An earthquake is a shaking of the ground caused by the sudden breaking and shifting（滑动） of large sections of Earth's rocky outer shell. Earthquakes are among the most powerful events on earth, and their results can be terrifying. A severe earthquake may release energy 10,000 times as great as that of the first atomic bomb.（一次强烈的地震释放的能量也许相当于第一颗原子弹威力的1万倍）
Rock movements during an earthquake can make rivers change their course.（河道） Earthquakes can trigger landslides that cause great damage and loss of life. Large earthquakes beneath the ocean can create a series of huge, destructive waves called tsunamis （海啸）that flood coasts for many miles. Magnitude (震级) means the degree of a earthquake, and it contains10grades: Richter Scale(1—10) (里氏震级)
Earthquakes almost never kill people directly. （地震本身是几乎不会造成人员伤亡的）Instead, many deaths and injuries in earthquakes result from falling objects and the collapse of buildings, bridges, and other structures. Fire resulting from broken gas or power lines is another major danger during a quake. Spills of hazardous（危险的） chemicals are also a concern during an earthquake.
The force of an earthquake depends on how much rock breaks and how far it shifts.（地震产生多大的力量取决于岩层崩裂和移动的程度）Powerful earthquakes can shake firm ground violently for great distances.
During minor earthquakes, the vibration（震动） may be no greater than the vibration caused by a passing truck. （而一些小级别的地震产生的震动还不如一辆卡车通过时的响动来的大）
On average, a powerful earthquake occurs less than once every two years. At least 40 moderate earthquakes cause damage somewhere in the world each year.
About 40,000 to 50,000 small earthquakes--large enough to be felt but not damaging--occur annually.（每年，地球上都会发生4万到5万次有感但却不会造成破坏的地震）
