MAPPING UPPER MANTLE VELOCITY ANOMALIES IN THE EASTERN UNITED STATES USING TELESEISMIC P-WAVE RESIDUALS

The impetus for this study is the observation by several authors of low shear-wave velocities in the uppermost mantle beneath New England (USA), referred to as the North Appalachian Anomaly (NAA). In previous work, we used an independent data set, P-wave teleseismic travel time residuals, to map the extent of the NAA and found that anomalously slow P-wave residuals extend from Vermont to eastern Maine. This paper extends the earlier paper by (1) adding travel time measurements for additional teleseisms, (2) using the relative residuals to estimate the thermal gradient across the western anomaly boundary, and (3) extending the measurements to the Appalachians south of New England.

Currently 15 teleseismic earthquakes large enough to produce clear, impulsive arrivals have been identified during the period when numerous seismometers have been deployed in the eastern United States. Most of these events occurred in South America and approached New York-New England from due south, fortuitously oriented along the proposed western NAA boundary. A key aspect of our residual analysis is that we account for and remove the contribution of the crust to the travel time. Thus, our algorithm isolates the contribution of the upper mantle velocity differences to the measured travel time residual.

The P-wave residual map shows a clear boundary between fast and slow residuals oriented nearly along the New York-Vermont border. The location and width of the boundary zone can be clarified by plotting individual average station residuals as a function of longitude, taking advantage of the South American locations of many of the events. The mean residual for New York stations west of the anomaly is approximately -0.1 s, while the New England stations within the anomaly have a mean residual of +0.65 s. Thus, the travel time difference is about 0.75 s across the anomaly boundary. Assuming a path length of 200 km through the mantle, a velocity reduction of 0.27 km/s in the upper mantle between New York and New England can account for this travel time difference. Using a partial derivative value of ∂Vp/∂T = -0.5x10^-3 km/s/°C implies a horizontal temperature difference of 540 °C between central New York and western New Hampshire. A critical, unresolved question is the vertical extent of the North Appalachian Anomaly.