Upper mantle surprises derived from the recent Virginia earthquake waveform data

Recent high resolution regional waveform modeling reveals that the lithosphere beneath the North American craton is subdivided into an upper nearly uniform layer and a lower layer with a high velocity gradient. The boundary occurs at about a depth of 115 km and is responsible for 8° discontinuity in seismic record sections that is often observed in craton environments. Unexpectedly, we find seismic velocities in the lower layer significantly reduced along a corridor from the New Madrid rift zone to Virginia. This reduced velocity in the lower lithosphere may be associated with a possible historic hotspot activity. We also find a well developed X-discontinuity that we model as a ∼3% increase in P velocity starting at a depth of ∼290 km. These anomalous features transition into a nearly 1D craton structure to the north with a strong low-velocity anomaly just above the 410 discontinuity along an east–west boundary. The latter two features may be relics of structures formed from the descending Farallon plate between Late Cretaceous and Early Tertiary.