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Earthquakes and seismic history...
STATE OF FLORIDA
Department of Natural Resources
Tom Gardner, Executive Director
Division of Resource Management
Jeremy Craft, Director
Florida Geological Survey
Walter Schmidt, State Geologist and Chief
Open File Report No. 40
(Revision of Information Circular No. 93)
EARTHQUAKES AND SEISMIC HISTORY OF FLORIDA
Florida Geological Survey
SEISMIC HISTORY OF FLORIDA
Earthquakes in Florida? Not likely -- that's true -- but their
effects have been felt in the historical past. With respect to
earthquakes, Florida is in a region that is classified as stable,
that is, earthquakes are not probable. However, this is a restless
earth and no area is truly stable, in the strictest sense of the
word. While earthquakes may occur in stable areas, they are
generally much milder than the catastrophic ones, such as the Loma
Prieta, California, earthquake of October 17, 1989.
Scientists and news media report earthquakes with terminology that
is useful for comparative purposes. The more common terms are
given here in bold type, with their definitions. An earthquake is
the oscillatory, and sometimes violent movement of the earth's
surface that follows the release of energy somewhere within the
earth's crust. This energy can be generated by a volcanic
eruption, a sudden dislocation or movements of segments of the
crust along faults, by manmade explosions, and even by the great
weight of water impounded behind dams. Usually, though, most
destructive earthquakes are caused by movements of the crust along
Faults are zones in the earth's crust where there has been
movement, such as the famous San Andreas fault zone in California,
which was responsible for the Loma Prieta earthquake. A fault
occurs when internal forces cause rocks in the earth's crust to
rupture and move against one another. These sudden ruptures and
grinding movements release the energy that causes the ground-
shaking, which we call an earthquake. Geologists have found that
earthquakes tend to concentrate and recur along faults, and the
fact that a fault zone has recently experienced an earthquake
offers no assurance the internal crustal stresses have been
relieved enough to prevent another earthquake. Aftershocks can
occur for minutes, days, or even months after the main shock.
Faults in Florida have been found during the exploratory drilling
for oil in the panhandle. These faults are associated with the
deeper parts of the Apalachicola Embayment and with the Foshee
Fault system which extends northward into the state of Alabama.
Some other faults have been postulated along the eastern seaboard
and elsewhere in Florida, based on inferred evidence. However,
there is no evidence that any Florida faults have caused
The location of an earthquake is described by the geographic
position of its focal depth, as shown on Figure 1. The focal depth
of an earthquake is the depth below the earth's surface to the
region (focus) where the earthquake's energy originates. The
epicenter of an earthquake is the point on the earth's surface
directly above the focus.
The energy released by an earthquake at its focus travels as
seismic waves through the earth and along the surface. The first
indication of an earthquake will often be a sharply felt THUD,
which signals the arrival of the seismic waves that travel through
the earth. This will be followed by the ground-roll or shaking
caused by the seismic waves that travel along the earth's surface.
Waves, similar to ocean waves, have been observed to travel across
the ground in response to the surface seismic waves, literally
flipping people and animals off their feet.
Vibrations caused by earthquakes are detected, recorded, and
measured by instruments called seismographs. Seismographs have
their frames securely anchored into bedrock and are very sensitive
to earth movements. The zigzag line recorded by a seismograph,
called a seismogram, reflects the variations in movement of the
rock beneath the instrument. From data compiled from seismographs
recorded at several locations, the time of occurrence, the
epicenter, the focal depth, and estimates of the amount of energy
released can be determined for each earthquake.
Seismic waves of energy passing through rock strata cause alternate
expansion and compression of the rocks. One result of this is that
the seismic waves can cause the water level to fluctuate in a cased
well, and a water level recorder installed in a water well can,
under special conditions, act as a crude seismograph. As the water
level changes, a record is preserved of the earthquake, as on a
seismogram. The Florida Geological Survey has such a sensitive and
instrumented well (Figure 2). It has recorded several of the
world's major earthquakes. Figures 3 to 8 show the fluctuations of
the water level in the Survey's well for some of these earthquakes.
Figure 9 shows the effect the great Alaskan earthquake had on
another instrumented well located north of iLke Butler, Union
The severity of an earthquake can be expressed in several ways.
The magnitude of an earthquake, as expressed on the Richter Scale,
is a measure of the amplitude of the seismic waves and is related
to the amount of energy released, an amount that can be estimated
from seismograms. Magnitudes on the Richter scale are expressed as
whole numbers and decimals. For example, the Alaskan earthquake of
March 27, 1964, was Richter magnitude 8.5. However, this scale is
logarithmic, so that each increase in whole number represents a
force approximately 30 times larger than measured by the previous
whole number. A magnitude 7 indicates that about 30 times more
energy was released by an earthquake, compared to a magnitude 6; a
magnitude 6 earthquake releases about 900 times the energy as a
magnitude 4 earthquake (30 x 30). An earthquake of magnitude 2 is
the smallest normally felt by some humans, while earthquakes with
magnitudes of 6 or larger are considered to be major earthquakes.
The intensity of an earthquake, as expressed by the Modified
Figure .i Generalized cross section of earth showing the release
of energy at an earthquake's focus generates seismic waves, which
propagate through the earth. The point on the earth's surface
directly over the focus is called the epicenter.
. I I IL
..- J well casing
with clock drive
Figure 2. Diagram of the Florida Geological Survey's analog water
level recorder. The float follows fluctuations of the water level
in the well, sending electrical signals to the remote recorder.
The strip chart is driven by a constant speed clock motor, giving
a time-related graph of the changes in water level.
5.4" earthquake shock
recorder trace of
-- f normal water
Figure 3. Largest earthquake in the world during 1978, Richter
magnitude 7.8, struck November 29 about 300 miles southeast of
Mexico City, causing at least 8 deaths. This earthquake caused the
water level to fluctuate 5.4 inches in the Survey's well.
p earthquake shock
Figure 4. The Colombian earthquake of December 12, 1979, Richter
magnitude 7.9 killed at least 600 people. It caused the water
level to fluctuate 10.8 inches in the Survey's well.
Figure 5. Mexico City was struck by a magnitude 8.1 earthquake on
September 19, 1985, and another 7.3 quake on September 20, 1985.
The 8.1 tremor caused the Survey's water well to fluctuate 11.4
inches; the magnitude 7.3 aftershock caused 6 inches fluctuation.
main earthquake shock
S approx. aftershock
---- 36 hrs.
11.4" '-- 6
Figure 6. On October 18, 1989, the Loma Prieta quake struck about
50 miles southeast of San Francisco, killing at least 62 people.
This magnitude 7.1 tremor caused 3 inches of water level change in
the Survey's well.
Figure 7. Two large earthquake tremors struck Costa Rica on March
25, 1990. The first shock of magnitude 5.5 and an aftershock of
6.9 occurred within 7 minutes of each other, too short a time for
the Survey's slow-moving recorder to separate their traces. They
caused 3.3 inches of water level fluctuation.
Figure 8. A magnitude 7.7 quake hit the central part of the main
Philippine island of Luzon, north of Manila, on July 16, 1990. It
caused about 1.8 inches of water level change. This is the first
known earthquake recorded by this well that occurred such a great
distance from Florida, some 13,000 miles across the Pacific Ocean.
This demonstrates the awesome amounts of energy released by major
earthquakes. After travelling half-way around the world, this
quake's seismic waves still had enough energy to compress the
limestone rocks of the Floridan aquifer to cause a water level
fluctuation of nearly 2 inches.
earthquake shock -
normal water level
over 10 feet,
- normal water
Figure 9. The Good Friday earthquake that struck Alaska on March
27, 1964, registered 8.4 on the Richter scale, and was the largest
instrumentally recorded earthquake ever to strike the North
American continent. It caused this water level recorder to go off-
scale, in both directions, a water level fluctuation of over 10
feet. The major shock and the aftershocks caused the water level
to fluctuate for more than two hours. This record is from a water
well located north of Lake Butler, Union County, Florida.
SZONE 0. Arm with no
reamonab expectncy of
SZONE 1. Anra that may
h;: mln d wge from
largl t expected ditant
** **' *t* *W'
MTA WLth ORALOOGA
Figure 10. Seismic risk map of Florida. (Modified from "Seismic
Risk Map of the United States," in: Earthquake Information
Bulletin, Nov.-Dec. 1974, U. S. Geological Survey).
; :*l~o' l k~rMI
Mercalli scale, written as MM, is much more meaningful to laymen
than the Richter scale since it is based on actual human
observations of earthquake effects at specific locations. Modified
Mercalli scale values of shock intensities are given as Roman
numerals and range from MM I to MM XII: MM I = not felt except by
very few people favorably situated, to MM XII damage total, lines
of sight disturbed, objects are thrown in the air. The maximum
intensity experienced in the Alaskan earthquake of 1964 was MM X;
in the San Francisco earthquake of 1906 it was estimated at MM XI.
Most of the earthquakes felt in Florida had estimated local
intensities of MM IV to MM VI. Generally, a tremor of intensity MM
IV is: felt indoors by many, outdoors by few. May awaken light
sleepers. Vibrations similar to the passing of heavy trucks.
Dishes, windows rattle; glassware clinks; houses may creak.
Intensity MM VI is: felt by everyone, indoors and outdoors.
Awakens all sleepers. Frightens many people; creates general
excitement. Persons move unsteadily. Trees shake slightly.
Liquids move strongly. Damage to some buildings. Church bells
ring. Windows break; pictures and books fall; furniture overturns.
Other natural or man-made phenomena cause rumblings or shaking of
the ground which are sometimes mistaken for earthquakes, such as
shock waves from an explosion or sonic booms from airplanes. Some
earth tremors in north and central Florida seem to occur during the
passage of cold-weather fronts. Although the exact relationship
has not been well established, apparently the rapid movement of
large, cold, air masses can shake houses violently enough to rattle
dishes, windows and doors, similar to a weak earthquake tremor of
about intensity MM IV.
Figure 10 shows zones of estimated risk of damage in Florida. It
should be pointed out that, if it had not been for the effects of
the major 1886 earthquake at Charleston, South Carolina, all of
Florida probably would be in Zone 0.
Official records and newspaper accounts document the occurrences of
many earthquakes and "tremors" since 1727 whose epicenters were
estimated to have been near or in Florida. These earthquakes are
listed in Table 1 with estimates of their epicentral locations and
intensities. Several are grouped together, since they are
aftershocks which commonly occur hours, days, or even weeks after
major shocks. Of the earthquakes felt in Florida, only six are
considered to have had epicenters in Florida, and even some of
these were possibly the effects of tremors from earthquakes outside
Florida. A seismographic station was established in 1977 at the
Geology Department of the University of Florida in Gainesville.
Since then, this station has not detected any earthquake that
originated in Florida (Dr. Douglas Smith, personal communication,
Table 1. List of known earthquakes and "tremors" felt in Florida,
from 1727 through January 1991, with estimated epicenters and
intensities. Compiled from Campbell (1943) and accounts from local
October 29, 1727:
February 6, 1780:
May 8, 1781:
February 8, 1843:
January 12, 1879:
Jan. 22-23, 1880:
August 31, 1886:
Sept. 1-9, 1886:
November 5, 1886:
June 20, 1893:
October 31, 1900:
January 23, 1903:
Unofficial sources reported a severe quake, MM
VI, in St. Augustine, but the original record
has not been located. New England had a
severe shock about 1.0:40 a.m. on this date,
and a quake was reported on the island of
Martinique on the same day.
Pensacola felt a tremor described as "mild."
Pensacola suffered a "severe" tremor that
shook ammunition racks from barrack walls,
levelled houses, but no fatalities.
Earthquake in West Indies, felt in United
States, intensity unknown.
Earthquake felt through north and central
Florida bounded by a line drawn from Fort
Myers to Daytona on the south, to a line drawn
from Tallahassee to Savannah, Georgia, on the
north, an area of about 25,000 square miles.
Intensity MM VI near Gainesville.
Earthquake in Cuba of intensity MM VII, about
120 miles east of Havana. Felt in Florida.
Several shocks of intensity MM VII to MM VIII
were felt in Key West resulting from a
disastrous earthquake at Vuelta Abajo, about
80 miles west of Havana, Cuba.
The great earthquake in Charleston, South
Carolina, MM X. Felt all over north Florida,
with an estimated intensity of MM V MM VI.
Bells rang in St. Augustine, and severe shocks
were felt along the east coast. Quake effects
felt in Tampa.
Jacksonville felt more aftershocks of
intensity about MM IV from the Charleston
Jacksonville felt another aftershock from the
Jacksonville felt a tremor at 10:07 p.m.
Estimated intensity MM IV.
U. S. Coast & Geodetic Survey recorded a local
shock of MM V at Jacksonville.
Shock of intensity MM VI felt at Savannah.
June 12, 1912:
June 20, 1912:
November 13, 1935:
January 19, 1942:
January 5, 1945:
December 22, 1945:
November 8, 1948:
November 18, 1952:
March 26, 1953:
October 27, 1973:
December 4, 1975:
January 13, 1978:
Effects felt in north Florida.
Strong shock felt at Savannah.
unknown. Felt in Florida.
Shock of MM V felt at Savannah; probably
associated with the above quake of June 12.
Felt in north Florida.
(Exact date unknown) An earth tremor was felt
over a wide area in central Florida near
LaBelle, Fort Myers and Marco Island. Thought
to be from an earthquake, but some persons
believed it was tremendous explosions, though
no explosions were known to have been
detonated. Estimated intensity at Marco
Island was MM V.
Two short tremors were felt at Palatka in the
early morning. The second shock was felt at
St. Augustine and on nearby Anastasia Island.
Estimated intensity at Palatka was MM IV or MM
Several shocks felt on south coast of Florida,
with some shocks felt near Lake Okeechobee and
in the Fort Myers area. Estimated intensity
was about MM IV.
About 10 a.m. windows shook violently in the
De Land courthouse, Volusia County.
Shock felt in the Miami Beach Hollywood area
at 11:25 a.m. Intensity MM I to MM III.
A sudden jar, accompanied by sounds like
distant explosions, rattled doors and windows
on Captiva Island, west of Fort Myers.
Windows and doors were rattled by a slight
tremor at Quincy, about 20 miles northwest of
Two shocks estimated as MM IV were felt in the
Shock felt in central east coastal area of
Seminole, Volusia, Orange, and Brevard
Counties, at 1:21 a.m., maximum intensity MM
Shock felt in Daytona and Orlando areas, 6:57
a.m., maximum intensity MM IV.
Two shocks reported by residents in eastern
- 10 -
part of Polk County south of Haines City.
Tremors were about one minute apart and each
lasted about 15 seconds, shaking doors and
rattling windows. The tremors occurred
between 4:10 and 4:20 p.m. No injuries or
November 13, 1978: Tremor felt in parts of northwest Florida near
Lake City. Seismic station at Americus,
Georgia, estimated it originated in the
1978 through January 1991: None reported by Gainesville
Campbell, Robert B., 1943. Earthquakes in Florida, Proceedings of
the Florida Academy of Sciences, vol. 6, no. 1, March, pp. 1-4.
Meadows, Paul, 1981. Records of water level recorders, U. S.
Geological Survey Water Resources Division, Tallahassee, Florida.
Mott, C. J., 1983. Earthquake history of Florida: 1727 to 1981,
Florida Scientist, vol. 46, pp. 116-120.
Smith, D. L. and Randazzo, A. F., 1989. History of seismological
activity in Florida: evidence of a uniquely stable basement,
Proceedings of the Second Symposium on Current Issues Related to
Nuclear Power Plant Structures, Equipment, and Piping With Emphasis
on Resolution of Seismic Issues in Low-Seismicity Regions. Electric
Power Research Institute document EPRI NP-6437-D, pp. 2-37 to 2-58.
Stover, C. W., B. G. Reagor, and S. T. Algermissen, 1979.
Seismicity Map of Florida, U. S. Geological Survey Map MF-1056.
U. S. Geological Survey, Earthquake Information Bulletin, Nov.-Dec.
1974. Seismic Risk Map of U. S.
- 11 -
FLRD GEOLOSk ( IC SUfRiW
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