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San Francisco in California, USA, is famous for its earthquakes. It is located on the San Andreas Fault, which is actually a fault zone containing many different faults. The San Andreas Fault system is more than 800 miles long and extends to depths of at least 15 kilometers (9.3 miles) within the Earth. It is a band of crushed and broken rock ranging from a few hundred feet to a mile wide.

The San Andreas Fault marks the boundary between two masses of land that are moving past each other. On one side of the fault, the landmass that makes up most of North America is moving southeast. On the other side, the floor of the Pacific Ocean and its attached islands and coastal land, including some of western edge of California, is moving northwest. As they slide past each other, the rate of movement is about 5.6 cm (two inches) per year. That is about as fast as your fingernails grow. At that speed, in about 15 million years Los Angeles and San Francisco will be neighbors!

There are faults in rocks all over the world. Most stopped moving millions of years ago, but in some parts of the world they are still moving and causing earthquakes today.

San Andreas is an example of a strike-slip fault, where two landmasses are moving sideways relative to each other.

» On August 12, 2000, a series of explosions sank the Russian submarine Kursk. The resulting seismic waves were recorded by seismograph stations at the Baltic Sea near Norway, a part of the world that rarely experiences any seismic activity from earthquakes. Seismologists used the data to reconstruct the disaster.

The Baltic Sea seismic stations recorded an explosion at the same time and place that the Kursk sank. According to the data, the force of the explosion was about 250 kg (550 pounds) of high explosive. This would be comparable to the warhead of a modern torpedo. This data suggests that a torpedo misfired or exploded prematurely.

A second explosion occurred 135 seconds later. This time, the force of the explosion was equivalent to about five tons of TNT. This would be about the same as four to eight of the ship-to-ship missiles carried by the Kursk. It is also about the same as a cruise missile tipped with high explosive warheads.

It has been concluded that the first explosion caused a fire that spread to other weapons on board, causing the second, larger explosion.

» During World War I, a German scientist named Dr. Ludger Mintrop designed a portable seismograph for the purpose of locating heavy guns used by the Allies. He measured the time it took for the sound from a large gun to reach each of three seismographs spaced a long distance apart. He hoped that these measurements would enable him to figure out exactly where the guns were located. However, he found errors in some of his calculations. He discovered that the sound waves did not always travel at consistent speeds. The variations in speed were the result of refraction. As the sound waves traveled, they encountered different geological formations. The waves bent as they met boundaries of layers of rocks with varying densities.

The first sound to reach a recording instrument may not have traveled directly along the surface of the ground. It may have gone down to an underground rock layer of a different density, where it would be refracted. It might then move along the top of that layer, before coming back up to the surface and the recording instrument. The farther away the instrument is from the sound source (in this case, a gun), the more likely such a detour is to occur.

After the war, Mintrop experimented with the reverse process. He set off explosions at a known distance from the seismograph and measured the time taken for the sound to travel that distance. From this data he was able to estimate the depths of rock formations. In 1921 he started a company that he called “Seismos,” the Greek word for earthquake. Seismos used the technique to identify potential oil-bearing rock formations for oil companies around the world. Today, Seismos is part of Schlumberger.

» The most deadly recorded earthquake took place in central China in 1557. At that time, most people in the area lived in caves. Many of the caves collapsed, and about 830,000 people died. Another quake in China—this one in 1976—killed more than 250, 000 people.

» In 1811, a quake on the New Madrid, Missouri, fault in the central USA was so strong that the seismic waves moving out from it rang church bells nearly 1000 miles away in Boston. Aftershocks followed for more than a year, some of them as powerful as the original quake.

» The strongest recorded earthquake so far occurred in Chile in 1960. It had a magnitude of 9.3 on the Richter scale. Other exceptionally strong South American earthquakes include another Chilean quake in 1922 (8.7) and one in Ecuador in 1906 (8.6).

» Alaska has more earthquakes than any other state in the USA. A magnitude 7 earthquake hits somewhere in the state nearly every other year, and a magnitude 8 or greater quake occurs on an average of every 14 years. The strongest recorded earthquake so far in the USA was a magnitude 9.2 quake in Anchorage in 1964.

» Just as the Earth has earthquakes, the moon has moonquakes. Most take place about halfway between the center and surface of the moon.

» Every year there are about half a million earthquakes in the world that are strong enough to be detected. One-fifth of those are strong enough to be felt by people. About 100 are strong enough to cause damage.

» Japan has more than 1500 active faults and has experienced more than 400 major quakes during the last 1000 years