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2017 Pohang earthquake

Coordinates: 36°07′N 129°22′E / 36.12°N 129.36°E / 36.12; 129.36
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2017 Pohang earthquake
Map
Map
2017 Pohang earthquake is located in South Korea
2017 Pohang earthquake
UTC time2017-11-15 05:29:31
ISC event611461139
USGS-ANSSComCat
Local date15 November 2017
Local time14:29:31 KST[1]
MagnitudeML(KMA) 5.4[1]
Depth7 km[1]
Epicenter36°07′N 129°22′E / 36.12°N 129.36°E / 36.12; 129.36
TypeReverse-Thrust
Areas affectedPohang, Ulsan, Daegu
Max. intensityMMI VII (Very strong)
Peak acceleration0.58 g
TsunamiNo
ForeshocksM 2.6 (14:22:32 KST)
M 2.2 (14:22:44 KST)
Casualties82 injured
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1.5km
1mile
Epicenter
Epicenter
EGS
EGS
Water injection well of an Enhanced geothermal system (EGS) might have triggered the earthquake

The 2017 Pohang earthquake (浦項地震), measuring magnitude 5.4 on the Richter scale, struck Heunghae, Pohang, North Gyeongsang Province, South Korea on November 15, 2017 and the epicenter was 7km above the surface.[2] It is tied with the 2016 Gyeongju earthquake as the country's strongest earthquake in modern history since seismological observations started in Korea in 1978 [3], and the most destructive ever recorded with "an estimated 300 billion won (US$290 million) in damage."[4]. It also recorded a VII intensity on the Modified Mercalli intensity scale, making it the second largest earthquake in recorded history [5].

Seismotectonic setting

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Although the Korean Peninsula is part of the Eurasian Plate, until the early Tertiary era (about 30 million years ago), it was made up of continental magmatic arcs along a plate border. With the coetaneous formation of smaller-scale basins, such as the Pohang, and the opening of the East Sea (or Japan Sea) as a back-arc basin in the early to middle Tertiary (~30 to 15 million years ago), north-northeast-striking strike-slip faults and NNE- to NE-striking normal faults developed primarily in southeastern Korea and adjacent offshore areas. The present compressional regime has caused some of these faults to reactivate as thrust and strike-slip faults. Throughout the southern Korean Peninsula, the axes of compression identified by focal mechanism solutions show modest plunges to the ENE [6][7]. With low rates of deformation and seismicity and a lengthy recurrence interval between destructive earthquakes, Korea's modern instrumental seismicity (since 1978) is a typical example of a stable continental zone [8].

Main earthquake

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The earthquake, along with several aftershocks, caused significant infrastructure damage in the southern port city of Pohang. At least 82 people were injured in the earthquake, including 15 people who were hospitalized. About 1,124 people stayed in temporary shelters after they had to leave their homes.[9]

  • Aftershocks of the earthquake
    Aftershocks of the earthquake
  • Shakemap for the earthquake
    Shakemap for the earthquake
  • Crumbled walls piled on damaged cars in Pohang, South Korea
    Crumbled walls piled on damaged cars in Pohang, South Korea

Earthquake development

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There were two main foreshocks for the earthquake which happened 7km north and northwest of Buk-gu Pohang respectively [6]. The mainshock occurred approximately 29 minutes and 31 seconds after the initial foreshock was detected. Several aftershocks were also detected following the main earthquake [6].

Main fault

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A previously unmapped fault that seems to be the opposite of the Gokgang fault system was reactivated by the seismic series, according to the three-dimensional distribution of aftershocks and the focal mechanism solutions of the bigger aftershocks that occurred right after the main events [10]. The focal mechanism of the earthquake was determined to be a strike slip fault with some thrust elements, indicating a complicated faulting sequence [11][7][12]. The estimated size of the Pohang earthquake's seismic fault is 4–5 km broad and 7–8 km long, based on the distribution of aftershocks that happened on the main fault plane [11]. This earthquake is known as a "runaway earthquake", where a chain reaction occurs: the slipping generates heat and more slipping occurs. The fault ruptured in 3 distinct steps, all occurring within 6 seconds, "In each step, the seismic moment was released as approximately 6%, 59%, and 35%, respectively." [13]

Cause

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Following the earthquake, it was determined that the earthquake was a "runaway" earthquake triggered by critical‐state fault as a result of fluid injection due to geothermal projects in the area [14]. An enhanced geothermal system (EGS) was located a mere 600 meters from the epicenter of the earthquake and part of the development included injection of high pressure fluid which can cause stress changes leading to earthquakes [11]. Although the magnitude of the earthquake was much higher than earthquakes expected with EGS, the Korean Government Commission determined that the earthquake was a result of fluid injection too [12] [15]. Further investigations found that the earthquake was not directly caused by fluid injection but rather resulted from a self-sustaining rupture process that released a significant amount of energy through tectonic loading on the fault zone that was reactivated by fluid injection [12] [7].

Aftershocks

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A 4.6 magnitude earthquake on February 10, 2018 was detected and was determined to be an aftershock of the main earthquake [16]. The aftershock occurred in the Southeast part of the fault unlike the mainshock which happened in the Northeast portion of the fault. Long term monitoring of the fault has determined that the frequency of aftershocks has decreased following the initial event. However, about 10 aftershocks associated with the original earthquake still happen every month, indicating that the initial earthquake may have changed the seismological characteristics of the area[16]. It was also determined that the number of aftershocks happening along the fault surface where the main earthquake also decreased, with most of them being concentrated along the rupture tips whilst trending predominantly in a Northeast-Southwest direction [10].

Result

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According to an initial assessment by the Ministry of the Interior and Safety, the earthquake damaged 2,165 private properties, including 1,988 private houses. Of those, 52 homes suffered severe damage and 157 suffered serious damage.[9]

Damage was also reported at 227 schools across the region, 107 of them in Pohang, 44 in Ulsan and 26 in Daegu. Furthermore, damage was reported at 79 public offices and parks, 23 port facilities, 7 roads, 90 shops, 77 factories, and 11 bridges.[9] Closer to the coast in Pohang, exterior walls of a building at Handong University had collapsed.[17]

Nearly 20,000 people, including soldiers, were mobilized to help clear debris and to assist in restoration works. More than 80 percent of damaged properties were restored within 4 days of the initial earthquake.[9]

A seismograph installed nearby epicenter of this earthquake measured peak ground acceleration (PGA) of 0.58 g; Due to Pohang City's poor subsoil area, seismic wave amplified while passing through, making the damage somewhat heavier than the 5.8 magnitude 2016 Gyeongju earthquake. 0.58 g of PGA is about equivalent to MMI Intensity VIII to IX.[18]

The mainly caused fault of this earthquake was under debate; initially Yangsan Fault was thought to be the cause of this earthquake, however days later Korea Meteorological Administration (KMA) announced that Jangsa fault – a branch fault of Yangsan fault – is main cause of the earthquake. However, the Korea Institute of Geoscience and Mineral Resources (KIGAM) analyzed that the unknown fault caused this earthquake.[19] Water injection in the ground by the geothermal plant in Pohang might have also triggered the earthquake.[20][21]

The College Scholastic Ability Test was delayed by one week both to change testing sites and allow nerves a chance to relax from the quake.[22][23]

Social Fallout from the Earthquake

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The Pohang local government requested in 2019 that the national assembly and the central government work closely together to draft special legislation that identifies and compensates the victims of the 2017 Pohang earthquake, as more and more citizens are joining the lawsuits. In March of 2024, an article published by one of Korea's largest business newspapers reported that 96% of the Pohang population had joined the lawsuit. All of the Pohang residents in the suit are expected to receive 3 million won each, or about $2100 USD, by order of the Daegu district court. The total amount of compensation could reach 2 trillion won. [24]

By 2019, a large number of the residents displaced by the earthquake were still living in tents in gymnasiums, because some buildings were still considered unstable. [25]

Nearly 80% of Pohang residents, according to Mayor Lee Gang-deok, complain about mental health issues, and 42% are currently dealing with post-traumatic stress disorder. As a result, the special law is intended to fully treat the emotional and bodily pain caused by the earthquake. Although bills have been presented for the National Assembly's consideration, discussions over the specifics of the legislation are still underway [25].

See also

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References

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  1. ^ a b c "발생시각 2017년 11월 15일 14시 29분 31초 규모 5.4". Korea Meteorological Administration. 15 November 2017. Retrieved 15 November 2017.
  2. ^ "South Korea earthquake: Tremors that rocked Pohang were second-strongest on record". The Independent. 16 November 2017. Archived from the original on 2022-05-01. Retrieved 18 November 2017.
  3. ^ "포항 인근 규모 5.4 지진, 서울도 흔들…경주 다음 역대 2위(3보)". 뉴스1 (in Korean). 2017-11-15. Retrieved 2024-11-20.
  4. ^ Zastrow, Mark (22 March 2019). "South Korea accepts geothermal plant probably caused destructive quake". Nature. doi:10.1038/d41586-019-00959-4. PMID 32203345. S2CID 159087199.
  5. ^ Naik, Sambit Prasanajit; Kim, Young-Seog; Kim, Taehyung; Su-Ho, Jeong (April 2019). "Geological and Structural Control on Localized Ground Effects within the Heunghae Basin during the Pohang Earthquake (MW 5.4, 15th November 2017), South Korea". Geosciences. 9 (4): 173. doi:10.3390/geosciences9040173. ISSN 2076-3263.
  6. ^ a b c Kim, Kwang-Hee; Seo, Wooseok; Han, Jongwon; Kwon, Joa; Kang, Su Young; Ree, Jin-Han; Kim, Sungshil; Liu, Kang (2020-01-05). "The 2017 ML 5.4 Pohang earthquake sequence, Korea, recorded by a dense seismic network". Tectonophysics. 774: 228306. doi:10.1016/j.tecto.2019.228306. ISSN 0040-1951.
  7. ^ a b c Kim, Kwang-Hee; Ree, Jin-Han; Kim, YoungHee; Kim, Sungshil; Kang, Su Young; Seo, Wooseok (June 2018). "Assessing whether the 2017 M w 5.4 Pohang earthquake in South Korea was an induced event". Science. 360 (6392): 1007–1009. doi:10.1126/science.aat6081. ISSN 0036-8075. PMID 29700224.
  8. ^ academic.oup.com https://academic.oup.com/gji/article/124/2/381/780578. Retrieved 2024-11-20. {{cite web}}: Missing or empty |title= (help)
  9. ^ a b c d "Pohang quakes damage some 2,000 houses, 227 schools". Yonhap. 19 November 2017. Archived from the original on December 1, 2017.
  10. ^ a b Kim, Kwang-Hee; Seo, Wooseok; Han, Jongwon; Kwon, Joa; Kang, Su Young; Ree, Jin-Han; Kim, Sungshil; Liu, Kang (2020-01-05). "The 2017 ML 5.4 Pohang earthquake sequence, Korea, recorded by a dense seismic network". Tectonophysics. 774: 228306. doi:10.1016/j.tecto.2019.228306. ISSN 0040-1951.
  11. ^ a b c Grigoli, F.; Cesca, S.; Rinaldi, A. P.; Manconi, A.; López-Comino, J. A.; Clinton, J. F.; Westaway, R.; Cauzzi, C.; Dahm, T.; Wiemer, S. (June 2018). "The November 2017 M w 5.5 Pohang earthquake: A possible case of induced seismicity in South Korea". Science. 360 (6392): 1003–1006. doi:10.1126/science.aat2010. ISSN 0036-8075. PMID 29700226.
  12. ^ a b c Woo, J.-U.; Kim, M.; Sheen, D.-H.; Kang, T.-S.; Rhie, J.; Grigoli, F.; Ellsworth, W. L.; Giardini, D. (December 2019). "An In-Depth Seismological Analysis Revealing a Causal Link Between the 2017 M W 5.5 Pohang Earthquake and EGS Project". Journal of Geophysical Research: Solid Earth. 124 (12): 13060–13078. doi:10.1029/2019JB018368. ISSN 2169-9313.
  13. ^ Cho, Eunbyeol; Woo, Jeong-Ung; Rhie, Junkee; Kang, Tae-Seob; Baag, So-Young (2023-04-01). "Rupture Process of the 2017 Mw 5.5 Pohang, South Korea, Earthquake via an Empirical Green's Function Method". Bulletin of the Seismological Society of America. 113 (2): 592–603. doi:10.1785/0120220161. ISSN 0037-1106.
  14. ^ Eunbyeol Cho, Jeong‐Ung Woo, Junkee Rhie, Tae‐Seob Kang, So‐Young Baag; Rupture Process of the 2017 Mw 5.5 Pohang, South Korea, Earthquake via an Empirical Green’s Function Method. Bulletin of the Seismological Society of America 2023;; 113 (2): 592–603. doi: https://doi.org/10.1785/0120220161
  15. ^ Korean Government Commission (2019). Summary report of the Korean Government Commission on relations between the 2017 Pohang earthquake and EGS project, Geological Society of Korea.
  16. ^ a b Heo, Dabeen; Ahn, Byeong Seok; Kim, Kwang-Hee; Rhie, Junkee; Sohn, Yujin; Kang, Tae-Seob (2024-09-01). "Spatial and temporal evolutions of aftershocks between 2017 and 2023 at the Pohang EGS site and its vicinity". Episodes Journal of International Geoscience. 47 (3): 611–623. doi:10.18814/epiiugs/2024/02403s07.
  17. ^ "Handong University campus dizzying situation...Emergency evacuation". Sports Kyunghyang. 15 November 2017. Retrieved November 15, 2017.
  18. ^ "연약한 지반이 지진 피해 키웠다…실제 충격은 규모 7.4".
  19. ^ "<포항 5.4 强震>"포항 강진, 장사단층 아닌 無名단층서 발생"".
  20. ^ Grigoli, F.; Cesca, S.; Rinaldi, A. P.; Manconi, A.; López-Comino, J. A.; Clinton, J. F.; Westaway, R.; Cauzzi, C.; Dahm, T.; Wiemer, S. (1 June 2018). "The November 2017 Mw 5.5 Pohang earthquake: A possible case of induced seismicity in South Korea". Science. 360 (6392): 1003–1006. Bibcode:2018Sci...360.1003G. doi:10.1126/science.aat2010. PMID 29700226.
  21. ^ Kim, Kwang-Hee; Ree, Jin-Han; Kim, YoungHee; Kim, Sungshil; Kang, Su Young; Seo, Wooseok (1 June 2018). "Assessing whether the 2017 Mw 5.4 Pohang earthquake in South Korea was an induced event". Science. 360 (6392): 1007–1009. Bibcode:2018Sci...360.1007K. doi:10.1126/science.aat6081. ISSN 0036-8075. PMID 29700224.
  22. ^ "South Korea quiet for quake-delayed college entrance exam". The Straits Times. 23 November 2017. Retrieved 15 March 2019.
  23. ^ Kim, Jung-hee; Cho, Han-dae (16 November 2017). "CSAT delay causes chain reaction of changes". JoongAng Ilbo. Retrieved 15 March 2019.
  24. ^ WOO, Sungduk. "Participation increased after the ruling on the payment of 3 million won in alimony. During the 2017 Pohang earthquake, the population reached 96%". Maeil Business Newspaper.
  25. ^ a b Lee, Juheon. "The Social Fallout From Pohang's 'Man-Made' Earthquake". The Diplomat.
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