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The 2008 Great Sichuan / Wenchuan Earthquake

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A screenshot of images for the 2008 Sichuan Earthquake

A screenshot of images for the 2008 Sichuan Earthquake

Earthquake map

Earthquake map

The 2008 Sichuan Earthquake was one of the most disastrous earthquakes in terms of destruction, cost, casualties and other factors. It is five years today (May 12th 2013) since this earthquake struck China on the 12th of May 2008 at 02:28:01 pm local time.

The quake struck with a magnitude of 7.9 on the Richter scale and lasted for about 2 minutes, destroying millions of homes and buildings including schools and hospitals, damaging dams, breaking water pipes and gas mains, causing power failures, disrupting telecommunications, creating dangerous quake lakes, destroying and damaging roads and railways, killing and injuring thousands of people, leaving millions homeless, leaving many children orphaned and many more.

The epicentre of the quake was in the Wenchuan county in the Sichuan province and hence it is also mentioned as the Wenchuan earthquake.

the-2008-great-sichuan-wenchuan-earthquake

Longmenshan fault system:

This earthquake occurred along the Longmenshan fault and hence we will briefly look at the details about this fault.

The Longmenshan fault zone or fault system is made up of a few faults; in other words, there are a few faults that lie within the Longmenshan fault zone. This fault is found at the base of the Longmen mountains in the Sichuan province of China. The cause of the fault is the continental convergence of the Indo-Australian plate and the Eurasian plate. The earthquake occurred due to a rupture on this fault which in turn ruptured two other faults in the fault zone. The rupture was due to the motion of the tectonic plates.

The Longmenshan fault is a thrust fault, where ground on one side of the fault moved up or over the ground on the other side. Thrust fault is a reverse fault at a very low angle. If you look at the pictures to the right hand side, you will get a better understanding of the different types of faults and also the difference between a reverse fault and thrust fault.

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A Modified Mercalli Intensity (MMI) scale vs a Richter scale

A Modified Mercalli Intensity (MMI) scale vs a Richter scale

Details of the 2008 Sichuan earthquake:

  • The epicentre of the earthquake was in the Wenchuan County, Sichuan which is a mountainous region in western china. The depth of focus of the quake was 19 km. The quake occurred on the 12th of May 2008 at 14:28 hours local time and lasted for about two minutes.
  • The magnitude of the quake was 7.9 with intensity XI on the intensity scale.
  • The quake was felt in the neighbouring countries of Beijing and Shanghai (more than 1,500 km away from epicentre) where buildings swung due to the tremors.
  • The fault broke to a length of 240 km
  • There were thousands of aftershocks even after months of the quake and killed more people and caused more damages.
the-2008-great-sichuan-wenchuan-earthquake

Cause of the 2008 Sichuan earthquake:

  • The earthquake occurred on the Longmenshan fault system that lies along the border of the Indo-Australian plate and the Eurasian plate in the eastern border of the Tibetan plateau. The quake occurred due to the rupture on this fault, which occurred starting from Wenchuan. The quake waves travelled north-east at an average speed of around 3.1 km per second.
  • The rupture was due to the impact of Indian plate striking on the Eurasian plate (reverse fault) pushing it northward. The total length of the rupture was 240 km and the maximum displacement was around 9 metres.
  • This fault system has many other faults in which two of them were ruptured; the Yingxiu-Beichuan Fault (YBF) and the Guanxian-Anxian fault (GAF).
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Reason for high intensity earthquake and heavy damage:

  • The earthquake occurred in a region of the fault that was along the place where the mountain and basin met. The Sichuan basin area is made up of soft sedimentary rocks, so when seismic waves travel through this area, they slow down and increase in amplitude.
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Note: Seismic waves travel faster in rigid terrain (this is the reason why the effect was felt in far away cities) and they travel slower in sedimentary plains (reason for more destruction in the Sichuan basin area).

General rule, soft rocks shake more (easier to shake) compared to rigid rocks, hence the higher intensity of the quake and severe damage in Sichuan area. More than 100 million people live in this area.

  • Also the depth of focus or focal depth (the depth of the quake) of the quake was shallow.
  • Again in the mountain region, earthquake caused hill slides and landslides and hence buried the towns in this area. This hindered relief work and also blocked rivers to form lakes which later broke due to accumulation of more water, leading to more destruction.

Note: Focal depth and epicentre are two different terms in an earthquake. The pictures to the right hand side will clearly help you distinguish between these two terms

Focal depth - is the point within the earth where the rupture initiates.

Epicentre - Point vertically above the focal depth of the earthquake on the surface of the earth

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Different views about why this earthquake occurred:

  • According to USGS, the displacement was around nine metres along the length of the fault and there were surface deformations greater than three metres.
  • According to the Japanese seismologist Yuji Yagi from the University of Tsukuba, the earthquake took place in 2 stages.
  1. In the first part, the Longmenshan fault was ripped apart by seven meters.
  2. In the second part, the fault sheared by four meters. The total quake lasted around two minutes. The severity of the earthquake was due to shallow focal depth of the quake.
  • According to Japanese seismologist Teruyuki Kato from the University of Tokyo, the seismic waves travelled long distances without losing their energy, because of the firm terrain in Central China.
  • It is also believed that this earthquake was triggered by the construction and the filling of the Zipingpu dam (50 stories tall situated 5.5 km from epicentre of earthquake) that released the tension between the two sides of the fault causing them to move apart. However this was not the cause, the dam could have just triggered the earthquake earlier than it was supposed to happen.
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Damages and casualties in the 2008 Sichuan earthquake:

  • This quake is considered to be one of the 21st deadliest quakes of all times with 69,195 casualties, 18,392 missing and 374,643 injured.
A study on the loss caused by the 2008 Sichuan earthquake

A study on the loss caused by the 2008 Sichuan earthquake

  • Around 4.8 million people were left homeless but estimates predict that it could be as high as 11 million as the government did not release the official figures. The population in the affected area was 15 million.
  • There were 149 to 284 major aftershocks and more than 42,719 aftershocks in total that occurred through to the 28th of May 2008.
  • More than eighty percent of the buildings were destroyed and most of this was due to the poor construction.
  • Almost all provinces in China except Xinjiang, Jilin and Heilongjiang were affected by the quake.
  • Two chemical plants collapsed in Shifang leading to leakage of Ammonia and sulphuric acid in large quantity and hundreds of people being buried under the debris.
  • A school in Dujjangyan collapsed with hundreds of students buried under the debris.
A panda from Wolong Reserve habitat, who has grown close to her caretaker while looked after in other centres while the destroyed panda habitat was rebuilt.

A panda from Wolong Reserve habitat, who has grown close to her caretaker while looked after in other centres while the destroyed panda habitat was rebuilt.

The Wenchuan China Earthquake May 12, 2008

  • Juyuan middle school collapsed and many students were buried.
  • The security guards at a Panda reserve were killed in the quake. One Panda died being smashed by the walls in her enclosure. The other pandas in two reserves close to the epicentre survived but many with injuries. Not much is known about the pandas in the wild.
  • The Zipingpu Hydropower plant had minor damages and is structurally safe and stable.
  • The Tulong reservoir is in danger of collapsing.
  • Around 391 to 400 dams were damaged due to the quake.
  • Thousands of school children died due to poor construction of school buildings. (19,065 students died and 546 disabled). Around 4,727 children were orphaned.
  • Around 7,000 school buildings collapsed throughout the provinces and a total of 21 million buildings were damaged.
  • An estimated 5 million people were left homeless, millions of livestock were killed and agricultural lands were damaged.
  • Around a million pigs died in the Sichuan province
  • The total loss due to the quake is estimated to be $123 billion as property loss and $147 billion (1 trillion yuan) as economic loss.
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the-2008-great-sichuan-wenchuan-earthquake

Other countries that felt the effect of this quake:

Hongkong and Macau felt tremors three minutes after the quake and that lasted for 30 seconds with intensity III, the shaking was felt in Vietnam, Thailand felt shaking from the tremors six minutes after the quake, with buildings swinging for few minutes, Taiwan and Mongolia felt tremors almost eight minutes after the quake, Nepal and Bangladesh felt tremors, India felt tremors almost nine minutes after the quake, and tremors were also felt in Russia and Pakistan.