EarthQuakes

EARTHQUAKE (-KWAK)n. a shaking or trembling of the crust of the earth, caused by underground volcanic forces or by breaking and shifting of rock beneath the surface.

Earthquakes are caused by the abrupt release of stored energy within the earth's crust. These sudden fractions occur when stresses build up to the breaking point of the breaking point of the rocks miles beneath the surface in fault zones.

In an earthquake, the actual movement of the ground is seldom the direct cause of death or injury. Most casualties result from falling objects and debris from buildings or other structures, which are damaged or demolished by the earthquake.

Earthquakes also can trigger landslides, cause fires and flooding and generate massive ocean waves called tsunamis.

WHY SHOULD I BE CONCERNED?

In an average year, dozens of earthquakes will occur in the United States. Yet most earthquakes, detectable only by sensitive instruments, will go unnoticed by the general public. Not to be overlooked is the fact that larger earthquakes can and do occur.

Outside California, the greatest potential for an earthquake in the United States exists in the seismic zone of the New Madrid Fault system, which extends some 120 miles through far southeast Missouri and into northeast Arkansas.

In a five-month period during the winter of 1811-1812, more than 2,000 earthquakes rocked the central United States near the New Madrid area. Although instruments for measuring them were not available, at least four earthquakes thought to be among the most intense ever to occur on this continent happened during this period. Using damage reports made at the time, experts have estimated the quakes had a magnitude of 8.0 to 8.8 on the Richter scale. The earthquakes were felt throughout two-thirds of the continental United States, from Canada to Mexico and from the Rocky Mountains to the East Coast, where they rang church bells in Boston.

Damaging quakes are not as frequent along the New Madrid seismic zone as in California, but when they do occur, the destruction can be much more widespread affecting up to 30 states. This is due to different geological characteristics. Unlike the relatively uniform crust of the central United States, the complex and fractured rock of the West Coast tends to dampen seismic waves as they travel, thereby reducing the area of damage.

Parts of Kansas and Oklahoma are in an earthquake hazard zone where moderate damage could be expected. In the last 100 years, about 25 minor earthquakes have been recorded in Kansas. Since 1900 in Oklahoma, scientists have recorded hundreds of small tremors and minor quakes, although only a handful of them were ever felt by humans.

Southeast Missouri University Center for Earthquake Studies.