地震作文：What is earthquake?

　　an earthquake is the result of a sudden release of energy in the earth's crust that creates seismic waves(地震波). earthquakes are recorded with a seismometer(地震检波器), also known as a seismograph(地震仪). the moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete richter magnitude(里氏量级), with magnitude 3 or lower earthquakes being mostly imperceptible 感觉不到的 and magnitude 7 causing serious damage over large areas. intensity of shaking is measured on the modified mercalli scale(麦加利震级, 麦氏震级).

　　at the earth's surface, earthquakes manifest themselves by a shaking and sometimes displacement (位移)of the ground. when a large earthquake epicenter (震中)is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami(海啸). the shaking in earthquakes can also trigger landslides and occasionally volcanic activity.

　　in its most generic sense, the word earthquake is used to describe any seismic event—whether a natural phenomenon or an event caused by humans—that generates seismic waves. earthquakes are caused mostly by rupture(破裂, 裂开) of geological faults(断层), but also by volcanic activity, landslides, mine blasts, and nuclear experiments.

　　an earthquake's point of initial rupture is called its focus or hypocenter. the term epicenter means the point at ground level directly above this.