User:Mikenorton/Geology of the 2008 Wenchuan earthquake

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Geology[edit]

According to a study by the China Earthquake Administration (CEA), the earthquake occurred along the Longmenshan Fault, a thrust structure along the border of the Indo-Australian Plate and Eurasian Plate. Seismic activities concentrated on its mid-fracture (known as Yingxiu-Beichuan fracture). The rupture lasted close to 120 seconds, with the majority of energy released in the first 80 seconds. Starting from Wenchuan, the rupture propagated at an average speed of 3.1 km/s (6,900 mph), 49° toward north east, rupturing a total of about 300 km (190 mi). Maximum displacement amounted to nine metres (30 ft). The focus was deeper than 10 km (6.2 mi).[1]

In a United States Geological Survey (USGS) study, preliminary rupture models of the earthquake indicated displacement of up to nine metres (30 ft) along a fault approximately 240 km (150 mi) long by 20 km (12 mi) deep.[2] The earthquake generated deformations of the surface greater than three metres (9.8 ft)[3] and increased the stress (and probability of occurrence of future events) at the northeastern and southwestern ends of the fault.[3] On May 20, USGS seismologist Tom Parsons warned that there is "high risk" of a major M>7 aftershock over the next couple weeks or months.[4]

Japanese seismologist Yuji Yagi at the University of Tsukuba said that the earthquake occurred in more than 1 stage: "The 250-kilometre (155 mi) Longmenshan Fault tore in two sections, the first one ripping about 6.5 metres (7 yd) followed by a second one that sheared 3.5 metres (4 yd)."[5] His data also showed that the earthquake lasted about two minutes and released 30 times the energy of the Great Hanshin earthquake of 1995 in Japan, which killed over 6,000 people. He pointed out that the shallowness of the epicenter and the density of population greatly increased the severity of the earthquake. Teruyuki Kato, a seismologist at the University of Tokyo, said that the seismic waves of the quake traveled a long distance without losing their power because of the firmness of the terrain in central China. According to reports from Chengdu, the capital of Sichuan province, the earthquake tremors lasted for "about two or three minutes".[6]

Tectonics[edit]

The extent of the earthquake and after shock-affected areas lying north-east, along the Longmen Shan fault.

The Longmen Shan Fault System is situated in the eastern border of the Tibetan Plateau and contains several faults. This earthquake ruptured at least two imbricate structures in Longmen Shan Fault System, i.e. the Beichuan Fault and the Guanxian–Anxian Fault. In the epicentral area, the average slip in Beichuan Fault was about 3.5 metres (11 ft) vertical, 3.5 metres (11 ft) horizontal-parallel to the fault, and 4.8 metres (16 ft) horizontal-perpendicular to the fault. In the area about 30 kilometres (19 mi) northeast of the epicenter, the surface slip on Beichuan Fault was almost purely dextral strike-slip up to about three metres (9.8 ft), while the average slip in Guanxian–Anxian Fault was about two metres (6.6 ft) vertical and 2.3 metres (7.5 ft) horizontal.[7]

According to CEA:[1]

"The energy source of the Wenchuan earthquake and Longmenshan's southeast push came from the strike of the Indian Plate onto the Eurasian Plate and its northward push. The inter-plate relative motion caused large scale structural deformation inside the Asian continent, resulting in a thinning crust of the Qinghai-Tibet Plateau, the uplift of its landscape and an eastward extrude. Near the Sichuan Basin, Qinghai-Tibet Plateau's east-northward movement meets with strong resistance from the South China Block, causing a high degree of stress accumulation in the Longmenshan thrust formation. This finally caused a sudden dislocation in the Yingxiu-Beichuan fracture, leading to the violent earthquake of Ms 8.0."[8]

Dr. Harley Benz, US Geological Survey
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According to the United States Geological Survey:[9]

The earthquake occurred as the result of motion on a northeast striking reverse fault or thrust fault on the northwestern margin of the Sichuan Basin. The earthquake's epicenter and focal-mechanism are consistent with it having occurred as the result of movement on the Longmenshan Fault or a tectonically related fault. The earthquake reflects tectonic stresses resulting from the convergence of crustal material slowly moving from the high Tibetan Plateau, to the west, against strong crust underlying the Sichuan Basin and southeastern China.

On a continental scale, the seismicity of central and eastern Asia is a result of northward convergence of the Indian Plate against the Eurasian Plate with a velocity of about 50 mm/a [2.0 in/year]. The convergence of the two plates is broadly accommodated by the uplift of the Asian highlands and by the motion of crustal material to the east away from the uplifted Tibetan Plateau. The northwestern margin of the Sichuan Basin has previously experienced destructive earthquakes. The magnitude 7.5 earthquake of August 25, 1933, killed more than 9,300 people.

According to the British Geological Survey:[10]

The earthquake occurred 92 km [57 mi] northwest of the city of Chengdu in eastern Sichuan province and over 1,500 km [930 mi] from Beijing, where it was also strongly felt. Earthquakes of this size have the potential to cause extensive damage and loss of life.

The epicenter was in the mountains of the Eastern Margin of Qing-Tibet Plateau at the northwest margin of the Sichuan Basin. The earthquake occurred as a result of motion on a northeast striking thrust fault that runs along the margin of the basin.

The seismicity of central and eastern Asia is caused by the northward movement of the India plate at a rate of 5 cm/year [2.0 in/year] and its collision with Eurasia, resulting in the uplift of the Himalaya and Tibetan plateaux and associated earthquake activity. This deformation also results in the extrusion of crustal material from the high Tibetan Plateaux in the west towards the Sichuan Basin and southeastern China. China frequently suffers large and deadly earthquakes. In August 1933, the magnitude 7.5 Diexi earthquake, about 90 km [56 mi] northeast of today's earthquake, destroyed the town of Diexi and surrounding villages, and caused many landslides, some of which dammed the rivers.

Intensities and damage area[edit]

The map of earthquake intensity published by CEA after surveying 500,000 km2 (190,000 sq mi) of the affected area shows a maximum liedu of XI on the China Seismic Intensity Scale (CSIS), described as "very destructive" on the European Macroseismic Scale (EMS) from which CSIS drew reference.[11] (USGS, using the Modified Mercalli intensity scale (CC), also placed maximum intensity at XI, "extreme".[12]) Two south-west-north-east stripes of liedu XI are centered around Yingxiu, Wenchuan (the town closest to the epicenter of the main quake) and Beichuan (the town repeatedly struck by strong aftershocks including one registering Ms 6.1 on August 1, 2008), both in Sichuan Province, occupying a total of 2,419 km2 (934 sq mi). The Yingxiu liedu-XI zone is about 66 km (41 mi) long and 20 km (12 mi) wide along Wenchuan–Dujiangyan–Pengzhou; the Beichuan liedu-XI zone is about 82 km (51 mi) long and 15 km (9.3 mi) wide along An County–Beichuan–Pingwu. The area with liedu X (comparable to X on EMS, "destructive" and X on MM, "disastrous") spans 3,144 km2 (1,214 sq mi). The area affected by earthquakes exceeding liedu VI totals 440,442 km2 (170,056 sq mi), occupying an oval 936 km (582 mi) long and 596 km (370 mi) wide, spanning three provinces and one autonomous region.[citation needed]

QLARM (Quake Loss Alarms for Response and Mitigation) issues near-real-time estimates of fatalities and number of injured for earthquakes worldwide. Recent alerts can be found on the web page of the International Institute for Earth Simulation Foundation.[13] Such an alert was issued 21 minutes after the May 12 Wenchuan earthquake of 2008. It had at first been assigned M7.5, internationally. This initial underestimate of the magnitude is a known problem with earthquakes of M8 and larger. Based on the M7.5 information, QLARM distributed an email to about 300 recipients estimating that 1,000 to 4,000 fatalities had occurred. After learning that the earthquake may measure M8, QLARM distributed a revised estimate of 40,000 to 100,000 fatalities. This information was distributed within 100 minutes of the Wenchuan earthquake.[citation needed]

The news and official reports of fatalities are often strongly misleading. After the Wenchuan earthquake, officials led the public to believe for more than a week that the fatalities numbered only a fraction of what they really were (Figure 1). At the very beginning, everyone expects the news reports to be an initial count that will grow, not however, after one week. After such a long time, the false news reports take on a reality in their own right and the theoretical loss calculations by experts are discarded.[citation needed]

Figure 1: Official fatality reports for the Wenchuan M8 earthquake as a function of time. Squares show fatalities, triangles show the sum of fatalities plus missing persons, which equaled the number of fatalities in the end. The diamond is the QLARM estimate 100 minutes after the earthquake, with the range of possible values given by the solid, vertical line through the diamond. The horizontal dash-dotted line indicates the average value of fatalities calculated by QLARM.
Figure 2: Map of settlements with the estimated mean damage due to the Wenchuan earthquake modeled as a line rupture extending as far as the aftershocks.

Once the extent of the rupture of the Wenchuan earthquake became known, QLARM calculated a more detail picture of the losses. Figure 2 shows a map of the expected mean damage of the settlements affected by the Wenchuan earthquake on a scale of 5. The resistance to shaking of buildings in large cities is assumed to be stronger than in villages, therefore the damage and percentage of fatalities in large cities is less than in villages.[citation needed]

Aftershocks[edit]

On the night of May 12, residents of Chengdu worried about potential aftershocks gathered in the street to avoid staying in buildings.

Between 64 and 104 major aftershocks, ranging in magnitude from 4.0 to 6.1, were recorded within 72 hours of the main quake. According to Chinese official counts, "by 12:00 CST, November 6, 2008, there had been 42,719 total aftershocks, of which 246 ranged from 4.0 MS to 4.9 MS, 34 from 5.0 MS to 5.9 MS, and 8 from 6.0 Ms to 6.4 MS; the strongest aftershock measured 6.4 MS."[14] The latest aftershock exceeding M6 occurred on August 5, 2008.

(The Ms  6.1 earthquake on August 30, 2008, in southern Sichuan was not part of this series because it was caused by a different fault. See 2008 Panzhihua earthquake for details.)

New sources[edit]

Thrust configuration and seismicity - Wang & Lin 2017[15]

Response of seismogenic faults to the Wenchuan Ms 8 earthquake - Wu et al. 2018[16]

Fault geometry and slip distributions from GPS and InSAR - Wan et al. 2017[17]

Effects of impoundment of the Zipingu reservoir and triggering of the 2008 Wenchuan earthquake - Tao et al. 2015[18]

Rupture process from surface ruptures and striations - Pan et al. 2014[19]

Coulomb stress changes induced by the 2008 Wenchuan EQ and triggering of the 2013 Lushan EQ - Wang et al. 2014[20]

Updated intensity map from co-seismic landslide abundance - Xu et al. 2013[21]

Link to Science Direct articles relevant to the 2008 earthquake

References[edit]

  1. ^ a b 汶川8.0级地震成因分析 [Analysis of the cause of the M8.0 Wenchuan earthquake] (in Chinese). China Earthquake Administration. May 30, 2008. Archived from the original on September 16, 2008. Retrieved September 7, 2008.
  2. ^ Ji, Chen; Hayes, Gavin. "Finite Fault Model of the May 12, 2008 Mw 7.9 Eastern Sichuan, China Earthquake". United States Geological Survey – National Earthquake Information Center. Archived from the original on May 17, 2008. Retrieved May 15, 2008.
  3. ^ a b Álvarez-Gómez, José A. (May 12, 2008). "Calculation of surface deformation and variation of static Coulomb forces for the earthquake of 7.9 MW on May 12, 2008, in Sichuan, China" (in Spanish). Complutense University of Madrid. Archived from the original on May 17, 2008. Retrieved May 20, 2008.
  4. ^ Barboza, David (May 20, 2008). "One Week Later, a Nation Pauses to Share Its Mourning and Grief". The New York Times. Archived from the original on December 11, 2008. Retrieved May 20, 2008.
  5. ^ Schumaker, Dave (May 15, 2008). "Sichuan Quake Ruptured in 2 Stages | The Geology News Blog". Geology.rockbandit.net. Archived from the original on July 24, 2011. Retrieved November 7, 2012.
  6. ^ "Massive quake kills nearly 10,000 in China". NBC News. May 12, 2008. Archived from the original on July 20, 2013. Retrieved May 20, 2008.
  7. ^ de Michele, Marcello; Raucoules, Daniel; de Sigoyer, Julia; Pubellier, Manuel; Chamot-Rooke, Nicolas (October 2010), "Three-dimensional surface displacement of the 2008 May 12 Sichuan earthquake (China) derived from Synthetic Aperture Radar: evidence for rupture on a blind thrust", Geophys. J. Int. (abstract), 183 (3): 1097–1103, Bibcode:2010GeoJI.183.1097D, doi:10.1111/j.1365-246X.2010.04807.x(subscription required)
  8. ^ Chinese: 汶川地震的发生及龙门山向东南方向推覆的动力来源是印度板块与欧亚大陆碰撞及其向北的推挤，这一板块间的相对运动导致了亚洲大陆内部大规模的构造变形，造成了青藏高原的地壳缩短、地貌隆升和向东挤出(图3)。由于青藏高原在向东北方向运动的过程中在四川盆地一带遭到华南活动地块的强烈阻挡，使得应力在龙门山推覆构造带上高度积累，以至于沿映秀-北川断裂突然发生错动，产生8.0级强烈地震 See Longmenshan Fault for more quotes from this study.
  9. ^ "Magnitude 7.9 – Eastern Sichuan, China". UGS. May 12, 2008. Archived from the original on September 11, 2008. Retrieved March 17, 2011.
  10. ^ "Eastern Sichuan Earthquake May 12, 2008". British Geological Survey. May 13, 2008. Archived from the original on June 5, 2011. Retrieved March 17, 2011.
  11. ^ "Seismic intensity map of the M8.0 Wenchuan earthquake (汶川8.0级地震烈度分布图)". CEA. August 29, 2008. Archived from the original on September 8, 2008. Retrieved October 9, 2003.
  12. ^ "Magnitude 7.9 – Eastern Sichuan, China and Hong Kong". USGS. May 12, 2008. Archived from the original on September 16, 2008. Retrieved September 16, 2008.
  13. ^ "Near Real-Time Earthquake Loss Estimates". Archived from the original on May 18, 2017. Retrieved January 16, 2017.
  14. ^ 据地震台网测定，北京时间2009-01-15 02:23 在四川汶川(北纬31.3,东经103.3) 发生5.1级地震。截止2009年01月14日12时，汶川7.9级地震余震区共发生42719次余震，其中4.0～4.9级246次，5.0～5.9级 34次，6.0～6.9级8次，最大震级为6.4级。 [According to surveys by the China Seismic Network, an earthquake of M(s) 5.1 struck Wenchuan, Sichuan at 02:23 (CST) on January 15, 2009. By 12:00 o'clock January 14, 2009 (CST), aftershock zone of the 8 Ms-Wenchuan earthquake had 42,719 total aftershocks, of which 246 between M4.0 and 4.9, 34 between M5.0 and 5.9, 8 between M6.0 and 6.9; the strongest being of 6.4 Ms] (in Chinese). Sichuan Earthquake Administration (SCEA). Archived from the original on July 30, 2008. Retrieved December 10, 2008. Another source is 汶川8.0级地震余震分布与统计 [Distributions and statistics of aftershocks of the M8.0 Wenchuan earthquake] (in Chinese). China Earthquake Administration. August 29, 2008. Archived from the original on September 16, 2008. Retrieved September 7, 2008. The text in the latter appears to be an exact copy of the former, only updated less frequently.
  15. ^ Wang, M.; Lin, A. (2017). "Active thrusting of the Longquan Fault in the central Sichuan basin, China, and the seismotectonic behavior in the Longmen Shan fold-and-thrust belt". Journal of Geophysical Research: Solid Earth. 122 (7): 5639–5662. Bibcode:2017JGRB..122.5639W. doi:10.1002/2016JB013391.
  16. ^ Wu, Y.; Jiang, Z; Liang, H.; Pang, Y.; Zhu, S.; Chang, L.; Chen, C.; Li, J. (2018). "Deformation Response of Seismogenic Faults to the Wenchuan MS 8.0 Earthquake: A Case Study for the Southern Segment of the Longmenshan Fault Zone". Remote Sensing. 10 (6): 894. Bibcode:2018RemS...10..894W. doi:10.3390/rs10060894.
  17. ^ Wan, Y.; Shen, Z.-K.; Bürgmann, R.; Sun, J.; Wang, M. (2017). "Fault geometry and slip distribution of the 2008 Mw 7.9 Wenchuan, China earthquake, inferred from GPS and InSAR measurements". Geophysical Journal International. 208 (2): 748–766. doi:10.1093/gji/ggw421.
  18. ^ Tao, W.; Masterlark, T.; Shen, Z.-K.; Ronchin, E. (2015). "Impoundment of the Zipingpu reservoir and triggering of the 2008 Mw 7.9 Wenchuan earthquake, China". Journal of Geophysical Research: Solid Earth. 120 (10): 7033–7047. Bibcode:2015JGRB..120.7033T. doi:10.1002/2014JB011766. PMC 5066322. PMID 27812436.
  19. ^ Pan, J.; Li, H.; Si, J.; Pei, J.; Fu, X.; Chevalier, M.-L.; Liu, D. (2014). "Rupture process of the Wenchuan earthquake (Mw 7.9) from surface ruptures and fault striations characteristics". Tectonophysics. 619–620: 13–28. Bibcode:2014Tectp.619...13P. doi:10.1016/j.tecto.2013.06.028.
  20. ^ Wang, Y.; Wang, F.; Wang, M.; Shen, Z.-K; Wan, Y. (2014). "Coulomb Stress Change and Evolution Induced by the 2008 Wenchuan Earthquake and its Delayed Triggering of the 2013 Mw 6.6 Lushan Earthquake". Wang. 85 (1): 52–59. doi:10.1785/0220130111.
  21. ^ Xu, C.; Xu, X.; Zhou, B.; Yu, G. (2013). "Revisions of the M 8.0 Wenchuan earthquake seismic intensity map based on co-seismic landslide abundance". Natural Hazards. 69 (3): 1459–1476. doi:10.1007/s11069-013-0757-0. S2CID 129918966.
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