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
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:
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://ufdc.ufl.edu/UF00001039/00001 . 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:
. 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.
Which tectonic plate is Florida situated on and what is its position on the plate?
Florida sits on the trailing margin of the North American tectonic plate. The trailing margin is also referred to as a passive margin. For this reason Florida is in a very stable position in terms of its location with respect to tectonic plates.
the last earthquake in Florida?
No known earthquake has ever had its epicenter
beneath Florida although several have been felt in the state. The
http://www.uflib.ufl.edu/ufdc/?b=UF00001154&v=00001 will take
you to Florida Geological Survey Information Circular No. 93,
Earthquakes and Seismic History of Florida. The most recent
earthquake to be felt in the state occurred on September 10, 2006.
It was a magnitude 6 with its epicenter in the Gulf of Mexico and
was not linked to any specific fault. You can read more about it at
the following link:
http://www.dep.state.fl.us/geology/news/newsletter.htm . Click
on the October, 2006 Newsletter.
What is the risk of a Tsunami in Florida?
The risk of a tsunami striking Florida
is considered to be low. The website for the National Oceanographic and Atmospheric
Administration lists the following states as being especially vulnerable to tsunamis,
in addition to the U.S. Caribbean Islands: Hawaii, Alaska, Washington, Oregon, and
California. There is currently no tsunami warning system for the east coast of
the United States.
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.