The earth’s crust is divided into a series of rigid plates that move in relation to one another. An earthquake is a shaking or trembling of the earth that often originates from movement of those rigid plates past one another, along surfaces called faults. Faults are fractures or breaks in the earth’s crust along which movement takes place. The shaking or trembling of the ground is caused by seismic waves that radiate from the place in the earth’s crust where sudden movement has occurred. Earthquakes are also associated with volcanic activity.

Florida is situated on the trailing (or passive) margin of the North American Plate while California is located on its active margin. The active margin is bounded by faults that generate earthquakes when there is movement along them. This is the fundamental reason that Florida has an extremely low incidence of earthquakes while California experiences many (mostly small) earthquakes.

Because earthquakes are usually associated with faults the FGS frequently receives inquiries concerning the location of faults in Florida. A number of faults have been proposed by various authors over the years based on various criteria. Because of the difficulties in defining faults in the state there is little agreement concerning the validity of those which have been proposed. Several problems interfere with the definition of faults. The layered rocks of Florida are mainly thick carbonate units that do not contain well-defined marker beds. The natural process that causes limestone to dissolve may obscure what was originally a faulted surface. Faults have been proposed based on unconventional criteria such as water quality changes. The low topographic relief in Florida is such that the features must be defined using subsurface data from wells. None of the proposed features in Florida are known to have any seismicity associated with them.

In a highly unusual event, some Floridians felt a strong (Magnitude 5.8) earthquake that occurred on September 10 at 10:56 a.m. Eastern Daylight Time in the Gulf of Mexico, about 250 miles (405 km) south-southwest of Apalachicola, Florida. A Tectonic Summary by the United States Geological Survey (USGS) noted that the earthquake was centered under the Gulf of Mexico far from the nearest active plate boundary. The earthquake is thought to result from forces applied at the plate boundary. The USGS has not linked the earthquake to any specific fault. Magnitude 5.8 earthquakes are typically unlikely to generate a destructive tsunami. For additional information visit: http://earthquake.usgs.gov/eqcenter/eqinthenews/2006/usslav/

Florida has, in historic times, “felt” the effects of about two dozen “seismic events”. Many of those events are described in Florida Geological Survey Open File Report Number 40, Earthquake and Seismic History of Florida, available online at: http://fulltext10.fcla.edu/cgi/t/text/text-idx?c=feol&idno=UF00001039&format=pdf . Researchers from the University of Florida undertook a critical review of all seismic data for Florida and concluded that of all reported events from 1879 to 1975 only five might qualify as possible earthquakes (Smith D.L. and Lord, K.M., 1997).

The subject of earthquakes in Florida is discussed in some detail in Florida Geological Survey Special Publication Number 35 beginning on page 50. That publication is available online at: http://fulltext10.fcla.edu/cgi/t/text/text-idx?c=feol&idno=UF00000124&format=pdf . The United States Geological Survey web site ( http://earthquake.usgs.gov/ ) provides interesting and useful information concerning earthquakes, their locations and the frequency with which they occur.

Suggested Reading:

• Smith, D.L. and Lord, K.M., 1997, Tectonic Evolution and Geophysics of the Florida Basement, in Randazzo, A.F. and Jones, D.S., eds.,
• The Geology of Florida, University of Florida Press, p. 13-36.