How Are Earthquakes Measured?
The crust of the earth is never absolutely quiet. Instruments called seismographs are used for detecting the vibrations on earth. These instruments are placed in different parts of the world to keep a record of vibrations every day, every year. The record sheets of two or more seismographs help seismologists to find out where and of what intensity an earthquake has taken place.
Earthquake vibrations can be very strong, and as we all know, can be very destructive. These vibrations may travel thousands of miles and so with the help of seismographs an earthquake in China can be measured in a city in America.The earthquake vibrations consist of three or more kinds of wave motions which travel at different speeds through the rocky crust of the earth. They also move in different directions. The primary waves vibrate lengthwise, the secondary waves crosswise, and the long waves around the earth’s surface. The damage we see on the earth’s surface is caused mostly by these long waves.
A seismograph has a delicately suspended weight which remains still when an earthquake shakes the other parts of the instrument. But the post which holds it moves with the disturbance and a chart attached to the post, underneath the weight also moves with it. As the chart moves, the movements are recorded on it by the weight. These record sheets can tell the time, intensity and even the direction of the waves of an earthquake.