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Bill Motz, ago 12, of Phoenix, Arizona, or his question:

What causes earthquakes?

The study of earthquakes is seismology. And seismology has taught us a lot about the earth’s crust where earthquakes occur. There is still more to be learned. For, although we know a lot about what happens during an earthquake, we do not yet know what deep underground forces cause thorn to happen.

Most earthquakes do little or no damage at all. But a big quake is one of Nature's most dramatic events. In a populated area the buildings heave and sway as if tossed by an angry sea. Even in a deserted area, the news is picked up by listening posts around the globe. For an earthquake sends out shook waves both through and around the globe. These vibrations are recorded an seismographs.

A big earthquake often changes the surface of the earth. It happens along a fault, which is a weak spot or kind of crack in the earthts crust. Sometimes a road crosses the fault. After a bird quake, such a road may break in two and the broken ends slide apart. This is caused by a horizontal movement in the earth's crust. Sometimes one side of the fault moves up, forming a cliff. This is a vertical movement in the earths crust.

The crust of the earth varies from 10 to 40 miles in depth. It is made of layers of the worlds lightest rocks. Below it era layers of denser, heavier materials. The crust seems to float an these denser materials, forming the continents and. the sea buds. It covers the entire globe, but is fitted together in sections like the plates of closely knit armor. Earthquake faults are at the weak points, joints in the crusty armor.

The crustal blocks, the sections of armor plating, are not permanently fixed. Maybe one is moving north and its neighbor moving south, ono may be rising and its neighbor sinking. These motions are very, very slow and we do not know what causes them.

The famous San Andreas fault runs under the San Francisco Bay area and for hundreds of miles south through California. After the big quake of 1906, the land on the western side of the fault was found to have moved more than ten feet to the northwest. The land on the eastern side of the fault had moved more than ton feet to the southeast. Fences were broken at the fault and the broken ends separated by 21 foot. The San Andreas is a joint between two massive crustal blocks slowly slowly moving in opposite directions.

Friction at the fault prevents the two crustal blocks from sliding. Tension builds up through the earth. The moving masses get stuck and bend at the fault. Finally the tension breaks like a steel trap and things are adjusted in a few moments.

In 1915, a big quake shook Nevada crustal blocks along a fault sprang apart vertically. A cliff 16 feet high and hundreds of miles long appeared in the Sonoma mountains. In 1891, a quake in Japan moved the crustal blocks both horizontally and vertically. A road crossing the fault was broken. The two ends slid apart, one ending in a cliff the other at the foot of the cliff. All these dramatic events are part of the moving crustal blocks. But we do not know what causes the massive sections of the earth's crust to move.

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