EVIDENCE FOR A LENGTHY LATE-QUATERNARY EARTHQUAKE HISTORY IN THE EAST TENNESSEE SEISMIC ZONE, TENNESSEE, GEORGIA, ALABAMA

The East Tennessee seismic zone (ETSZ) is the second most active intraplate region in the eastern U.S. Earthquakes in the ETSZ occur in basement rocks at depths of 5–26 km in a region ~50 km-wide and 300 km-long extending from NE AL and NW GA to NE of Knoxville, TN. Its earthquake frequency and size led the USGS to rate it capable of M = 7.5 events, although no events >M = 4.8 have been recorded. Detailed studies of paleoseismology had not been made here prior to 2009. We began a reconnaissance and detailed paleoseismic investigation in 2009 to determine: (1) if large prehistoric earthquakes have occurred in the ETSZ, and (2) if so, how frequently they recur. Our paleoseismic work around Douglas Lake (E of Knoxville) provided evidence of several prehistoric events that produced: (1) strike-slip, thrust, and normal faults and numerous water-bleached fractures in several different late-Quaternary terraces; 2) minor paleoliquefaction (small dikes) in terrace alluvium; and 3) anomalous fractures and disrupted features in terrace alluvium at three locales attributed to liquefaction and forceful groundwater expulsion during one or more major late-Quaternary earthquakes. Possible seismogenic features have been observed at additional localities in Sequatchie Valley to the W of the ETSZ and farther S into NW GA.

We have proved that the ETSZ has a long history of late Quaternary movement, with the recurrence interval yet to be determined. It is now clear that the ETSZ is not an ephemeral seismic zone, but one of continuing strong seismic activity. This seismicity mostly occurs immediately E of the New York–Alabama magnetic lineament (NYAL, 048 trend); some have suggested that the NYAL concentrates the ETSZ seismicity. Lines along maxima in computer-generated contoured density plots of ETSZ earthquakes trend primarily 039, with branches oriented SW and SE at high angles to the main line, suggesting little or no relationship to the NYAL lineament.