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
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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 a fault.
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 groundshaking, 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 earthquakes.
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 aelsmlc 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 wall 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 Uke Butler, Union County, Florida.
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
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of energy at an earthquake's focus generates seismic waves, which
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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.
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K U recorder trace of S normal water level changes
TIME -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.
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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 inghes in the Survey's well.
I main earthquake shock approx. aftershock
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level TIME 0
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.
normal water level V
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.
normal water level I
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.
earthquake shock 1.8"
normal water level
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.
over 10 feet, aftershocks'
- 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 of fscale, 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.
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meroalli 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, January 1991).
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 newspapers.
October 29, 1727: 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. February 6, 1780: Pensacola felt a tremor described as "mild."
May 8, 1781: Pensacola suffered a "severe" tremor that shook ammunition racks from barrack walls, levelled houses, but no fatalities.
February 8, 1843: Earthquake in West Indies, felt in United States, intensity unknown.
January 12, 1879: 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.
Jan. 22-23, 1880: Earthquake in Cuba of intensity MM VII, about 120 miles east of Havana. Felt in Florida.
January 27, 1880: 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.
August 31, 1886: 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.
Sept. 1-9, 1886: Jacksonville felt more aftershocks of intensity about MM IV from the Charleston quake.
November 5, 1886: Jacksonville felt another aftershock from the Charleston quake.
June 20, 1893: Jacksonville felt a tremor at 10:07 p.m.
Estimated intensity MM IV.
October 31, 1900: U. S. Coast & Geodetic Survey recorded a local shock of MM V at Jacksonville.
January 23, 1903: Shock of intensity MM VI felt at Savannah.
Effects felt in north Florida.
June 12, 1912: Strong shock felt at Savannah. Intensity unknown. Felt in Florida.
June 20, 1912: Shock of MM V felt at Savannah; probably associated with the above quake of June 12. Felt in north Florida.
1930: (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.
November 13, 1935: 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 V.
January 19, 1942: 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.
January 5, 1945: About 10 a.m. windows shook violently in the De Land courthouse, Volusia County.
December 22, 1945: Shock felt in the Miami Beach Hollywood area at 11:25 a.m. Intensity MM I to MM III.
November 8, 1948: A sudden jar, accompanied by sounds like distant explosions, rattled doors and windows on Captiva Island, west of Fort Myers.
November 18, 1952: Windows and doors were rattled by a slight tremor at Quincy, about 20 miles northwest of Tallahassee.
March 26, 1953: Two shocks estimated as MM IV were felt in the Orlando area.
October 27, 1973: Shock felt in central east coastal area of Seminole, Volusia, Orange, and Brevard Counties, at 1:21 a.m., maximum intensity MM V.
December 4, 1975: Shock felt in Daytona and Orlando areas, 6:57 a.m., maximum intensity MM IV.
January 13, 1978: Two shocks reported by residents in eastern
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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 damages.
November 13, 1978: Tremor felt in parts of northwest Florida near Lake City. Seismic station at Americus, Georgia, estimated it originated in the Atlantic Ocean.
1978 through January 1991: None reported by Gainesville seismographic station.
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.
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