A NEW SET OF FOCAL MECHANISMS AND A GEODYNAMIC MODEL FOR THE EASTERN TENNESSEE SEISMIC ZONE

We present a new set of 26 focal mechanisms for the eastern Tennessee seismic zone (ETSZ) and discuss the implications for regional uplift through geodynamic modeling. The mechanisms are for earthquakes with magnitudes 2.5 and greater occurring after 1999. The ETSZ is the second largest naturally occurring seismic zone in the central and eastern US and the seismicity is attributed to reactivation of a major Grenville-age shear zone. P- and S- wave velocity models, the distribution of hypocenters, focal mechanisms, and potential field anomalies suggest the presence of a basement shear zone. The new focal mechanism solutions supplement and are consistent with a previously calculated set of 26 focal mechanisms for the period 1983-1993. Focal mechanisms fall into two groups. The first group shows strike-slip motion on steeply dipping nodal planes striking N-S/E-W and NE-SW/NW-SE. Mechanisms in the second group display primarily dip-slip motion and are constrained geographically to the southern portion of the seismic zone. Events in the second group are among the shallowest in the dataset (8-12 km). A receiver function study in the southern portion of the seismic zone cannot detect a distinct Moho boundary, suggesting delamination of the lower crust. We examine the stress regime, which may be changing with depth, using a geodynamic model. Preliminary results suggest a possible relationship between normal faulting and uplift caused by delamination.