We present three-dimensional P and S wave velocity models for the active eastern Tennessee seismic zone (ETSZ) using arrival time data from more than 1000 local earthquakes. A nonlinear tomography method is used that involves sequential inversion for model and hypocenter parameters. We image several velocity anomalies that persist through most of the inversion volume. Some anomalies support the presence of known features such as an ancient rift zone in northern Tennessee. Other anomalies reveal the presence of basement features that can be correlated with regional gravity and magnetic anomalies. We image a narrow, NE-SW trending, steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the prominent New York-Alabama magnetic lineament. The low-velocity zone may have an apparent dip to the SE at depths exceeding 15 km. The low-velocity zone is interpreted as a major basement fault juxtaposing Granite-Rhyolite basement to the NW from Grenville southern Appalachian basement to the SE. Relocated hypocenters align in near-vertical segments suggesting reactivation of a distributed zone of deformation associated with a major strike-slip fault. We suggest that the ETSZ represents reactivation of an ancient shear zone established during formation of the super continent Rodinia.

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[1] A three-dimensional P wave velocity model has been developed for the Charlevoix seismic zone (CSZ). The CSZ is located along the St. Lawrence River $100 km northeast of Quebec City, Canada, and is one of the most active seismic zones in eastern North America. Five earthquakes with magnitudes equal to or exceeding 6.0 have occurred in the CSZ in historic time, and around 200 earthquakes occur annually. Hypocenters are located in Precambrian basement rocks. Basement rocks have been affected by numerous tectonic events including Grenvillian collision, Iapetan rifting, and meteor impact. We performed a sequential, tomographic inversion for P wave velocity structure based upon 3093 P wave arrivals from 489 earthquakes recorded by 12 stations. High velocity is associated with the center of the impact crater. The region of high velocity is surrounded by low velocities interpreted to be highly disrupted rocks. An elongated, high-velocity region is present at midcrustal depths that trends parallel to the St. Lawrence River. Earthquakes avoid the high-velocity body and separate into two bands, one on either side of the feature. Larger earthquakes (magnitude ! 4) have occurred along the northern edges of the high-velocity region.INDEX TERMS: 7205 Seismology: Continental crust (1242); 7215 Seismology: Earthquake parameters; 7230 Seismology: Seismicity and seismotectonics; KEYWORDS: tomography, Charlevoix, intraplate seismic zone, velocity model Citation: Vlahovic, G., C. Powell, and M. Lamontagne, A three-dimensional P wave velocity model for the Charlevoix seismic zone,

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The eastern Tennessee seismic zone (ETSZ) is the second most seismically active area in the central and eastern United States after the New Madrid seismic zone, but the relatively weak seismicity and the absence of correlation between the seismicity distribution and the surface geology make its seismogenic potential controversial. In this work we investigate the structure of the upper crust in the ETSZ by means of group velocity tomography maps from seismic noise data. Results show that the seismic activity is associated with a relatively low velocity anomaly mainly located in one or more basement blocks. These blocks, bounded to the NW by the NY-AL lineament and to the SE by the Clingman lineaments, are buried beneath low velocity strata consistent with the presence of a relatively thick sedimentary cover. The imaged low velocity anomaly migrates towards the SE at increasing periods, suggesting a possible SE dipping weak structure where most of the seismic activity takes place.