Large earthquake paleoseismology in the East Tennessee seismic zone: Results of an 18-month pilot study
Published:January 01, 2013
Robert D. Hatcher, Jr., James D. Vaughn, Stephen F. Obermeier, 2013. "Large earthquake paleoseismology in the East Tennessee seismic zone: Results of an 18-month pilot study", Recent Advances in North American Paleoseismology and Neotectonics East of the Rockies, Randel Tom Cox, Martitia P. Tuttle, Oliver S. Boyd, Jacques Locat
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The East Tennessee seismic zone in the southern Appalachians is an ~75-km-wide, 350-km-long region of seismicity that extends from NE Alabama and NW Georgia to NE of Knoxville, Tennessee. It is the second most active seismic zone east of the U.S. Rocky Mountains. Although the East Tennessee seismic zone has not recorded historical earthquakes of M > 5, researchers have used hypothetical and theoretical relationships to suggest that it may be capable of generating an “infrequent” M ~7.5 quake. To help clarify the late Pleistocene earthquake history and the earthquake potential of the East Tennessee seismic zone, we conducted an 18-mo pilot study to seek evidence of paleoseismic activity and have made important discoveries.
ENE of Knoxville, Tennessee, in late Pleistocene French Broad River alluvium, we discovered: (1) strike-slip, thrust, and normal faults involving bedrock and alluvium at three sites, and widespread bleached or clay-filled fractures; (2) paleoliquefaction; and (3) anomalous fractured and disrupted features at three sites attributable to liquefaction and forceful groundwater expulsion and fluidization during or immediately after two or more major late Quaternary earthquakes. All of these features were produced by seismic events with a probable minimum M ~6.5. Optically stimulated luminescence dates at four sites provide maximum ages of 73–112 ka for at least two events. Upward penetration of at least two generations of fractures, clastic-sediment intrusions, and faults into the Bt horizons of Ultisols at several sites implies that two strong shocks occurred sometime after ~73 ka, and possibly much later than 73 ka.
Two exposures in terrace alluvium E and W of the Tennessee Highway 92 bridge S of Dandridge, Tennessee, were graded and geologically mapped at 1 in. = 5 ft. The site W of the bridge revealed at least three sets of crosscutting fractures that terminate upslope against the base of an overlying late Pleistocene colluvium. The E site revealed numerous fractures and a fault with ~20 cm of sinistral displacement. Moreover, several “fluidization boils” containing shale clasts from below are cut by younger, red, clay-filled fractures. Few of these fracture sets in the Quaternary sediments parallel those in bedrock of the Tennessee Valley and Ridge and Blue Ridge geologic provinces that host the East Tennessee seismic zone, and these fractures are poorly aligned with the present-day N70E maximum principal stress orientation. A third site, 5 km SW of Dandridge on the NW side of Douglas Reservoir, contains at least two NW-vergent thrust faults that transported weathered bedrock 25–50 cm over late Pleistocene alluvium. At the same site, a 12-m-long mode 1 branching fracture in Sevier Shale is filled with Quaternary sediment, and is truncated by the largest thrust fault at 1–2 m depth. This structure, including the Quaternary sediment it contains, is also displaced 10 cm along a NW-trending sinistral fault. The discovery of faults at the ground surface that displace both bedrock and terrace alluvium contrasts with the modern seismicity, which occurs at 5–26 km depth in rocks below the basal décollement of major Paleozoic thrust sheets.
Collectively, these initial findings imply that the East Tennessee seismic zone has produced coseismic surface faulting and generated at least two strong (M > 6.5) earthquakes during the late Quaternary.