chart of earthquake

When we think of an earthquake, we think of buildings toppling, huge cracks opening up in the earth, and so on. What is there to be “measured?” Well, an earthquake is a trembling or vibration of the earth’s surface. And it iS these vibrations that are measured. The cause of an earthquake is usually a “fault” in the rocks of the earth’s crust, a break along which one rock mass has rubbed on another with very great force and friction. Much of the vast energy in this rubbing is changed to vibration in the rocks. This vibration may travel thousands of miles, which is why an earthquake in Tokyo can be detected and measured in England.

Earthquake vibrations consist of three or more types of wave motion, which travel at different speeds through the earth’s rocky crust. The waves move in different directions. The primary waves vibrate length wise; the secondary waves crosswise; and the long waves travel around the earth’s surface. The long waves move more slowly, but they have a larger motion and cause all the damage that may be seen by man.

Instruments called seismographs placed in different parts of the world record vibrations every day in the year, for the earth’s crust is never still. The record sheets of two or more seismographs help seismologists to see where the quake took place.

A seismograph is a delicately hung weight, which remains still when an earthquake shakes the surrounding parts of the instrument. In other words, this weight, which hangs down from a fixed post, doesn’t move during an earthquake. But the post holding it moves, and attached to the post, underneath the weight, is a chart. As the chart moves, a record is made on it by the weight.

Record sheets indicate the time the wave arrives, the force of the motion, and can even indicate the direction from which the wave comes.