外国人最想和你聊的时事话题：地震

　What is Earthquake　什么是地震
     An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011 (as of March 2011), and it was the largest Japanese earthquake since records began. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.
     The effects of earthquakes include, but are not limited to, the following:
     Shaking and ground rupture
     Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from the epicenter, and the local geological and geomorphologic conditions, which may amplify or reduce wave propagation. The groundshaking is measured by ground acceleration.
     Specific local geological, geomorphologic, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.
     Landslides and avalanches
     Earthquakes, along with severe storms, volcanic activity, and coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue.
     Fires
     Earthquakes can cause fires by damaging electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, more deaths in the 1906 San Francisco earthquake were caused by fire than by the earthquake itself.
     Soil liquefaction
     Soil liquefaction occurs when, because of the shaking, watersaturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits. This can be a devastating effect of earthquakes. For example, in the 1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves.
     Tsunami
     Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across Open Ocean and wreak destruction on far shores hours after the earthquake that generated them.
     Ordinarily, earthquakes under magnitude 7.5 on the Richter scale do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.
     Floods
     A flood is an overflow of any amount of water that reaches land. Floods occur usually when the volume of water within a body of water, such as a river or lake, exceeds the total capacity of the formation, and as a result some of the water flows or sits outside of the normal perimeter of the body. However, floods may be secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to dam rivers, which collapse and cause floods.
     Tidal forces
     Research work has shown a robust correlation between small tidally induced forces and non-volcanic tremor activity.
     Human impacts
     An earthquake may cause injury and loss of life, road and bridge damage, general property damage, and collapse or destabilization(potentially leading to future collapse) of buildings. The aftermath may bring disease, lack of basic necessities, higher insurance premiums. Earthquakes can also cause volcanic eruptions, bringing further problems.
     地震分为天然地震(natural earthquake)和人工地震(artificial earthquake)两大类。此外，某些特殊情况下也会产生地震，如大陨石冲击地面(陨石冲击地震meteorolite shocking earthquake)等。引起地球表层振动的原因有很多，根据地震的成因，可以把地震分为以下几种：
     1. 构造地震(tectonic earthquake)
     2. 火山地震(volcanic earthquake)
     3. 塌陷地震(collapse earthquake)
     4. 诱发地震(induced earthquake)
     5. 人工地震(man-made earthquake)
     地震波发源的地方，叫做震源(focus)。震源在地面上的垂直投影(vertical projection)，即地面上离震源最近的一点称为震中(epicenter)。它是接受振动最早的部位。震中到震源的深度叫做震源深度(hypocentral depth)。通常将震源深度小于60公里的叫浅源地震(shallow-focus earthquake)，深度在60公里至300公里的叫中源地震(intermediate-depth earthquake)，深度大于300公里的叫深源地震(anatectic earthquake)。对于同样大小的地震，由于震源深度不一样，对地面造成的破坏程度也不一样。震源越浅，破坏越大，但波及范围也越小，反之亦然。