User:Mikenorton/Geology of the 2008 Wenchuan earthquake

Geology
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, 49° toward north east, rupturing a total of about 300 km. Maximum displacement amounted to 9 m. The focus was deeper than 10 km.

In a United States Geological Survey (USGS) study, preliminary rupture models of the earthquake indicated displacement of up to 9 m along a fault approximately 240 km long by 20 km deep. The earthquake generated deformations of the surface greater than 3 m and increased the stress (and probability of occurrence of future events) at the northeastern and southwestern ends of the fault. 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.

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)." 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".

Tectonics
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 m vertical, 3.5 m horizontal-parallel to the fault, and 4.8 m horizontal-perpendicular to the fault. In the area about 30 km northeast of the epicenter, the surface slip on Beichuan Fault was almost purely dextral strike-slip up to about 3 m, while the average slip in Guanxian–Anxian Fault was about 2 m vertical and 2.3 m horizontal.

According to CEA:

"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."

According to the United States Geological Survey:

"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/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:

The earthquake occurred 92 km northwest of the city of Chengdu in eastern Sichuan province and over 1500 km 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 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 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
The map of earthquake intensity published by CEA after surveying 500000 km2 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. (USGS, using the Modified Mercalli intensity scale (CC), also placed maximum intensity at XI, "extreme". ) 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  6.1 on August 1, 2008), both in Sichuan Province, occupying a total of 2419 km2. The Yingxiu liedu-XI zone is about 66 km long and 20 km wide along Wenchuan–Dujiangyan–Pengzhou; the Beichuan liedu-XI zone is about 82 km long and 15 km wide along An County–Beichuan–Pingwu. The area with liedu X (comparable to X on EMS, "destructive" and X on MM, "disastrous") spans 3144 km2. The area affected by earthquakes exceeding liedu VI totals 440442 km2, occupying an oval 936 km long and 596 km wide, spanning three provinces and one autonomous region.

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. 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.

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.



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.

Aftershocks


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." The latest aftershock exceeding M6 occurred on August 5, 2008.

(The 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
Thrust configuration and seismicity - Wang & Lin 2017

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

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

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

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

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

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

Link to Science Direct articles relevant to the 2008 earthquake