Preliminary Results and Future Plans of the Electromagnetic Component of EarthScope Transportable Array Experiments

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 US. The Backbone MT survey consists of 7 stations spaced throughout the continental US with preliminary installation at Soap Creek, Oregon; Parkfield, California; Braden, Missouri; Blacksburg, Virginia and Socorro, NM. Permitting is either underway or completed at stations in eastern Montana and northern Wisconsin. These stations will be recording for at least five years to determine electrical conductivities at depths that extend into the mantle transition zone. The first transportable array experiment took place in 2006 in central and eastern Oregon. Preliminary 3D inverse models indicate several lithospheric electrical conductivity anomalies, including a linear zone marked by low conductivity transition along the Klamath-Blue Mountain Lineament associated with a linear trend of gravity minima. High electrical conductivity values occur in the upper crust under the accreted terrains in the Blue Mountains and the lower crust of the High Lava Plains. The second transportable array experiment was performed in 2007 and completes coverage of Oregon, Washington, and western Idaho. These stations allowed for the imaging of conductivity structures beneath the High Cascades, Columbia River Plateau (CRP) and Precambrian terranes in western Idaho. The High Cascades are marked by high conductivities and the CRP has higher conductivities in the upper mantle that correspond to lower velocities determined from refraction experiments. The edge of the Precambrian North American margin was imaged beneath the CRP with lower conductivities to the east of 1190 W. The 2008 transportable MT experiment will focus on the Snake River Plain and western Montana. Time series and MT transfer functions are available now from the IRIS Data Management Center cataloged under the network code EM (www.iris.edu/mda/EM).