Man's physical universe

xanabras

156 THE EARTH AS MAN'S ABODE

1891, 1896, and 1923. The last earthquake killed more than 140,000

people and destroyed property worth billions of dollars.

California has had four great earthquakes during the same period.

San Francisco was all but destroyed by the fire following the great

earthquake in 1906, and Santa Barbara and Long Beach experienced

considerable damage in 1925 and 1933. The Imperial Valley experienced,

in 1940, the fifth most destructive earthquake recorded in the

United States.

Charleston, South Carolina, experienced a disastrous

earthquake in 1886, and Helena, Montana, experienced one in 1935.

Nearly a hundred thousand people were killed in Messina in Sicily

by an earthquake that occurred in 1908.

During this same fifty-year period similar great disastrous earthquakes

have occurred in India, China, the West Indies, and South

America.

Earthquakes, small or large, are occurring at some place on the

earth's surface all the time; and they have been taking place since

history began to be recorded.

There are probably about 150 large

earthquakes a year, although very

few of them make the front pages

of newspapers because most of

them occur under the sea. It has

been estimated from the records

that 13,000,000 people have lost

their lives in earthquakes during

„,^ Af L-rju the past 4000 years.

Fig. 43. A fence shifted by an ^ ^ . , . , .

,i ^ r

It is only withm the past few

earthquake. (Courtesy of the U. S.

Geol. Survey.)

decades that man has learned the

nature and cause of earthquakes.

The earthquake is now known to be a vibration of the earth's crust.

Such vibrations could be produced by friction or jarring, but this would

be insufficient to cause anything but minor tremors.

The majority of

earthquakes are caused by the sudden slipping along fractures of portions

of the earth's crust or by the fractures themselves. Rocks are

somewhat elastic under the stress of great pressures and bend slowly

for centuries until they reach the breaking point.

Sometimes great blocks of material, separated from others by cracks,

are elevated or lowered, producing faults. In 1899 a block of forest on

the Pacific coast of the United States was submerged into the sea,

whereas a neighboring block was elevated forty-seven feet. In the

great earthquake of 1811-1812 in the Mississippi Valley, a lake twentyfour

miles long was formed by the subsidence of a section of land in

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