If you could look at a map that showed the entire surface of the earth and where earthquakes occur most often, you’d see a wavy ribbon that twisted up and down as it twisted around the earth.
Some parts are missed entirely, others seem to have a habit of having frequent earthquakes. The single region of the earth that has the most frequent earthquakes is Japan.
There is an earthquake there almost every day of the year! Of course, most of these are very minor quakes and do no damage at all. Another region which has frequent earthquakes is the Mediterranean area.
By contrast, consider the New England States. There have been no destructive earthquakes there since the Ice Age, many thousands of years ago! The explanation for this is that the crust of the earth is not the same everywhere.
In certain regions, the crust has not quite settled down firmly, and there is a fault. A fault is a break in the rocks of the earth’s crust. Where the break exists, one rock mass rubs against another with great force and friction.
The energy of this rubbing is changed to vibration in the rocks. While this vibration may travel thousands of miles, the earthquake is strongest, as you might imagine, right along the line of the fault made by the shifting of the earth blocks.
The sides of the fault may move up and down against each other, or the sides of the fault may shift lengthwise. Most of the changes that take place on the surface of the earth after an earthquake are seen along this fault line.
The part of the fault line where the vibration is felt most strongly is called the epicenter of the earthquake. And if this is near a city, the destruction may be very great. The loss of life is usually due to falling buildings and fires that may be started by broken gas mains under the streets.
The earthquake regions of the earth and the areas of recent volcanic activity are roughly the same. This is because both of these are regions where the earth’s crust is not at rest.