SPECULATION ON QUATERNARY STRESS FIELD CHANGES IN THE EAST TENNESSEE SEISMIC ZONE

The seismic hazard posed by the NE-trending East Tennessee seismic zone (ETSZ), lying within the southern Valley and Ridge province of the Appalachians, is poorly understood. Neotectonism in the region is suggested by previously reported geomorphic and thermochronology evidence that an episode of late Cenozoic unroofing of the southern Appalachians began in Miocene and is ongoing. Previously reported ETSZ focal mechanism solutions show mostly NE-striking right-lateral and E/W-striking left-lateral faulting with subordinate thrust and normal faulting, with cataloged earthquakes less than M 5. Paleoseismic data collected this decade in the ETSZ by the Hatcher team has shown that magnitudes of Pleistocene paleo-earthquakes were significantly stronger, but the occurrence of strong Holocene paleoseismicity in the ETSZ remains an open question. At several field investigation sites, systematic sets of Mode I (opening) joints and sand dikes in Pleistocene alluvium strike sub-parallel to the current (and average Quaternary) ENE-WSW greatest principal stress orientation in the eastern United States, consistent with a tectonic origin of these fractures. In addition, slickensides on some fractures in bedrock saprolite indicate slip accommodating ENE-WSW shortening, also consistent with a regional Quaternary stress regime. The timing of this ENE-WSW shortening can only be constrained to Late Pleistocene/Holocene. The Hatcher team also observed faults with slickensides and systematic sets of Mode I joints in Pleistocene alluvium that indicate NW-SE shortening. At one field investigation site, slickensides show that NW-transport of a thrust hanging wall is coeval with Late Pleistocene alluvium deposition, suggesting that NW-SE shortening may have characterized the last glacial maximum (LGM). Such a clockwise rotation of the local stress field may have resulted from compression at the foot of the SE flank of a NE/SW-trending glacial forebulge in Kentucky (to the SE of the LGM ice-sheet lobe in Ohio and Indiana). Regardless of the cause, a difference between the current stress field and the stress field during a paleoseismic event implies there also may be a temporal change in seismic potential of the fault, and thus in the hazard.