Joint hypocenter‐velocity inversion for the eastern Tennessee seismic zone

Abstract

A joint hypocenter‐velocity inversion for the eastern Tennessee seismic zone (ETSZ) has resolved velocity features in basement rock below detached Appalachian thrust sheets. P and S wave arrival times from 492 earthquakes have been inverted for one‐(1‐D) and three‐dimensional (3‐D) velocity models to midcrustal depths. The 3‐D P and S wave velocity solutions are computed independly and are very similar. In relation to the 1‐D model, velocity anomalies range from −8% to +16% in the first layer (upper 5 km) and between ±7% in deeper layers. Prominent velocity anomalies parallel the seismic zone and are consistent from layer to layer. The most persistent anomaly is a low‐velocity region that borders the seismic zone to the northwest and is flanked on either side by regions of anomalously high velocity. The New York‐Alabama (NY‐AL) magnetic lineament coincides with or lies close to the southeast boundary of the prominent velocity low in both the P and S wave velocity images. The spatial coincidence between velocity, gravity, and magnetic gradients suggests that major discontinuities are present in the basement. Relocation in the 3‐D velocity model reduced the number of very deep earthquakes (below 20 km) and further accentuated differences in seismogenic properties on either side of the NY‐AL lineament. After relocation, most earthquakes occur in a vertically bounded region roughly 30 km wide extending from 4 to 22 km in depth. Most earthquakes occur in regions characterized by either average velocity or small velocity anomalies.