Rather often, in one place or another, the land begins to shake. We say that there is an earthquake.
In an earthquake the earth may not shake very hard. The quake may do no damage. Perhaps it only rattles Windows, or dishes on shelves, and frightens people a little. But an earthquake sometimos does an enormous amount of damage. It may shake down buildings. It may destroy a whole city and kill thousands of people. An earthquake out at sea may send great waves rushing in over seacoasts near by.
The worst earthquake in the history of the United States was in 1906. It almost ruined the city of San Francisco. It killed nearly 500 people and destroyed about $500,000,000 worth of property. Much of this damage was done, not by the quaking itself, but by fires that broke out as a result of the earthquake.
Another famous earthquake took place in Kansu Province in China in 1920. This earthquake covered 300 square miles and killed 200,000 people.
Sometimes an earthquake occurs in connection with a volcanic eruption. Just be-fore a volcano erupted on one of the islands of Japan in 1914 there were more than 400 earthquakes on the island. Sometimes an earthquake is caused by the sliding of huge masses of rock along a great crack in the earth's crust. The San Francisco earthquake was caused in this way.
Places far away from mountains are rather safe from earthquakes. Earthquakes and mountain-building go together.
Scientists have invented an instrument that makes a record of earthquakes. It is called a seismograph. A seismograph can make a record of an earthquake thousands of miles away.
Engineers have learned how to build special kinds of houses in the places where earthquakes are likely to happen. These houses are able to shift slightly as the earth shakes, and so do not collapse. Reinforced concrete is a good material to use in building these houses.
