Foundation: What is an earthquake?
An earthquake is a sudden tremor or movement of the Earth’s crust caused by underground movement along a fault plane or by volcanic activity.
Magnitude
The magnitude of earthquakes, which is the energy released by earthquakes, is commonly measured using the Richter scale which compares the duration of their seismic waves as well as the maximum heights of the seismic waves at a distance of 100 kilometres from the point on the Earth’s surface directly above the source of the earthquake, the epicentre.
Types of seismic waves
Earthquakes emit two types of seismic waves, categorised into body waves and surface waves, which have their own characteristics.
Body waves
1. Primary waves are identical in character to sound waves. The ground is forced to move forwards and backwards as it is compressed and decompressed.
2. Secondary waves travel more slowly and arrive after the primary waves. They move in all directions, at speeds which depend upon the density of the rocks through which they are moving.
Surface waves
Surface waves are formed close to the epicentre and can only travel through the outer part of the Earth’s crust. They are responsible for the majority of the buildings damaged by earthquakes.
Interesting Fact
Earthquake vibrations travel very fast, up to 14 kilometres per second. The fastest seismic waves take less than 20 minutes to reach the other side of the earth, a distance of almost 13,000 kilometres!
The following interesting animation elaborates on the various characteristics of an earthquake. You even get to create your own earthquake and see how it causes destruction. Right-click to open animation in a new window:
Causes of earthquakes
There are two main causes of earthquakes. They can be linked to explosive volcanic eruptions or can be triggered by tectonic activity, with the latter being the cause of most earthquakes.
According to the Plates Tectonics theory, deep in the Earth’s interior, convection of the rocks, caused by temperature variations in the Earth, induces stresses that result in movement of the transform boundaries. The rates of plate movements range from about 2 to 12 centimeters per year. The stresses from convection deform the brittle portions of transform boundaries, thereby storing tremendous energy within the plates. When the accumulating stress exceeds the strength of the rocks comprising these brittle zones, the rocks break suddenly, releasing the stored elastic energy called seismic waves as an earthquake.
The following animation shows how earthquakes are created. Right-click to open the animation in a new window:
Similarly, the following video describes how earthquakes occur.
Where do earthquakes occur at?
Some 80 percent of all the planet’s earthquakes occur along the rim of the Pacific Ocean, called the “Ring of Fire” because of the preponderance of volcanic activity there as well. Most earthquakes occur at fault zones, where tectonic plates collide or slide against each other. Other quakes can occur far from faults zones when plates are stretched or squeezed.
When two plates collide into each other, the area where a plate subducts beneath another is known as the subduction zone. Areas with the white squares on the diagram above are places where the main subduction zones are found.
Here is a diagram of what a subduction zone looks like:
The following diagram shows the distribution of earthquakes.
Impacts of earthquakes
Social Impact
– Destruction of infrastructures
Collapsing buildings claim by far the majority of lives, but the destruction is often compounded by mud slides, fires, floods, or tsunamis. Smaller temblors that usually occur in the days following a large earthquake can complicate rescue efforts and cause further death and destruction.
– Possibility of other hazards
Earthquakes also can cause a tsunami, or a series of waves which can cross an ocean and cause extensive damage to coastal regions. In areas where there are steep slopes, vibrations resulting from earthquakes may cause landslides.
Environmental Impact
– Effects of earthquakes
The effects of an earthquake depend on many factors, such as the distance from the epicentre and the local ground conditions. Generally, for locations near the epicentre, the following effects may be observed:
Measures to be taken during an earthquake
Loss of life can be avoided through emergency planning, education, and the construction of buildings that sway rather than break under the stress of an earthquake.
Safety Tips
• Have an earthquake readiness plan.
• Locate a place in each room of the house that you can go to in case of an earthquake. It should be a spot where nothing is likely to fall on you.
• Know how to turn off your gas and water mains.
If Shaking Begins
• Drop down; take cover under a desk or table and hold on.
• Stay indoors until the shaking stops and you’re sure it’s safe to exit.
• If you are outdoors, find a clear spot away from buildings, trees, and power lines. Drop to the ground.
• If you are in a car, slow down and drive to a clear place. Stay in the car until the shaking stops.
About Sichuan earthquake
On May 12, 2008, a 7.9-magnitude earthquake hit Sichuan Province, a mountainous region in Western China, killing about 70,000 people and leaving over 18,000 missing. Over 15 million people lived in the affected area, including almost 4 million in the city of Chengdu.
Thousands of the initial quake’s victims were children crushed in shoddily built schools. It was estimated that up to 10,000 students died in the collapse of 7,000 classrooms and dormitory rooms.
What caused the Sichuan earthquake?
The Sichuan Province is a very active tectonic area due to the northward indentation of the Indian plate into the Tibetan crust after the collision between the Indian and Eurasian plates which induced uplift and deformation of the Tibetan Plateau.
The earthquake was caused by movement along a thrust fault located on Longmenshan Fault (yellow line). The deformation of the rocks in this region was caused by the the collision between the Indian and Eurasian tectonic (crustal) plates and the continued movement of the plates relative to each other.
This map shows the tectonics in the Sichuan Province, and in particular the Longmenshan thrust fault(yellow line) and the epicenter of the earthquake (red star).
The Longmenshan thrust zone mainly consists of three thrusts and associated fault-propatation folds. This zone is approximately 60 km wide, and constitutes the topographic boundary between the eastern Tibetan Plateau and the Sichuan basin. The convergence rate across the thrust zone is inferred to be 4-6 mm/yr.
Details of the Sichuan earthquake
China: Earthquake – Situation Map
Earthquake-affected provinces
Earthquake in Sichuan Province:
CCTV-9 China Earthquake:
Most terrifying LIVE footage of Sichuan earthquake by far(2)(in chinese):
About video: The live footage taken by a Peruvian tourist in Qingchengshan, Sichuan Province, China when the earthquake happened.
China Earthquake May 12 ’08 – New Footage:
Camera catches the quake hitting the Chengdu Airport:
About video: This is the security camera catching the situation at Chengdu Shuangliu International Airport when the huge earthquake happened.The airport is one of the top 100 most busiest airports in the world, so when the earthquake hit there were more than ten thousand passengers had passed the Security Gate in the security zone,and much more passengers in the Waiting Hall.This video shows us the evacuation of those passengers.
Quake hits western China:
Impacts of the Sichuan earthquake
Social Impact
During the quake, a large number of schools collasped and a lot of houses were destroyed, leaving about 4.8 million of people homeless. Moreover farms and crops are also destroyed and since villagers depend on their farms for living, when the farms are destroyed, they lost their main source of income causing them to fall into poverty.
Extent of the most damaged areas from the Sichuan earthquake
Earthquake in China cause 900 children buried in school 2008:
Interactive map. Right-click to open the map in a new window and find out more about some of the worst-affected places.
Economical Impact
The earthquake caused massive infrastructure damages hence affecting China’s economical development. It is estimated that a total amount of direct economic losses caused by the earthquake is at RMB852,309 million. As a result, a huge amount of money that could be used to improve other aspects of China now had to be pumped into rebuilding the properties, therefore slowing down the economic development of China. Also, the earthquake actually brought down the tourism as people will not go there since the place will be filled with chaos, debris and corpses.
Earthquake epicentre, aftershocks and impacts of earthquake
Psycological Impact
When people found their loved ones dead, they become devastated due to the sudden lost of loved ones. Also, those that survived may actually be traumatized after experiencing the earthquake.
One family’s loss in Sichuan earthquake:
Measures taken after the Sichuan earthquake
Rebuilding structures
Some of the hardest hit areas, like Beichuan County were destroyed and 99% of the houses in Qingchuan County needed reconstructions. These infrastructure damages add to the financial and environmental cost of the quake and also complicate the procedure of other rebuilding efforts.
On June 8th, Premier Wen Jiabao signed a regulation on reconstruction (Regulation on Recovery and Reconstruction of the Wenchuan Earthquake) which emphasized that settlements should occupy as little as possible farmland, avoid pollution of drinking water sources, and protect ecological areas.
Transitional settlements
A huge number of houses collapsed in the quake and this resulted in an increase in number of homeless people. Many have been living in tents since the disaster, and hence the government has released guidelines for the construction of permanent transitional settlements (Earthquake-stricken Area Transitional Settlements: Technical Guidelines for Environmental Protection).
A New Model for the Future
China’s relief organizations, researchers, and government officials will also be improving disaster prevention and response in the future so as to help evacuate the people faster in case of emergencies like the earthquake and save as much people as possible.
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Sichuan earthquake 
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