Homeowner's Guide to Earthquake Hazards in Georgia
By, Leland Timothy Long
School of Earth and Atmospheric Sciences
Georgia Institute of Technology
Atlanta, Georgia 30332-0340
When earthquake hazards are discussed, Georgia is not the first state to be mentioned. Earthquakes in Georgia are rare compared to the long history of damaging earthquakes which are associated with California's active San Andreas Fault zone. Movements along active faults like the San Andreas explain 85% of the earthquakes in the world. The rest are scattered over areas like Georgia that lack clearly defined active faults. These scattered earthquakes in Georgia the eastern United States have caused significant damage and can be an important consideration for homeowners.
Because earthquakes are less frequent in the eastern United States than in California, we are not constantly reminded of our seismic hazard by frequent small earthquakes. Never-the-less the historical record of earthquakes in the southeastern United States and Georgia (figure 1) makes it clear that earthquakes and their associated seismic hazards exist. Damages from 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. Also, some seismologist argue that while earthquakes in the eastern United States are less frequent, the large earthquakes cause damage over much larger areas and would affect more people than earthquakes of similar size in the western United States. In Georgia, calculations of seismic hazard indicate that large earthquakes outside our borders are as likely to cause damage in Georgia as earthquakes of any size occurring within Georgia.
The map of Georgia shows the location of all earthquakes that are known to have occurred within 25 km (15 mi.) of Georgia. The earthquakes across northwestern Georgia are part of the Southeastern Tennessee Seismic Zone (STSZ) that extends northeast through Knoxville. The STSZ lies primarily in the Valley and Ridge Province of the Southern Appalachians. Earthquakes in the STSZ are at depths of 14 ± 10 km and do not appear to be correlated with surface geology or near-surface faults. On the basis of seismicity, the STSZ is second only to the New Madrid Seismic Zone in the eastern United States for its size and rate of earthquake production. Earthquakes in the Blue Ridge and Piedmont Provinces occur in clusters with notable concentrations near reservoirs such as Lake Sinclair and Clarks Hill. They may occur any place that has unweathered and slightly fractured granitic rock near the surface. The small Piedmont earthquakes are unique in that they represent movements along shallow fractures that have been weakened, perhaps by penetrating fluids or weathering. Few earthquakes are known to occur in the Coastal Plane Province of South Georgia.
Small earthquakes are often no more alarming than the many vibrations that originate near or in a home, such as thunder, heavy trucks nearby, sonic booms, objects falling, and unbalanced washing machines. These all have unique characteristics that we learn to recognize through experience. Likewise, we can identify the small magnitude 2.0 or less earthquakes and explosions of a similar size by their unique characteristics. They usually start with a jolt, build rapidly in amplitude within a couple of seconds and then decay. The total felt duration of the typical small Georgia earthquake is usually less than 10 seconds and it sounds like a muffled dynamite explosion. Also, the rattling of loose objects may generate earthquake sounds. The typical small earthquake will be felt by many within 15km (10mi.) so that consultation with neighbors should eliminate most non earthquake sources within the home. The events in northern Georgia are 8 to 15 km deep and do not shake the surface as hard as the Piedmont earthquakes that are typically within 2.0 km of the surface. In the Piedmont, the earthquakes may occur as part of a 2 to 4 month long swarm, such as in the Norris Lake Community swarm of 1993. The time during a swarm is a good opportunity to eliminate hazards that could cause damage and injury during an earthquake, particularly because earthquake swarms are often followed by isolated events as large as the largest event in the swarm.
If homeowners live near a quarry, they may be very familiar with vibrations from blasts that feel very much like small earthquakes. If the homeowner feels vibrations that seem unusually large or occur at night, the homeowner could be experiencing earthquakes. If unusually large vibrations are from quarry blasts, usually occurring during the lunch hour or in the late afternoon during the week, the homeowner may contact the State Fire Marshal obtain help in determining if the vibrations exceed the legal limit.
Moderate and Larger earthquakes are usually immediately identified because they are both recorded by regional networks and felt by people who have previously experienced an earthquake. Most transplants from California identify these earthquakes immediately.
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 news media will usually be quick to distribute information on the identity and size of these earthquakes. 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 cracks in cement blocks or brick facing falling off buildings. The Piedmont and northwestern Georgia each experience about one magnitude 4.0 event every 10 years.
Large earthquakes are those with magnitudes about 5.5. These 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. 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. In the eastern United States water heaters and furnaces are not routinely protected against being knocked 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 and have caused as much damage in Georgia as the earthquakes occurring within Georgia. The probability for a magnitude 6.0 or larger earthquake somewhere in the eastern United States is about 61% in the next 25 years. We have experienced one magnitude 7.0 once every hundred years in all the eastern United States. There is but only one chance in 1000 per year for a magnitude 7.0 in Georgia. These distant earthquakes provide the greatest threat. The damage in Georgia would be similar the damage caused by the 1886 Charleston earthquake, if the event occurred in a neighboring state. Near the epicenter the damage would be like that experienced in Charleston In 1886 or in the "World Series" California earthquake on October 18, 1989. The Charleston earthquake nearly devastated the city, and killed about 60 people. One should expect extensive damage in a radius of 10 to 30 miles of the epicenter. Outside this zone of major damage to a distance of 150 miles the damage will be moderate. Buildings may be damaged sufficiently to collapse in the larger aftershocks. Many people will be displaced from their homes. Transportation may be interrupted by broken rail lines and bridges. Chimneys will be knocked over, windows broken and plaster cracked. The four New Madrid earthquakes of 1811 and 1812 were the largest of their type 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 sturdy log cabins. A similar event today, perhaps in southeastern Tennessee, could generate extensive damage to all of Georgia.
Seismic monitoring in the United States is coordinated by the United States Geological Survey (http://gldage.cr.usgs.gov). The USGS maintains station GOGA. In Georgia, The Georgia Institute of Technology maintains a small network including station ATL just south of Atlanta. The University of Tennessee (http://tanasi.gg.utk.edu), 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(http://www.ceri.memphis.edu/) maintains a southern Appalachian Regional Network. Worldwide earthquakes are posted as they occur at http://www.iris.edu. There also exists a growing number of amateur seismologists and high schools that maintain working seismographs. High schools sponsored by Georgia Tech that will soon be recording earthquake data in 1999 are shown in figure 3 as a "*" with existing seismic stations.
Our current understanding of the earthquake process is limited and routine use of scientific data to predict the location, time and magnitude of an earthquake is unlikely in the near future. Earthquake hazards instead are estimated from the statistical properties of earthquakes, which are assumed to remain constant through time. 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. In fact, 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 an increased probability of being the location of the next major earthquake; but the next earthquake could surprise us and occur outside of these currently active zones.
Although there is some disagreement and uncertainty among seismologist on how to estimate future seismicity, most agree that statistical estimates of historical seismicity provide the best measure of seismic hazard available today. Consequently, the historical seismicity was used as the basis for the new hazard maps being prepared by the United States Geological Survey. In these maps, earthquake hazards are expressed in terms of the level of vibration that has a given probability of being experienced during some time period. The example in Figure 4 defines the level of vibration (in percentage of the acceleration of gravity) that has a 10 percent probability of occurring in 50 years based. A 10 percent probability in 50 years is equivalent an average of once every 450 years. These USGS ground-shaking maps will be used by the Building Seismic Safety Council in its revisions to the seismic risk maps that will be adapted for use in State and local building codes. The hazard indicated by these maps is the 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 infrequent and limited to a few questionable historical accounts. The hazard is greater toward South Carolina, showing the influence of the continuing activity near Charleston, South Carolina.
The best insurance against earthquake damage is to eliminate those hazards in the home that could cause significant damage during an earthquake. An earthquake rider on your home insurance policy could reduce the impact of financial losses in an earthquake. To be effective in Georgia, earthquake riders 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.
When the earth shakes in an earthquake, falling objects can cause injury or start a fire. Many of the hazards associated with falling objects can be eliminated or minimized now, before an earthquake strikes. The following checklist can help earthquake proof your home.
During an earthquake "Duck, Cover and Hold"
Steps to follow after a large earthquake
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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., pp 8-22.
Slemmons, D.B., Engdahl, E.R., Blackwell, D., and Schwartz, D., (1991) Neotectonics of North America, Decade Map Volume, The Geological Society of America, Boulder, Colorado, 493p.