층을 기준으로 약 1.1 m의 겉보기 수직변위가 인지되며, 단층면과 단층조선 등을 활용하여 계산된 실변위량은 약 1.41 m이다. 이를 최대변위-모멘트 지진규모의 경험식에 적용하면, 최후기 단층운동에 의한 모멘트 지진규모는 약 6.6 로 추정된다. 절단된 퇴적층에서 획득한 OSL 연대는 약 88,000년으로 도출되었고, 14C를 활용한 연대는 약 10,000 ~ 41,000 Cal yr BP로 도출되었다. 호계지점에서는 굴착단면의 절단된 퇴적층을 기준으로 최소 약 4 m의 수직 누적변위가 인지된다. 절단된 퇴적층에서 획득한 OSL 연대는 약 161,600년으로 도출되었다. 추후 단층의 연장지점에서 정밀한 조사를 통해 신뢰도 높은 단층변수들이 더 축적된다면, 울산단층대 남부지역의 신뢰할 만한 지진재해도 평가가 가능할 것이다.

지진재해 대비를 위한 활성단층 연구의 중요성

권오상(Ohsang Gwon),Sambit Prasanajit Naik,김영석(Young-Seog Kim) 대한지질학회 2018 대한지질학회 학술대회 Vol.2018 No.10