Sharon Pacione, aged 13, of Peoria, I11.,for her question:
What causes an earthquake?
An earthquake happens when the old earth rearranges her coats. The heaviest part of the earth is in the center. This round core begins about 2,000 miles under our feet. It is a ball almost 4,000 miles in diameter. It is enfolded in a shell of lighter rocks almost 1,000 miles thick. Above that is a layer of still lighter rocks made of ancient lavas called basalt.
The outer shell of the earth is the ground on which we live. It may seem like solid, heavy rocks to us. Hut it is the lightest of earth's stoney blankets. It caries in depth from ten to sixty miles. It is a wonderful mixture of all kinds of stones, rocks, minerals and metals.
The old earth likes to keep her rocky coats smooth and even all over. But, like the blankets on your bed, they tend to wrinkle. When this happens, the weight of the top blankets becomes out of kilter. The earth has a number of tricks for straightening her blankets. One of them is to twitch with an earthquake.
Naturally, most earthquakes happen where the outer layers of rocks are uneven. This is where growing mountains border on deep hollows. It is necessary to redistribute the weight in these areas. Certain patches of the earth are called earthquake belts. There the twitching earthquakes go on all the time,
One earthquake belt is along the American coast of the Pacific. Another runs through Japan and Indonesia. Still another affects the Mediterranean area. This one is really at the foot of the high Himalayas. Peoria, Illinois is not in an earthquake belt,
All told, there are about 8,000 earthquakes recorded each year. Most of them are harmless tremors, too small to notice. But once in a while we hear of a devastating earthquake. The ground rocks and buildings tremble: Gas and water mains are torn and twisted. Fire
follows an earthquake in the cities. The surface of the ground may
change. Huge cracks may appear in the ground. Rocks and landslides may come tumbling down mountainsides. The whole area is in disaster.
The earthquake really starts deep below the surface of the ground. Masses of rock, maybe 60 miles down, are moving from place to place to adjust the balance. They lumber and blunder and tumble and crash. They shake the earth and the rocks above them. They set vibrations pulsing through and around the whole earth.
The actual moving job is usually done in less than a minute. The earth has just twitched one of her blankets to make the weight more comfortable. Experts check earthquakes on a seismograph. This instrument records the speed and intensity of the vibrations. It also brings the experts news of the rocky layers deep below ground level.