Simultaneous determination of hypocenter location and crustal structure in the Virginia seismic region
Authors: Koch Manfred , Kalata Jody 
Published: February 2008

Simultaneous inversions for one‐ (1‐D) and three‐dimensional (3‐D) seismic structure and hypocenters (SSH) for both the central and western Virginia seismic regions are performed, using direct and refracted phases from about 60 earthquakes. Optimal inversion models are computed by employing regularization techniques of Tikhonov and of Backus and Gilbert. Improvements for the residual sum squared, of up to 70% are obtained for the 3‐D models and somewhat less for the 1‐D models. For the central Virginia seismic region the SSH 1‐D velocity models show a strong negative bias in the deduced Pn velocity which could only be alleviated after a ray‐tracing travel time correction for the Pn phases due to the east‐west downdipping Moho has been applied. A 4° Moho dip turns out to be optimal for best explaining the observed travel time residuals and returning a realistic Pn velocity of 8.2–8.3 km/s. However, the deduced velocity in the lower crust is only ∼ 6.35 km/s, which is lower than that predicted from gravity modeling but can be explained in terms of a lower crustal petrology of anorthosite‐rich granulite. A more detailed analysis of the lower crust shows that the hypothesized high‐velocity layer at the bottom of the crust (Pratt et al., 1988) is not clearly identifiable by the present travel time data and so can neither be corroborated nor refuted. Many of the 1‐D crustal results are further substantiated by the 3‐D model, which also provides a reduced lower crustal velocity under the Goochland Terrane. Positive velocity anomalies, which are found in the upper and middle crust under the Blue Ridge/Piedmont boundary, correlate qualitatively with the high‐density, thrust‐faulted slab deduced by Pratt et al. (1988) from gravity modeling. The vertical movements of the relocated hypocenters in the central region are of O(2–5 km) and are such as to further widen the aseismic gap under the Goochland Terrane. In the eastern section of the seismic zone (at the Piedmont/Atlantic Coastal Plain boundary) the downward focal shifts at the lower seismic boundary place events well into the Grenville basement, indicating a lowering of the brittle‐ductile transition boundary. For the western Virginia seismic region the SSH 1‐D velocity models result in a gradual increase in velocity with depth. Significant shifts in the hypocentral relocations are obtained which indicate a further deepening of the brittle‐ductile transition. This and results of thermomechanical creep models favor a quartz‐poor, diorite rich, middle and lower crust in the western region. The 3‐D velocity model of the western region shows a variety of features which can be related to the geology and the tectonics of the Valley and Ridge province. In the surficial layer a velocity low in the Mississippian‐Permian molasse of the West‐Virginian Valley and Ridge province and a velocity high in the thicker Cambrian‐Ordovician carbonate thrust and fault sheets of the Giles county area of the Valley and Ridge are obtained. In the lower crust, higher velocities are detected in the western Valley and Ridge province, which correlates well with the thickening of the crust in the western section of the Virginian Appalachian.