什么引起了地震？

>>>沪江汶川地震专题 

An earthquake is a sudden tremor or movement of the earth's crust, which originates at or below the surface.
地震是地壳一次突然的震动或运动，起源于地球表面或以下。
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.
地震由两种主要原因引起。他们都跟火山爆发有关或由建筑活动引起的。后者是大多数地震的原因。
Earthquakes in the UK are all tectonic quakes, triggered when the Earth’s crust is subjected to strain and eventually moves.
英国的地震都是由建筑物震动，是由地壳绷紧时最终移动引起的。
At the start of a quake there is a sudden movement within the Earth's crust and shock waves move out from that point.
震动开始时地壳内部有一些突然的移动，随后地震波开始在那一点传开来。
The focus of the earthquake or tremor is often deep below the surface and difficult to map, so what is normally called the epicentre is taken as the point on the Earth directly above it.
震中心通常在表面以下深处，很难用地图表示，所以通常我们所称的正中心为地球最上方。
There are two scales for defining the strength of an earthquake, the Mercalli Scale and the Richter Scale.
地震强度用两个级别来定义，Mercalli级别和Richter级别。
Earthquakes emit three types of waves which have their own characteristics.
地震发出两种波，它们有各自的特征。
Primary waves are identical in character to sound waves. The ground is forced to move forwards and backwards as it is compressed and decompressed.
主要波跟声波的特征相同，当地面被加压或减压时强迫向前和向后移动。
Secondary waves travel more slowly and arrive after the P-waves. They move in all directions, at speeds which depend upon the density of the rocks through which they are moving. They cannot move through liquids. They can move walls and fences, leaving them 'S' shaped.
次要波传播的更慢，发生在P波之后。它们根据所穿过岩石的密度以不同的速度向各个方向移动。他们不能穿过液体。但可以穿过墙和篱笆，留下S形状的痕迹。
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. The ground is made to move in a circular motion, causing it to rise and fall as visible waves move across the ground.
表面波在正中心形成，它只能穿过地壳外部，是大多数建筑物在地震中损坏的原因。地面环形运动，引起上升或下降，成为可视波横穿地面。

>>>沪江汶川地震专题