DETAILED GEOLOGIC MAPPING OF PALEOSEISMIC FEATURES: AN ADDED TOOL FOR SEISMIC HAZARD ASSESSMENT IN THE EAST TENNESSEE SEISMIC ZONE

The East Tennessee seismic zone (ETSZ) is the second most active in the central and eastern U.S. It encompasses an area ~50 km wide and 300 km long that extends from NE AL through NW GA into eastern TN, to NE of Knoxville. Modern earthquakes in the ETSZ occur in autochthonous basement at depths from 5 to 26 km; faults do not break the surface and have not produced large (M > 5) historic earthquakes, so our field studies have focused on searching for paleoseismic evidence. Our work has identified several thrust and strike-slip faults that displace late Quaternary alluvium; some propagated from shale bedrock into alluvium, providing evidence of M ≥ 6.5 earthquakes. These data should change the probability assessment for the ETSZ. Our goals are to determine the recurrence interval and maximum magnitude event that has occurred here.

We have employed techniques used for decades to map foundations of major engineered structures, but most paleoseismic features outside of reservoirs are not suitably exposed to permit this kind of study, and depend on vertical exposures along streams and in trenches. Multiple sites along the banks of Douglas Lake (French Broad River) near Dandridge, Tennessee, were cleared of surficial materials and the paleoseismic features recognized during reconnaissance foot traverses were mapped in detail. The lack of vegetation on excavated banks permits very detailed, centimeter-scale mapping of faults, fractures, and other paleoseismic features in bedrock and Quaternary sediments. We target late Quaternary deposits that overlie shale bedrock, to eliminate the possibility that karst features could produce features that mimic seismic damage. After excavation, each site is gridded and photographed, then mapped at a scale of 1:125; these detailed geologic maps have helped pinpoint areas in the field sites for trenching. Our geologic maps provide an important areal (2D) perspective of the geology, and are employed to locate trenches that provide insight into the third dimension. By making detailed geologic maps of these sites, we have been able to trench along and across features of interest in Quaternary alluvium. These methods have increased the probability of uncovering paleoseismic evidence in trenches, which provide more useful information to assess the capability of the ETSZ to produce large earthquakes.