Mantle Tomography of Central-Eastern USA: Influence of Inversion Volume Size

Teleseismic tomography is a powerful tool to study the 3-D structure of the upper mantle beneath a local seismic network, but the robustness of its images has been challenged by the increasing size of the study volume in recent studies. This is because teleseismic tomography uses relative travel-time residuals (RTTRs) that are contaminated more by the whole-mantle heterogeneity for a larger study region. In this work, we correct the RTTRs of teleseismic events for the mantle heterogeneities outside the study volume using two global tomographic models. Our results show that the whole-mantle correction is necessary when a study region is wider than ∼10°, and the maximum outside residual reaches 1.2 s for a study region of ∼35° wide. After the whole-mantle correction, those teleseismic events with a large ray spread angle are still useful for tomographic imaging. Applying this approach to teleseismic travel-time data recorded by the USArray, we obtain a high-resolution 3-D P-wave velocity (Vp) model beneath the central and eastern United States. It shows three low-velocity anomalies beneath the New Madrid Seismic Zone, the East Tennessee Seismic Zone, and the South Carolina Seismic Zone, suggesting that lithospheric weakness induced by fluids may play an important role in reactivating the intraplate seismic zones. The passage of the Bermuda hotspot and remnants of the subducted Farallon slab in the lower mantle might be responsible for the fluid release and lithospheric weakening, which reactivated ancient rifts and suture zones in the crust and facilitated the formation of the intraplate seismic zones.