Lester Finley Jr., age 12, of Lowell, Mass., for his question:
WHY DO WE HAVE EARTHQUAKES?
An earthquake is a sudden vibration of the ground that starts in one place called the epicenter and then spreads outward in all directions. We have earthquakes because many parts of the earth's crust are weak.
A minor cause of earthquakes is the movement beneath the surface of molten rock in volcanic parts of the world. Such movements usually start as tremors and seldom travel more than 100 miles.
In an earthquake, there is usually a shifting in the crust of the earth along a crack or fault.
Most earthquakes seem to originate at depths of 10 to 20 miles below the epicenter. Some are deeper but none come from a greater depth than 450 miles. Underground forces cause rocks to slowly but positively flow or creep.
Rocks behave much as a spring that is being bent. Sometimes the limit of its strength is reached and it breaks. Some of the energy of the deforming forces is released by one rock mass rubbing on another. This causes vibrations that can be felt for many hundreds of miles.
There are actually as many as a million earthquakes in the world each year. Some are small shakes that are hardly noticed while some are destructive shocks that cause a lot of damage.
Some parts of the world have earthquakes more than others. They are especially common in Japan, where there are an average of 200 earthquakes each day. Greenland, on the other hand, hardly ever has an earthquake.
The study of earthquakes is called seismology. It depends on sensitive instruments, or seismographs, that are positioned in different parts of the world. They report when there is any movement in the earth's crust.
A simple seismograph is made of a delicately hung weight on which a pen is mounted. The weight and pen tend to remain still as the earth vibrates under it. A record sheet, mounted near the pen, is free to move with the earth. The motion of the earth causes a wavy line to be drawn on the record sheet. The wavy line tells the scientist the time of the wave, the amount of the motion and even the direction from which the wave comes.
The most disastrous earthquake in North American history was the San Francisco earthquake of 1906. Actually, earthquake damage was rather small but gas pipes broke and the city burned for many days. About 700 lives were lost and the damage ran to something like $500 million.
The 1755 earthquake in Lisbon, Portugal, produced giant waves called tsunamis. They destroyed the city and killed about 60,000 people.
A giant earthquake hit Tokyo and Yokohama, Japan, in 1923, killing more than 70,000 people.
Perhaps the most deadly earthquake in history is the one that hit Huasien Country in China in 1556. More than 830,000 people were killed in that great disaster.
The worst earthquake in recent times is the one that hit northeastern China in 1976. It was centered near the city of Tangshan. Strong shocks drove millions from their homes and more than 600,000 people died in it.