Since 2006, the magnetotelluric (MT) component of EarthScope’s USArray has deployed instruments which measure the Earth's naturally occurring electric and magnetic fields. Its backbone array consists of seven stations (MT-BB) operated across the United States since 2008 as a reference network. In addition, each summer up to twenty portable MT systems (MT-TA) are deployed campaign-style occupying large footprints in areas of special interest as proposed by the MT community (2006-2011 in the Pacific Northwest, 2011-2013 in the Mid-Continent Rift). Each MT-TA site is occupied for approximately three-weeks with a nominal 70-km grid spacing for imaging of crustal and lithospheric conductivity structure. These data can also be integrated with other geophysical data to identify North America's thermal structure and study the significance of fluids in the crust. Data are archived with IRIS, and processed into transfer functions that are available for a significant number of EarthScope and other MT stations. The USArray MT program also provides instruments and modest support (i.e. instrument preparation and data handling) for PI-driven Flexible Array (MT-FA) studies.
The MT instruments are operated and maintained by the National Geoelectromagnetic Facility at Oregon State University under a subaward (PI Adam Schultz) from IRIS. Andy Frassetto serves as the MT program lead at IRIS. The scientific and operational goals of the MT program are overseen by members of the EarthScope community serving on the Electromagnetic Advisory Committee (EMAC), chaired by Rob Evans.