Geology, Seismology and Earthquake Hazard of the New Madrid and Southern Appalachian Seismic Zones: A Comparative Overview

Jeffrey W. Munsey, Tennessee Valley Authority, Knoxville, TN 37902

The Tennessee Valley is impacted by two significant intraplate seismic zones: (1) the New Madrid Seismic Zone of the central Mississippi Valley; and (2) the Southern Appalachian Seismic Zone, which extends from Alabama to Virginia. The New Madrid Seismic Zone (NMSZ) exhibits the highest seismicity rate of any area in the United States east of the Rocky Mountains. The Southern Appalachian Seismic Zone (SASZ), in particular the portion from northwestern Georgia through most of eastern Tennessee, is the next most active seismic region in the eastern U.S. (Powell, et al., 1994).

The New Madrid Seismic Zone has produced damaging earthquakes in historical time including at least three earthquakes estimated to have had moment magnitudes (M) of 8.0 or greater in the 1811-12 sequence. Paleoseismic studies by the U.S. Geological Survey and the University of Memphis have revealed evidence of additional episodes of earthquakes large enough to cause damage (M > 6.0) prior to the 1811-12 sequence (Eugene Schweig, verbal communication, 1994).

The New Madrid Seismic Zone is contained within the Reelfoot rift. The Reelfoot rift has been interpreted to encompass much of the central Mississippi Valley including parts of the lower Ohio River Valley and the Wabash River Valley. Reactivation of faults within the rift system are thought to be responsible for the continuing seismicity of the zone. Most earthquakes occur in the crystalline basement at depths between 4 and 12 kilometers. This ancient failed rift has subsequently been covered with younger sediments including the late Mesozoic and Cenozoic sedimentary rocks of the Mississippi embayment. The “soft” and often times sandy and loosely consolidated nature of the embayment sediments mean that these areas are not only subject to the direct effects of ground shaking, but also the possibility of significant ground deformations such as slides, slumps and liquefaction.

Johnston and Nava (1984) have determined recurrence intervals for NMSZ earthquakes based on historical and instrumental data. On average, this zone produces about six earthquakes of magnitude 3.0 or larger per year (most earthquakes of this size or larger are “felt”). An earthquake of magnitude 6.0 or larger is expected somewhere in the zone about every 70 years. Considering that a magnitude 6.0 or larger earthquake has not occurred in the NMSZ since 1895, Johnston and Nava (1984) estimated a 40-63% chance of a magnitude 6.0 or larger earthquake in the NMSZ by the year 2000, and an 86-97% chance of this size earthquake by the year 2035.

The Southern Appalachian Seismic Zone (SASZ) is an intraplate seismic zone whose dimensions are similar to the NMSZ. As in the NMSZ, most of the seismicity is thought to be caused by reactivation of Precambrian age faults in the crystalline basement rocks buried beneath a younger veneer of sedimentary rocks. The tectonic stress regime is thought to be essentially the same for the SASZ and NMSZ, namely, the maximum stress is oriented nearly horizontal and trends east-northeast to west-southwest. Focal mechanisms (fault plane solutions) from SASZ earthquakes indicate predominantly strike slip and reverse faulting on steeply dipping surfaces oriented north- northeast to south-southwest or east-southeast to west-northwest (Munsey, et al., 1985, Teague, et al., 1986, and Nava, et al., 1989).

In spite of many similarities, there are a number of important differences between the NMSZ and SASZ. The most notable difference between the two zones is the lack of any known earthquakes with magnitudes of 6.0 or greater in the SASZ. The largest known earthquake in the SASZ was the Giles County, Virginia earthquake of 1897, which had an estimated magnitude of 5.8. The most active portion of the SASZ during the past 15 years, and perhaps longer, extends from northwestern Georgia through east Tennessee, hereafter termed the East Tennessee Seismic Zone (ETSZ). Given the rate of seismicity in the ETSZ, it is somewhat surprising that the largest known earthquake in the ETSZ was the 1973 Alcoa, Tennessee earthquake which had a magnitude of only 4.6. Seismicity rates for the SASZ, although significant by intraplate standards, are considerably less than the rates for the NMSZ. Only one or two earthquakes with magnitudes equal to or greater than 3.0 would be expected in the SASZ per year. The extrapolated, expected recurrence time for earthquakes with magnitudes of 6.0 or greater in the SASZ is 186 years (Bollinger et al., 1989).

The scarcity of easily deformable rocks and sediments in the SASZ has made identification of prehistoric earthquakes very difficult. To date, only limited paleoseismic studies have been conducted in the SASZ and these studies have produced no conclusive evidence of damaging prehistoric earthquakes. Thus, definition of a maximum earthquake for the SASZ, and especially for the ETSZ, remains problematic.

The same factors that limit investigations of possible large prehistoric earthquakes in the SASZ have the positive effect of limiting the extent of secondary earthquake hazards in the SASZ. Most of the SASZ region is covered by hardened, Paleozoic sedimentary rocks or crystalline rocks of Precambrian age. Therefore, rockslides are probably the most important secondary hazard, with little opportunity for liquefaction.

The preponderance of karst conditions in the SASZ may cause occasional instances of small, localized earthquakes when underground limestone caverns collapse or shift. Examples of this phenomenon may include the Bristol, Tennessee, earthquakes of February 1994, the Maryville, Tennessee, earthquakes of April 1994, and the Greeneville, Tennessee, earthquakes of March 1995, as well as some sporadic seismicity along the eastern flank of the Tellico reservoir. These events typically involve relatively small energy releases, and although they may be felt quite strongly in small areas, they do not represent a significant component of the regional seismic hazard.

References

Bollinger, G.A., F.C. Davison, Jr., M. S. Sibol and J. B. Birch, "Magnitude Recurrence Relations for the Southeastern United States and its Subdivisions," Journal of Geophysical Research, 94, 2857-2873, 1989.

Johnston, A. C., S. J. Nava, "Recurrence Rates and Probability Estimates for the New Madrid Seismic Zone," in Proceedings of the Symposium on "The New Madrid Seismic Zone," U.S. Geol. Survey Open File Report 84-770, 279-329, 1984.

Munsey, Jeffrey W., and G. A. Bollinger, "Focal Mechanism Analyses for Virginia Earthquakes," Bulletin of the Seismological Society of America, Vol. 75, No. 6, pp. 1613-1636.

Nava, Susan J., Jeffrey W. Munsey, Arch C. Johnston, "First Fault Plane Identification in the Southern Appalachians: The mbLg 4.2 Vonore, Tennessee Earthquake of March 27, 1987," Seismological Research Letters, Vol. 60, No. 3, pp. 119-129.

Powell, Christine A, G. A. Bollinger, Martin C. Chapman, Matthew S. Sibol, Arch C. Johnston, Russell L. Wheeler, "A Seismotectonic Model for the 300-Kilometer-Long Eastern Tennessee Seismic Zone," SCIENCE, Vol. 264, April 29, 1994, pp. 686-688.

Teague, Alan G., G. A. Bollinger, and A. C. Johnston, "Focal Mechanism Analysis for Eastern Tennessee Earthquakes (1981-1983)," Bulletin of the Seismological Society of America, Vol. 76, pp. 95-109.