Measuring the Earth's Changes
As far as historians can tell, the world's deadliest recorded earthquake occurred in central China in 1556. It struck a region where most people lived in caves carved from soft rock. When these dwellings collapsed, an estimated 830,000 people were killed.
The largest recorded earthquake in the United States struck Prince William Sound in Alaska on Good Friday, March 28, 1964. It had a magnitude of 9.2 on the Richter scale, the most common standard of the amount of energy released by a quake. It was developed by Charles F. Richter of the California Institute of Technology. He first recorded the occurrence with a seismograph — an instrument measuring the seismic waves, i.e., the amount of energy released by the shaking of the ground. The first "pendulum seismoscope" to measure the shaking of the ground during an earthquake was developed in 1751, and it was not until 1855 that faults were recognized as the source of earthquakes.
The largest recorded earthquake in the world had a magnitude of 9.5 when it occurred in Chile on May 22, 1960. At that time, seismographs recorded seismic waves traveling all over the earth's surface for several days. One of the potentially most earthquake-prone areas is the San Andreas Fault in California. It is actually a fault zone made up of several segments that move separately at any time. The San Andreas system of faults is over 800 miles long and in some areas as much as 10 miles deep.
The East African Rift is a 37-mile-wide zone of volcanic activity extending more than 1,800 miles from Ethiopia in the north to Zambia in the south. The average rate of movement across the San Andreas Fault was about 2 inches per year during the past 3 million years. If this rate continues, geologists project that San Francisco and Los Angeles will be next to each other in about 15 million years.
The 240-mile-long Wasatch range, which runs north-south through Utah, has experienced about one earthquake above the 6.5 magnitude every 350 years. In 1812, some very powerful volcanic activity occurred along the New Madrid fault in the Mississippi Valley. Most quakes, however, occur at depths of less than 50 miles below the earth's surface.
An underwater ocean wave caused by an earthquake tends to cause a tsunami while tidal waves occurring in shallow waters are triggered by the gravitational interaction between the Sun, moon and Earth. Estimates assume that every year there are some 500,000 earthquakes detectable all over the world. While about 100 of them tend to cause damage, only 100,000 can be felt. Actually, the earthquake that hit the San Francisco area in 1906 may have caused more damage by the subsequent fire than by its impact on the built-up area.
The cause of earthquakes was first stated in 1760 by the British engineer John Mitchell, one of the first fathers of seismology, in a memoir in which he wrote that earthquakes and the waves of energy that they make are caused by "shifting masses of rock miles below the surface." Many earthquakes and volcanic eruptions occur along plate boundaries, as between the Pacific plate and the North American plate. One of the most active seismic boundaries where earthquakes and eruptions are frequent is situated around the massive Pacific plate, commonly referred to as the Pacific Ring of Fire.
The interior of Antarctica has "icequakes," which, although they are much smaller, are perhaps more frequent than earthquakes. Icequakes are similar to earthquakes, but occur within the ice sheet itself instead of the land underneath the ice. Some polar observers have explained that they can observe the icequakes at the South Pole's seismograph.
Wolf D. Fuhrig, a professor emeritus of political science and criminal justice, has been a columnist since 1981.