Joint Analysis of Seismic and Electrical Observables Beneath the Central Appalachians Requires Partial Melt in the Upper Mantle

The Central Appalachian Anomaly (CAA) is a region of the upper mantle beneath eastern North America that exhibits pronounced anomalies in its seismic velocity, seismic attenuation, and electrical conductivity structure. The CAA clearly expresses itself in low velocity, high attenuation, and high conductivity values; however, the present-day composition and state of the asthenospheric upper mantle in the anomalous region remains imperfectly known. The collection of data from densely spaced, co-located seismic and magnetotelluric arrays during the Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment affords the opportunity to probe the structure and properties of the upper mantle in the CAA region in detail using multiple types of geophysical observations. Here, we present new observations of P and S wave travel times from teleseismic earthquakes measured at MAGIC stations, including a determination of how travel times deviate from the predictions of a standard 1-D reference model. These observations constrain the ratio of the P to S wave travel time perturbations associated with the CAA, which in turn allows us to estimate the ratio of P and S wave velocity anomalies. We combine these observations with previously published estimates of seismic attenuation and electrical conductivity in the upper mantle beneath the MAGIC array, and carry out forward modeling to determine reasonable ranges of temperature, partial melt fraction, water content, and composition for the CAA. Our results suggest that 1%–2% partial melt is required to simultaneously explain the velocity, attenuation, and electrical conductivity observations beneath the MAGIC array.