填空题

Earthquake
Every year earthquakes are responsible for a large number of deaths and a vast amount of destruction in various parts of the world. Most of these damaging earthquakes occur either in a narrow belt which surrounds the Pacific Ocean or in a line which extends from Burma to the Alps in Europe. Some of the destruction is directly caused by the quake itself. An example of this is the collapse of buildings as a result of the quake itself. Other damage results from landslides or major fires which are initiated by the quake.
These are about a million quakes a year. Fortunately, however, not all of them are destructive. The intensity of an earthquake is measured on the Richter Scale, which goes from upward. The highest scale recorded to date is 8.9. Major damage generally occurs from quakes ranging upward from 6.0.
The actual cause of the quake itself is the breaking of rocks at or below the earth’s surface. This is produced by pressure which scientists believe may be due to a number of reasons, two of which are the expansion and contraction of the earth’s crust and continental drift.
In order to limit the damage and to prevent some of the suffering resulting from earthquakes, scientists are working on ways to enable accurate prediction. Special instruments are used to help people record, for example, shaking of the earth. Scientists are trying to find methods that will enable them to indicate the exact time, location and size of an earthquake.
Certain phenomena have been observed which are believed to be the signs of imminent earthquakes. These include strange behaviors of some animals, the changes in the content of mineral water, etc. The magnetic properties of rocks may also display special pattern before earthquakes happen.
Not all earthquakes are strong enough to cause______