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.

Paleoseismic investigations in fluvial deposits frequently use large-scale (many centimeters to decimeters wide) ground-failure features of liquefaction origin as indicators of larger earthquakes (i.e., exceeding M ~6). Such large features are not the only signature of seismicity, however. Seismic shaking often produces an abundance of small-scale features (millimeter to centimeter in size) such as sills, small clastic dikes, and ground fractures, which can vary widely in height and range from paper-thin to a few centimeters wide. These small-scale seismic signatures commonly form in field settings where large liquefaction features are absent, such as regions with a reduced susceptibility for liquefaction or sites far from earthquake meizoseismal regions where shaking levels were lower. Thus, these small signatures have the potential to significantly expand the geographic area useful for paleoseismic studies, yet they are not typically sought in most paleoseismic field studies because many can develop nonseismically, and interpreting their formative origin can be challenging. We examined small-scale features that occur in association with large liquefaction features at a variety of field sites across the United States. We present new criteria, with many photographic examples, to evaluate whether small-scale features and ground fractures were seismically generated. Although this research was done primarily in fluvial settings in the United States, these criteria should be applicable worldwide in many field settings with clastic sediments, potentially giving the study of small-scale seismic features and fractures a significant role in future paleoseismic investigations.

Abstract The lower Wabash and Ohio River valleys have experienced seismicity throughout geologic time. The rocks and sediments in southern Indiana, southern Illinois, and western Kentucky provide records of these past seismic events in the form of various types of filled fractures. In the field these features occur either as downward penetrating, surface-filled fractures created by tectonic deformation or seismicity, or as upward penetrating liquefaction features such as clastic dikes and sills created by strong earthquakes. The fractures are widespread and abundant in many places, and are usually seen in natural exposures such as stream banks and less commonly in man-made excavations. In contrast, their causative faults are rarely observed. Thus, compared to searching for faults, the study of filled fractures is a useful and relatively inexpensive technique for assessing the seismic history of a region. The fractures discussed are clearly of seismic origin on the basis of morphology, sediment characteristics, regional patterns, and proximity to known faults. Further research is needed to determine whether additional types of features, which we discuss and examine in the field, can also serve as paleoseismic indicators.

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Abstract Washington, D.C., is the first and largest planned city in the United States. The city lies along the Fall Line at the boundary between the Atlantic Coastal Plain and the Piedmont Plateau and at the head of navigation on the estuary of the Potomac River. This site combines the engineering complexities of two vastly different geologic terranes with the other complications introduced by the terraces and channels of a major river-estuary system. The western part of the city and most of the suburbs to the west and north are on the Piedmont Plateau, an upland underlain by complexly deformed metasedimen-tary and metaigneous rocks of late Precambrian or early Paleozoic age. These crystalline rocks are mantled by soil, saprolite, and weathered rock to depths of as much as 50 m, which adds both to their geologic inscrutability and to the problems of excavation and design of structures. The Atlantic Coastal Plain is underlain by unmetamorphosed and little deformed fluvial and marine strata of Cretaceous through Miocene age. These deposits form a prism that thickens southeastward from a wedge edge at the Fall Line to as much as 450 m in the southeastern part of the metropolitan area. Unconformities, facies changes, and variations in physical properties with age and depth of burial add spice to the life of the engineering geologist dealing with these strata. Terrace deposits ranging in age from Miocene(?) to Holocene bevel across the contact between the Coastal Plain deposits and the crystalline rocks of the Piedmont. The oldest deposits underlie a broad, deeply dissected upland that stands at an elevation of 80 to 90 m southeast of the Fall Line; isolated outliers cap hills and interfluves at elevations of as much as 150 m northwest of the Fall Line. Lower and younger terraces flank the major drainages and occur at various levels down to the modern flood plains. Much of the central city is built on low terraces of Sangamon or Wisconsin age. These younger terraces locally fill and conceal deep bedrock channels cut by the ancestral Potomac during low stands of sea level during the Pleistocene. The terrace deposits show conspicuous differences in degree of weathering and soil development, depending on their age and physiographic position. Estuarine and marsh deposits flank the tidal reaches of the Potomac and Anacostia Rivers, and considerable parts of the central city are built on artificial fill over these deposits. Considerable experience in underground excavation has been gained in the last decade during construction of METRO, a regional rapid transit rail system. Tunneling techniques have been developed for both crystalline rocks and Coastal Plain deposits, but cut and cover methods are generally used in the young materials, which are generally weakest. Foundation and slope stability problems are widespread in some geologic units in the metropolitan area and are locally serious. They affect structures ranging from single family dwellings to the Washington Monument.