In the case of plates, however, the uneven surfaces between the plates cause the plates to remain locked in place for years. Huge pressure builds up behind each plate. Periodically, a shudder, or tremor, is recorded as some of this energy is released。

　　Finally, after about 50 years, rock in the seismic gap either suddenly breaks or moves under the great stress. This sudden release of energy sends shock waves through the rock layers above. The ground shakes, sidewalks crack, and buildings tumble. A mighty quake has struck。

　　Gaps Used To Predict Quakes

　　Many geologists have used what is called the seismic gap technique to accurately predict earthquakes. The technique was first developed by Soviet earthquake expert Dr. V. Fodotov during his studies of ancient and recent Japanese earthquakes. Dr. Fodotov was marking the location, size, and date of all known quakes in Japan when he noticed a striking pattern。

　　All major earthquakes were found to occur in only a few isolated spots in Japan. Each of these spots, he noted, experienced a major quake only once every 50 to 60 years. Dr. Fodotov concluded that spots that hadn�餿 had a quake in more than 50 years were “ripe” for a quake. The Russian scientist named these locations seismic gaps。

　　In the past several years, geologists from other countries have found seismic gaps in other parts of the world. After making detailed studies of past quakes in these regions, the geologists were able to make an accurate prediction of when a quake would occur。

　　How Do Animals Know When an Earthquake Is Coming?

　　Scientists who try to predict earthquakes have gotten some new helpers recently—animals。

　　That’s right, animals. Scientists have begun to catch on to what farmers have known for thousands of years. Animals often seem to know in advance that an earthquake is coming, and they show their fear by acting in strange ways. Before a Chinese quake in 1975, snakes awoke from their winter sleep early only to freeze to death in the cold air. Cows broke their halters and tried to escape. Chickens refused to enter their coop. All of this unusual behavior, as well as physical changes in the earth, alerted Chinese scientists to the coming quake. They moved people away from the danger zone and saved thousands of lives。

　　One task for scientists today is to learn exactly which types of animal behavior predict quakes. It’s not an easy job. First of all not every animal reacts to the danger of an earthquake. Just before a California quake in 1977, for example, an Arabian stallion became very nervous and tried to break out of his stall. The horse next to him, however, remained perfectly calm. It’s also difficult at times to tell the difference between normal animal restlessness and “earthquake nerves”. A zoo keeper once called earthquake researchers to say that his cougar had been acting strangely. It turned out that the cat had an upset stomach。

　　A second task for scientists is to find out exactly what kind of warnings the animals receive. They know that animals�� sense far more of the world than humans do. Many animals can see, hear, and smell things that people do not even notice. Some can detect tiny changes in air pressure, gravity, or the magnetism of Earth. This extra sense probably helps animals predict quakes。

　　A good example of this occurred with a group of dogs. They were penned up in an area that was being shaken by a series of tiny earthquakes. (Several small quakes often come before or after a large one。) Before each quake a low booming sound was heard. Each boom caused the dogs to bark wildly. Then the dogs began to bark during a silent period. A scientist who was recording tile quakes looked at his machine. It was acting as though there were a loud noise too. The scientist realized that the dogs had reacted to a booming noise. They also sensed the tiny quake that followed it. The machine recorded both, though humans felt and heard nothing。