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Urban Seismic Hazard Mapping for Memphis, Shelby County, Tennessee

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Urban Seismic Hazard Mapping for Memphis, Shelby County, Tennessee UNDERSTANDING EARTHQUAKE HAZARDS IN URBAN AREAS Urban Seismic Hazard Mapping for Memphis, Shelby County, Tennessee Earthquakes cannot be predicted, Why are there Different Map but scientists can forecast how Types? strongly the ground is likely to shake Just as weather forecast maps come in as a result of an earthquake. Seismic a variety of types to suit different needs, so hazard maps provide one way of do seismic hazard maps. There are maps conveying such forecasts. The U.S. showing the shaking expected for waves Geological Survey (USGS), which of different frequencies. This is because different structures are tuned to respond produces seismic hazard maps to particular seismic waves, much like for the Nation, is now engaged in different musical instruments resonate at developing more detailed maps for particular sound frequencies. One seismic Figure 1. Photograph of part of the mapped vulnerable urban areas. The first set area: the view from downtown Memphis, hazard map may show shaking from long of these maps is now available for Tenn., of Mud Island between the Wolf and period (for example, 1.0 second or longer) Memphis, Tenn. (fig. 1). Mississippi Rivers, and the I-40 bridge cross- waves that cause the ground to move ing from downtown to West Memphis, Ark. slowly over a large area. A long bridge or tall building might be most affected by these waves. Shorter period waves (for Who Uses Urban Seismic Hazard example, 0.2-second period) are more Maps? likely to damage single-family houses. A number of public and private groups use hazard maps to minimize losses 38°N resulting from earthquake shaking. The maps are used to guide city planners and Illinois engineers in designing safe structures and determining where they should be located. Emergency responders use hazard maps in planning post-earthquake relief efforts. Kentucky Insurance companies use hazard maps Missouri to examine the risk to their portfolios in Arkansas order to ensure financial recovery after damaging earthquakes. 36°N What is Seismic Hazard? Seismic hazard refers to a mea- MEMPHIS sure of the shaking, or ground motion, r e Tennessee from earthquakes that can damage our iv R Alabama man-made environment. The hazard Mississippi depends on the magnitudes and locations i p ip of likely earthquakes, how often they ss si 0 100 KILOMETERS is occur, and the properties of the rocks and M sediments that earthquake waves travel 34°N through. 92°W 90°W 88°W In the central U.S., earthquakes occur Figure 2. Map of the region surrounding Memphis, Tenn. The darker orange area is covered along the New Madrid seismic zone; by thick sediments, called the Mississippi embayment. These sediments significantly affect active faults in this region are buried by how the ground shakes during earthquakes. White lines indicate likely locations of major the thick sedimentary deposits of the faults (buried beneath the sediments), and black dots show the locations of earthquakes Mississippi embayment (fig. 2). recorded since the mid-1970s. Blue line is Mississippi River. U.S. Department of the Interior Fact Sheet 2005–3142 U.S. Geological Survey Printed on recycled paper July 2006 M M I S BARTLETT IS 14 BARTLETT S 14 S 51 I I S 51 70 S 70 S S I I P WOLF R P WOLF R P 40 P 40 IV 40 IV 40 g I E I E R 1.0 R R R I 0.9 I 35°10'N V 35°10'N MEMPHIS V MEMPHIS E 0.8 40 40 E R R 0.7 0.6 240 240 240 0.5 240 0.4 55 55 0.3 GERMANTOWN 240 GERMANTOWN 0.2 35°5'N 240 35°5'N 0.1 72 72 NONCONNAH CREEK NONCONNAH CREEK 0.0 385 385 61 61 55 76 55 78 90°5'W 90°W 89°55'W 89°50'W 90°5'W 90°W 89°55'W 89°50'W 0 10 KILOMETERS Figure 3. Two of the probabilistic urban seismic hazard maps for the Memphis, Shelby County, Tenn., area. These maps show accelerations with oscillation periods of 1.0 second, with a 2 percent (LEFT) and 10 percent (RIGHT) chance of being exceeded in 50 years. The color scale shows the acceleration level, measured in gravity, “g” units; 1 g is the acceleration of a freely falling object due to gravity. Seismic hazard maps may also be when conservative estimates and worst- Probabilistic seismic hazard maps either probabilistic or deterministic. In case scenarios are called for, such as attempt to account for all earthquakes an earthquake-prone area, an engineer when building critical facilities. Deter- that might affect an area. They incorpo- might assume a particular earthquake will ministic maps do not take into account the rate information about seismic activity occur and needs to know how hard it is likelihood of such a scenario occurring. and the locations of active faults, as well likely to shake the ground. These needs When considerations of likelihood as estimates of the time interval between call for “deterministic,” or “scenario,” become important, a “probabilistic” earthquake recurrences. forecast maps. Such maps might be used forecast map may be more appropriate. Figure 4. The Memphis maps show a reduc- tion in short-period (0.2 second) shaking by as much as 30 percent (TOP RIGHT INSET MAP) and an increase in longer period shaking, Highest hazard nearly doubling the motions in the 1.0-second- period map (TOP LEFT INSET MAP), compared 320+ 240-320 to the National Seismic Hazard map (BOTTOM 160-240 MAP and TOP BACKGROUND MAPS). These %g 80-160 differences result directly from including 40-80 the effects of the sediments, which are not 20-40 accounted for in the National Seismic Hazard 0-20 maps. The color scale shows the acceleration Lowest hazard level, measured in percent of gravity or “g” units; 100 percent g is the acceleration of a freely falling object due to gravity. M M I S BARTLETT IS 14 BARTLETT S 14 S 51 I I S 51 70 S 70 S S I I P WOLF R P WOLF R P 40 P 40 IV 40 IV 40 g I E I E R 1.0 R R R I 0.9 I 35°10'N V 35°10'N MEMPHIS V MEMPHIS E 0.8 40 40 E R R 0.7 0.6 240 240 240 0.5 240 0.4 55 55 0.3 GERMANTOWN 240 GERMANTOWN 0.2 35°5'N 240 35°5'N 0.1 72 72 NONCONNAH CREEK NONCONNAH CREEK 0.0 385 385 61 61 55 76 55 78 90°5'W 90°W 89°55'W 89°50'W 90°5'W 90°W 89°55'W 89°50'W 0 10 KILOMETERS Figure 5. Scenario maps for the Memphis, Shelby County, Tenn., area. The map on the LEFT is for a magnitude 7.7 earthquake, a possible repeat of the December 16, 1811, earthquake, and the map on the RIGHT is for a magnitude 6.2 earthquake, like the Marked Tree, Ark., event in 1843. The ground motions shown correspond to shaking with an oscillation period of 1.0 second. The USGS has developed a series A comparison of the Memphis and greater vulnerability of tall buildings or of probabilistic seismic hazard maps the National Seismic Hazard maps (fig. long bridges to earthquake shaking. The for the Nation. These maps consider a 4) highlights the importance of includ- average motions for both maps (about time period of 50 years, a typical build- ing the influence of shallow geologic 0.7 g each; 1.0 g is the acceleration of ing lifetime. At all locations the maps structure when estimating ground motion. a falling object due to gravity) are large answer the question, “At some specific The national maps show an increase in enough to cause significant damage. probability, what level of shaking is likely hazard toward the northwest because the The Memphis seismic hazard maps to be exceeded?” (that is, the probability faults of the New Madrid seismic zone lie show estimated values of ground accel- of exceedence). For example, the maps northwest of Memphis. As a result of the eration (an indicator of the force of the of the Memphis area (fig. 3) show the enhanced dissipation of waves traveling seismic waves). These maps display ground motions that have a 2 percent (left) through thick sediments, the Memphis ground acceleration in two ways: (1) peak and a 10 percent (right) chance of being maps show reductions in short-period ground acceleration (PGA), the largest exceeded within the next 50 years. The shaking (right map, fig. 4), imply- value of the total motion, consisting of predicted motions shown on the 2 percent ing that single family homes and other many types of seismic waves, and (2) maps are larger because they allow for small buildings are not as vulnerable as spectral accelerations (Sa), consisting of the more infrequent, large earthquakes. previously thought. However, the thick waves with periods of either 0.2 or 1.0 sediments also resonate at some frequen- second. Peak ground accelerations of as The Memphis Maps cies and cause increases in longer period little as 0.1 g may be strongly felt and shaking (left map, fig. 4); this implies a Although the USGS National Seismic do light damage, and severe shaking and Hazard maps cover the entire Nation, moderate to heavy damage may occur for they are not sufficiently detailed to values above about 0.3 g. include near-surface geologic conditions, The USGS Memphis urban seismic which can greatly affect ground shaking.
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