Emergency Manager's Guide to Earthquakes in Georgia

Prepared for: Georgia Emergency Management Agency

February 1999

Prepared by: Leland Timothy Long

School of Earth and Atmospheric Sciences

Georgia Institute of Technology

Atlanta, Georgia 30332-0340



When earthquakes are discussed, Georgia is not the first state to be mentioned. Earthquakes in Georgia are rare, particularly when they are compared to the long history of damaging earthquakes which are associated with California's active San Andrea fault zone and other fault zones bounding the tectonic plates of the Earth's crust. Movement of the Earth's crust along these plate boundaries explains most earthquakes. Georgia, like all the other states east of the Rocky Mountains, does not have active faults, and is not on a tectonic plate boundary. However, damaging earthquakes do occur in the interior of tectonic plates and these intraplate earthquakes can be an important consideration for emergency managers.

Earthquakes are less frequent in the eastern United States than in California. Consequently, fewer small earthquakes occur to remind us of the potential for damage from the less frequent great earthquakes. Never the less the historical record of earthquakes in Georgia (Figure 1) makes it clear that earthquakes and their associated seismic hazards exist. Damages from the great eastern United States earthquakes are largely forgotten because the last great earthquake was over 100 years ago. The 1886 Charleston, S.C., earthquake killed nearly 60 people and devastated the city. Although large earthquakes are less frequent, some seismologist argue that earthquakes cause damage over much larger areas in the eastern United States than earthquakes of similar size in the western United States. Also, the greater population density in the eastern United States increases the damage potential of eastern United States earthquakes over western United States earthquakes. Hence, in Georgia, as in most of the eastern United States, calculations of seismic hazard indicate that large distant earthquakes are likely to cause as much damage in Georgia as earthquakes of any size with epicenters within Georgia. For more details on the seismicity of the United States, see Slemmons et al. (1991).


Figure 1. Location of earthquakes within 15 miles of Georgia.

Earthquakes versus Quarry Explosions

Residents who live near a quarry may be very familiar with vibrations from blasts. Quarry blasts feel very much like small earthquakes. Occasionally earthquakes occur in the vicinity of quarries and require local seismograph records to discriminate them from quarry explosions. If the events occur at night and in a range of sizes, they are likely earthquakes. Quarry explosions are usually detonated during daylight hours, on a regular schedule (typically 12: noon and near 5:00 PM or closing time) and are usually the same size. If the vibrations from a quarry blast seem too large, the State Fire Marshal may be contacted for more information or to obtain help in determining if the vibrations exceed the legal limit.

Earthquakes may be Felt in any area of Georgia. No area in Georgia is immune from the earthquake threat, but northern Georgia has experienced the most earthquakes in recent history. Earthquakes large enough to cause damage could be felt in most if not all of Georgia's Counties. When a damaging earthquake occurs, it will affect an area covering many surrounding counties. Figure 2 gives Modified Mercalli (MM) intensity maps for two of Georgia's earthquakes, indicating that the footprint of an earthquake is large and that the vibrations do not respect County or State boundaries. Modified Mercalli intensity is described in Appendix II.

Figure 2.0 Intensity distribution for the Norris Lake Community magnitude 2.7 earthquake and Intensity distribution for the Covington-Mansfield earthquake.

Based on the historical seismicity (which was shown in Figure 1.0) three levels of seismic activity are apparent in Georgia. The least active area is the Coastal Plane of south Georgia, where one significant earthquake has been experienced in the last 30 years, the 1976 Reidsville earthquake. The northern half of Georgia has experienced moderate seismicity, with a magnitude 4 earthquake about every 10 years. When the details of the seismicity contained in the more frequent smaller earthquakes are included in a hazard assessment, two areas of northern Georgia stand out as being unusually active. These are the central Georgia seismic zone and the extension of the southeastern Tennessee seismic zone across northwest Georgia. The maximum damage from an earthquake will occur in the epicentral area and thus the counties located in these two zones have the greatest earthquake hazard in Georgia. The Counties with the greatest indicated hazard in Georgia are identified in Figure 3.0.

Figure 3.0 Location of Georgia Counties with greatest likelihood of experiencing an earthquake. The contours are the particle velocities in mm/s that have a 10 percent chance of being exceeded in 50 years. MM intensity VII is approximately 30 mm/s.




Measuring Earthquakes

Seismologists measure earthquakes in two ways; by magnitude, and by intensity. Magnitude is an instrumental determination of the relative size of an earthquake. Magnitudes are used for statistical comparisons of earthquakes and in studies of earthquake mechanisms. Intensity is an observational determination of the degree of shaking and damage at any one point. Intensity is used in studies of damage from earthquakes and to compare contemporary earthquakes with historical earthquakes which predate local seismograph records.

Magnitudes, when given to the news media, are traditionally referred to as "Richter Magnitude" out of respect for Dr. Charles Richter, who in 1935 developed one of the first magnitude scales. Because seismologists currently use many magnitude scales, this tradition also avoids confusion by the general public. Although each magnitude scale has a particular application, all are developed from the same principles introduced by Richter and all give magnitude values that are similar for most earthquakes of interest to the public. The different magnitude scales share certain common properties and differ only in that each may measure a different part of the seismogram written by the earthquake. First, magnitude scales are logarithmic because the magnitude equations define magnitude as being proportional to the log (base 10) of the amplitude of ground motion. Hence, a magnitude 4 earthquake is 10 times larger than a magnitude 3 earthquake and 100 times larger than a magnitude 2 earthquake. Second, they require a distance correction factor to correct for the normal decay with distance of earthquake waves. Third, the size of a "0" magnitude earthquake is arbitrary, although most are tied in some way to the original Richter magnitude. Fourth, magnitude is based on the average of measurements from many stations. Hence, when news media report magnitude, the first magnitudes reported come from individual stations and the official, or average, magnitude is not generally available until a few days after the event. Appendix I gives a descriptive comparison of the effects observed for events with different magnitudes.

Seismologists estimate intensity by comparing reports of shaking and damage to the Modified Mercalli Intensity Scale that is given in Appendix II. The intensity scale extends from I (felt lightly) to XII (total damage). Comparisons with instrumental data suggest that Intensities are roughly logarithmic estimates of the amplitude of shaking. Most estimates of intensity have an uncertainty of plus or minus one unit and, hence, the choice of Roman Numerals for the intensity scale discourages an unjustified refinement of intensity values. Intensity also depends strongly on the local soil conditions. Soft deep soil is associated with higher intensities than hard soil or near-surface rock. In an earthquake, one can expect greater damages in areas of soft and deep soil such as commonly occur in river valleys or in the areas of Charleston, SC that were made by filling in the swamp. By combining the size and location of an earthquake with knowledge of local soil conditions, an emergency manager can identify areas more likely to sustain damage.

Predicting Earthquakes

At this time earthquakes can not be predicted. Routine use of scientific data to predict earthquakes is unlikely with our current limited understanding of the earthquake process. Hence, for emergency management purposes, no warning can be expected now or in the near future for an earthquake. Seismologists have had to rely instead on statistical estimates of rates of occurrence of earthquakes. These statistical measures provide the best guesses on how often and where earthquakes will occur (but not when). Estimates of seismicity assume that past earthquakes will indicate where and how often future earthquakes will occur. Unfortunately, the length of our historical record is too short to give us confidence that every

seismic zone has been identified or that the potential of each seismic zone to generate a large earthquake is known. A disturbing fact is that most of the major historical earthquakes in the eastern United States occurred in areas that were not known for prior historical seismic activity. According to recent studies, significant seismic zones like that at Charleston, S.C. often show evidence of major earthquakes over periods of hundreds or thousands of years. Hence, the seismic zones that are active today have a greater probability of being the location of the next major earthquake; but the next earthquake could surprise us and occur outside of those active zones that are currently known.



Identifying Earthquakes

Small earthquakes, those with magnitudes less than 2.0, are difficult to identify if one has not previously experienced such an event. Vibrations from many sources can be felt; such as thunder, heavy trucks on nearby roads, sonic booms, objects falling and unbalanced washing machines. When the source is not obvious, an occasional resident may become alarmed by such sounds, and may call emergency services. The non-seismic vibrations are usually easy to distinguish from earthquakes and quarry explosions because earthquakes and explosions have unique characteristics. Earthquakes within a distance of 50 km usually start with a jolt, build in amplitude rapidly within a couple of seconds and then decay. The duration of felt motion in the typical near-by Georgia earthquake is only a few seconds. A small earthquake is often described as a muffled dynamite explosion. The sounds of an earthquake are transmitted to the air from the ground vibrations and the rattling of loose objects. The typical small earthquake will be felt by many in a neighborhood so that consultation with neighbors should eliminate most sources within a single residence. The sparse seismograph network in Georgia will not reliably register one of these small earthquakes unless the nearest station is closer than 25 km.

Larger earthquakes are usually immediately identified because they are both recorded by regional networks and felt by people who have previously experienced earthquakes. Most transplants from California identify these earthquakes immediately. The wide area of felt vibrations will also generate many calls to emergency services. The novelty of earthquakes in the eastern United States often generates an unusually high volume of such calls.




Earthquakes in Northwest Georgia

Earthquakes in northwestern Georgia are clustered along a northeast trending line that represents the southwest extension of the Southeastern Tennessee Seismic Zone. On the basis of seismicity, the Southeastern Tennessee Seismic Zone is second only to the New Madrid Seismic Zone in the eastern United States for its size and rate of earthquake production. In both seismic zones the earthquake hypocenters are at mid-crustal depths (14 ± 10 km) and outline a 150 mile long narrow active zone. These similarities and the existence of the great 1811-12 New Madrid earthquakes suggest that southeastern Tennessee or Northwest Georgia could also be the site of a similar great earthquake. This area currently experiences one magnitude 4.0 earthquake about every 10 years. A magnitude 4.0 earthquake is generally perceived as a startling vibration that may rock objects off shelves and may cause some cracking of plaster.


Earthquakes in Central and South Georgia

Earthquakes in the Piedmont and Blue Ridge Provinces of Georgia are scattered. However, they occur most often in places with unweathered rock near the surface. The locations of the Piedmont earthquakes do not define any active faults. Although the Piedmont of Georgia contains many spectacular faults, such as the Brevard Fault in figure 1, these faults indicate only active tectonic movements in the distant past when Georgia was on a plate boundary. These faults are not active today now that Georgia is in the interior of a tectonic plate. The shallow Piedmont earthquakes also occur frequently in areas of recently filled reservoirs or other sources for change in the pressure of water in the earth. The Piedmont earthquakes are unique in that they represent movements along shallow fractures that have been weakened, perhaps by penetrating fluids or by weathering. Because they are typically within 0.5 to 3.0 km (2 miles) of the surface they are easily felt and heard. They often occur in an earthquake swarm, so that many may be felt over a time period of 1 to 3 months. In the Piedmont, they are most common in areas of weakly fractured granitic rock. The Piedmont experiences about one magnitude 4.0 event every 10 years. A magnitude 4.0 earthquake in the Piedmont will be both felt and heard, along with many foreshocks and aftershocks. In the immediate epicentral zone, plaster and cement block walls will be cracked, merchandise will fall off store shelves, and minor structural damage will occur in buildings not designed to withstand earthquake forces. Earthquakes in the Coastal Plain of southern Georgia are too sparsely distributed to define a pattern.


Earthquakes outside Georgia's borders

The Charleston earthquake of 1886 and the New Madrid earthquakes of 1811-12 have caused as much damage in Georgia as all the earthquakes occurring within Georgia. In many models for hazard, these distant earthquakes provide the greatest threat. In most of Georgia the Charleston earthquake of August 31, 1886, knocked over chimneys, broke windows, and cracked plaster. The Charleston earthquake was a magnitude 7.0 event similar in size to the "World Series" California, earthquake of October 18, 1989.

The four New Madrid earthquakes of 1811-12 were the largest intraplate earthquakes in the world. The Mississippi River changed its course, the land surface sunk to form new lakes and the violent shaking snapped off trees. At the time settlements were sparse and limited to log cabins. A similar event today, perhaps in southeastern Tennessee, could generate extensive damage to the eastern United States and all of Georgia.

Major events like Charleston and New Madrid have occurred about once every 100 years in all of the eastern United States. The probability that such an event could cause at least some damage in Georgia within the next year is only about one in a thousand. The damage would be much like that experienced in Georgia during the 1886 Charleston earthquake, if the event occurred in a neighboring state. However, near the epicenter of the large event the damage would be like that experienced in Charleston or in the San Francisco Bay area on October 18, 1989. For such a major earthquake, the zone of extreme damage, MM intensity VIII and higher, could be in excess of 100 miles in radius.




Hazard Maps

Although some controversy exists in the use of current seismicity to estimate long-term trends, these statistical estimates are the only non-speculative measure of seismic hazard available today. Consequently, the historical seismicity was used as the principal basis for the new seismic hazard maps being prepared by the United States Geological Survey. In these maps, hazard is expressed in terms of the probability of experiencing a given level of vibration. In areas of more frequent seismicity higher levels of seismic hazard are expected, but the computations integrate the effects of distant as well as nearby earthquakes. The example in Figure 2 defines the level of vibration (in percentage of the acceleration of gravity) that has a 10 percent probability of occurring in 50 years based on historical seismicity. Also, in a statistical sense, this is the level of vibration one should expect to experience once every 450 years. These USGS seismic hazard maps will be used by the Building Seismic Safety Council in its revisions to the seismic hazard maps used in building codes. The seismic hazard indicated by these maps is greatest in northwest Georgia, decreases in the Piedmont and is minimal in the Coastal Plane. Except for the 1976 Reidsville, GA. event, earthquakes in the Coastal Plane are unknown except for a few questionable historical accounts. However, the seismic hazard is greater toward South Carolina, showing the influence of the continuing activity near Charleston, South Carolina.

Figure 4. Seismic hazard map for the eastern United States from the U.S. Geological Survey (from Frankel, 1995).

Emergency Response to Earthquakes

For convenience of discussion, earthquake response scenarios may be divided into those for small, moderate, large, and great earthquakes. In all cases, the first task is to determine the size and location, because these parameters will determine the extent and location of emergency services that will be needed. Unlike hurricanes and other weather-related disasters, there will be no opportunity for advanced preparation or mobilization.

Small earthquakes are of magnitude less than 2.5. These are typically felt only within a 15km radius from the epicenter in central Georgia and typically contained within one or two counties. In central Georgia, these could generate calls to emergency response agencies, but in northern Georgia small earthquakes generate few calls. The events in northern Georgia are deeper and do not shake the surface as hard as the much closer shallow Piedmont earthquakes. If the event is part of a typical Piedmont earthquake swarm, such as in the Norris Lake Community swarm of 1993, the continuing occurrence of minor seismicity may cause alarm. Actions, such as town meetings, may be needed to explain the events to the population. Also, the time following an earthquake or during a swarm provides a good opportunity to instruct the population in methods to minimize damage and injury during earthquakes, particularly because earthquake swarms are often followed by isolated events as large as the largest event in the swarm.

Moderate earthquakes are those with magnitudes about 4.0. These will be noticed by almost everyone in the epicentral area and will be felt as far away as100 miles. The switchboard may become swamped with calls, but the news media will usually be quick to distribute information on the identity and size of these earthquake. Some weak structures may experience minor damage, such as cracked plaster and items knocked off shelves. In rare incidences there may be some minor structural damage, such as brick facing falling off buildings. Life threatening situations would be rare for these moderate events and any associated emergencies should be easily handled as routine events.

Large earthquakes are those with magnitudes about 5.5. These will be widely noticed and will cause widespread minor damage in well-built structures. A few structures will suffer major damage and could require examination for safety, but these will be rare. Again, life- threatening situations would be restricted to the immediate epicentral zone and to weak structures that are located on poor foundation material. These events will be felt from 100 to 500 miles away. As with moderate earthquakes, the news media will distribute information about the felt area and damage areas. Some effort may be needed to control traffic in damage areas. Some disruption to traffic may be caused by damage. In rare cases, a bridge or road structure may be damaged. The possibility of damage after the earthquake from fires is possible. In the eastern United States, water heaters and furnaces are not routinely protected against falling over and these could start fires.

Damaging and great earthquakes are those with magnitude 6.0 and larger. The Charleston, 1886, and New Madrid, 1811-12, earthquakes are of this size. Expect extensive damage and loss of life in a radius of 10 to 30 miles of the epicenter. Outside the epicentral zone of major damage, the effects, to 150 miles, will be like those of the large earthquakes. Buildings will need to be examined for safety because these large earthquakes may have aftershocks that can cause more damage, particularly to weakened structures. Many people will be displaced from their homes and field or tent communities will need to be set up and maintained for 1 to 2 months. Transportation may be interrupted by broken rail lines and bridges. Also, clutter from buildings in the intensely damaged areas could inhibit rescue efforts. A systematic search of collapsed buildings will have to be undertaken to find survivors. The probability for the repeat of an event like Charleston, 1886, somewhere in the eastern United States is about 25% in the next 25 years. (one chance in 1000 per year in Georgia).


Seismic Monitoring

Seismic monitoring of significant earthquakes in the United States is coordinated by the United States Geological Survey (http://wwwneic.cr.usgs.gov). This includes most earthquakes larger than magnitude 3.5 and those that are felt widely. For small local earthquakes, it is generally necessary to rely on data from a nearby regional network. In Georgia, The Georgia Institute of Technology maintains a small network including station ATL just south of Atlanta. The University of Tennessee, University of Florida, University of North Carolina at Chapel Hill, and the University of South Carolina maintain seismic stations surrounding Georgia. Also, the Center for Earthquake Research and Information at Memphis State University maintains a Southern Appalachian Regional Network. These networks generally record events of magnitude greater than 1.5 and routinely distribute information on these events directly to the public or over the internet. There is also a growing number of independent stations using home-made instruments or less-expensive commercial instruments.


Earthquake Insurance

Most home insurance policies allow an earthquake rider. To be effective, they should protect the homeowner against the most likely damage expected from a small or distant earthquake, such as the failure of brick facing experienced by a homeowner in a small earthquake near Lake Sinclair. These riders vary in price depending on the deductible and company pricing practices. Clearly, a high deductible would protect only against the very rare large earthquake in Georgia.


Appendix I. Descriptive comparison of earthquake magnitudes with their observed effects.

The rate at which earthquakes have occurred in Georgia is shown in figure 5a. We experience a magnitude 3.0 every year or two and a magnitude 4.0 every 8 years. The best way to estimate the area of potential damage is to use the observed relation between magnitude and area of intensity VII. Modified Mercalli Intensity VII is the lowest level of shaking at which damage requiring some emergency response would be expected. The relation for the eastern United States is approximately, Log(AVII)= M - 2. The intensity VII area for a magnitude 4.0 is 100 square kilometer (a radius of 5.6 km or 3.5 mile) and a magnitude 6.0 is10,000 square kilometer (a radius of 56 km or 35 mile).

Figure 5.0 a) Number of events versus magnitude and b) relation between magnitude and Intensity VII area (Bollinger et al. 1993)





Magnitude 0.0 earthquakes that occur at shallow depths in the Piedmont are occasionally heard by people when they are within a few miles of the epicenter. Their sounds are like a distant cannon. These are usually ignored.

Magnitude 1.0 earthquakes that occur at shallow depths in the Piedmont are usually heard by people when they are within a few miles of the epicenter. These and smaller earthquakes are rarely reported by people in areas of northwest Georgia where the earthquake focus is deeper.

Magnitude 2.0 (e.g. Norris Lake Community, Georgia, summer 1993) earthquakes are typically described as a large quarry blast by residents in the Piedmont. Vibrations are felt near the epicenter. People in northwest Georgia occasionally report vibrations from events of this size.

Magnitude 3.0 (e.g. Heard County, Georgia, February 10, 1997, or the largest Norris Lake Community earthquakes) earthquakes are maximum intensity III in northwest Georgia and V in the Piedmont. Vibrations are like a heavy truck. Their sounds and vibrations are like an explosion. Sometimes two shakes are felt, with the first a higher frequency vibration and the second following within a few seconds a rocking vibration. In the Piedmont, they sound like a cannon. The vibration decays with time.

Magnitude 4.0 earthquakes (e.g. Clarks Hill Reservoir, Georgia, August 2, 1974) have maximum intensities in the VI to VII range. These events are just large enough to cause some minor damage in the epicenter area and groceries may off shelves. Felt over many counties, typically out to a distance of 100 miles.

Magnitude 5.0 earthquakes (e.g. Sharpsburg, Kentucky, July 27, 1980) are noted for widespread damage. The Sharpsburg earthquake was particularly noted for damage to chimneys. Intensity VI and higher within a radius of 30 miles. Felt over many states, a radius of over 300 miles.

Magnitude 6.0 earthquakes (e.g. Massena, New York, September 5, 1944) are characterized by intensity VIII and higher near the epicenter. The Massena earthquake was felt from Canada south to Maryland and from Maine west to Indiana. It caused property damage estimated at $2 million. Many chimneys required rebuilding, and several structures were unsafe for occupancy until repaired. Residents of St. Lawrence County reported that many water wells went dry. At Massena, 90 percent of the chimneys were destroyed or damaged and house foundations, plumbing, and masonry were damaged severely. Cracks formed in the ground, and brick-masonry and concrete structures were damaged.



Magnitude 7.0 earthquakes (Charleston, South Carolina, August 2, 1886) generate intensities of IX and above. Effects in the epicentral region include more than 1,300 square kilometers of extensive cratering and fissuring. Damage to railroad tracks, about 6 kilometers northwest of Charleston, included lateral and vertical displacements, formation of S-shaped curves and longitudinal movement. Strong alarming vibrations are felt. Many building will sustain damage, a few will fall or be rendered useless. Some lives will be lost in collapsed buildings or in fires following the earthquake. Communications and transportation will be interrupted significantly.


Appendix II. Modified Mercalli Intensity Scale of 1931 (Abridged)

I Not felt except by a very few under especially favorable circumstances.

II Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.

III Felt quite noticeably indoors, especially on upper floors of buildings, but most people do not recognize it as an earthquake. Standing motorcars rock slightly. Vibration like passing truck. Duration estimated.

IV During the day, felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, and doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing motorcars rocked noticeably.

V Felt by nearly everyone; many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbance of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.

VI Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.

VII Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures. Some chimneys broken. Noticed by persons driving motorcars.

VIII Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motorcars disturbed.

IX Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.

X Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.

XI Few, if any (masonry), structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.

XII Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.



Appendix III. Glossary of terms.

Acceleration: Rate of change in velocity with time. In earthquake ground shaking, acceleration is measured relative to the acceleration of gravity (g).

Active Fault or Active Seismic Zone: A fault that has exhibited movement in recent time and that is expected to move in the future. The movement may be indicated by earthquakes in a seismic zone or by displacements within the last 10,000 years of young soil or other deposits along a fault trace.

Aftershocks: Smaller earthquakes following a large event and occurring in the same fault zone. Generally, aftershocks decrease in magnitude and frequency-of-occurrence with time.

Aseismic Region. A region lacking earthquakes and also assumed to lack a potential for future earthquakes.

Capable Fault: A fault that is considered active for seismic hazard computations.

Creep: Slow slip along a fault without producing earthquakes.

Crust of the Earth: The top 30 km of the Earth that is brittle and the area of occurrence of most earthquakes. Mid-crustal depths represent the strongest part of the Earth's crust and are at depths of 10 to 20 km.

Duration: The duration of strong shaking is the time interval between the first and last peaks of strong (usually felt) ground motion.

Eastern United States: All states in the continental United States east of the Rocky Mountain Front, approximately 105E West.

Earthquake: The sudden release of stress along a fault and the resulting vibrations of the earth. The vibrations propagate away from the epicenter.

Earthquake Prediction: An earthquake prediction is a qualified determination of the magnitude, location, and time of a future earthquake.

Earthquake Swarm: An earthquake swarm is a prolonged series of small events. In a swarm, earthquake activity usually increases until the largest event occurs.

Epicenter: The location on the earth's surface directly above the focus (or hypocenter) for an earthquake.

Fault: (or Fault Zone) a zone of weakness or fractures in the earth along which the two sides have been displaced relative to each other parallel to the fracture. The total fault offset may range from centimeters to kilometers.

Focal Depth: The depth below the surface of the hypocenter, the point where an earthquake initiates movement.

Focal Plane: The area of movement on a fault during an earthquake. The Focus may be any place on the focal plane.

Focus: (or hypocenter) The place at which rock failure commences in an earthquake.

Foreshocks: Smaller earthquakes preceding a large event and occurring in the same fault zone.

Hazard Map: A map showing locations of areas where a defined level of vibration is expected to be felt in a given time period. For example, areas where an acceleration of 0.1 g or greater would be expected once every 450 years.

Hypocenter: see Focus.

Intraplate Earthquake: Earthquake that occurs in the interior of recognized tectonic plates, often not associated with major active fault zones. All eastern United States earthquakes are intraplate earthquakes.

Intensity: A measure of ground shaking obtained from the damage done to structures built by man, changes in the earth's surface and felt reports. The Modified Mercalli Intensity scale measures intensity in Roman Numeral units from I (felt slightly) to XII (total damage).

Isoseismal: Lines that surround zones in which an earthquake generated a given intensity.

Magnitude: Earthquake magnitude is an instrumental determination of the relative size of an earthquake. The Richter Magnitude was the first commonly used measure of earthquake size. All subsequent magnitude scales are tied to the Richter magnitude scale. Magnitudes released in news reports are often referred to as Richter Magnitude, although that term can only be applied strictly to southern California earthquakes.

Microseism. Weak, almost continuous seismic waves or earth noise; often caused by surf, ocean waves, wind, or industrial activity.

New Madrid Seismic Zone: An area of continuing seismic activity along the Mississippi River in Tennessee and Missouri. Also, the location of the epicenters of the four great New Madrid earthquakes of 1811-12.

P-wave: The primary or fastest wave traveling away from a seismic event through the earth. and consisting of a train of compressions and dilatations of the material.

Plate Tectonics: The Earth's crust consists of many rigid plates, such as the North American Plate. Plate Tectonics is the description of plate movement and interaction that explains earthquakes, volcanoes, and mountain building as consequences of horizontal surface motions of rigid portions of the Earth's crust.

San Andrea fault zone: A zone of movement between the North American Plate and the Pacific Plate, extending through southern California.

S wave: The secondary, or shear, seismic wave, traveling more slowly than the P wave, and consisting of elastic vibrations that are transverse to the direction of travel. It can not travel in a fluid.

Surface Waves: Seismic waves that are confined to the earth's surface. Surface wave velocities are less than S-wave velocities.

Seismicity: Generally, the occurrence of earthquakes in space and time. Usually given as the number of earthquakes of a given magnitude in a specified time, such as the number of zero magnitude events per year.

Seismogram: The record of an earthquake written by a seismograph.

Seismograph: An instrument for recording the motions of the Earth's surface.

Seismologist: Scientist trained in interpreting ground motion from earthquakes and in using the waves from explosions to determine the structure of the Earth. Seismologists are found in major universities and in the oil industry.

Seismology: The study of earthquakes, seismic sources, and wave propagation through the Earth.

Seismometer: The sensor part of the seismograph.

Tectonic Earthquakes: Earthquakes resulting from sudden release of energy stored by deformation of the Earth's tectonic plates.


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Bolt, Bruce A., (1993). Earthquakes, W.H. Freeman and Company, New York, New York, 331 p.

Building Seismic Safety Council, (1995). A nontechnical explanation of the 1994 NEHRP recommended provisions, Federal Emergency Management Agency publication 99, 82p.

Frankel, A., (1995) Mapping Seismic Hazard in the central and eastern United States. Seismological Research Letters, Vol 66, No. 4., pg 8-22.

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