THE EARTHSCOPE MAGNETOTELLURIC EXPERIMENT: PROGRESS AND FUTURE EXPERIMENTS

USArray (http://www.iris.edu/USArray) is installing magnetotelluric (MT) stations as part of Earthscope. The MT component of Earthscope consists of permanent (backbone) and transportable long period stations to record naturally occurring, time varying electric and magnetic fields to produce a regional lithospheric/asthensospheric electrical conductivity map of the United States. Twenty-eight long period MT instruments allows for the final installation of the backbone stations throughout the US and yearly transportable array studies. The current transportable experiments have been concentrated in the northwestern part of the U.S. with the closest station to the Midwest region being in eastern Montana. To date the data have been useful in distinguishing resistivity minima along the Snake River Plain, the Cascades and the High Lava Plains. Resistivity maxima have been observed along the Precambrian continental margin under the Columbia River Plateau basalts in Washington. The lack of equipment and money limit the regions to be imaged and currently the MT committee is considering experiments in the Midwest including the Midcontinental Rift in Iowa, the Oklahoma Aulacogen/Midcontinental Rift in Oklahoma/Texas and Kansas and the New Madrid region. The backbone MT survey consists of 7 stations spaced throughout the continental US with stations in the Midwest being in southwestern Missouri, eastern Montana and northwestern Minnesota. These stations will be recording for at least five years to determine electrical conductivities at depths that extend into the mantle transition zone. The station to produce data was at Braden, Missouri where data out to 100,000 seconds were recorded. Preliminary one-dimensional models indicate that the crust is relatively resistive, the crust-mantle boundary is at 40 km, and the lithosphere/Asthenosphere boundary is at 210 km. The addition of the other six backbone stations will allow for a three-dimensional image of the mantle’s electrical structure in the U.S.