38 (2021-2027): Pilot study for deep MT Observations under the embayment

• University of Memphis

During the winters of 2021-2023, a magnetotelluric pilot study of the New Madrid seismic zone was conducted by CERI to gather an initial MT dataset. Four short shallow profiles across the Axial Fault in Missouri and one long deep profile across the eastern Reelfoot Rift faults and the Axial Fault in Tennessee and Missouri were acquired. Four new broad frequency-band MT recorders were borrowed from the Earthscope Primary Instrument Center (ERIC, formerly IRIS PASSCAL) the first winter and one the second winter for recording this data. The resulting MT data showed improved resolution of resistivity structures both near the Axial Fault from the short profiles and across the east bounding faults of the Reelfoot Rift into the Mississippi embayment from the long profile. Gravity data along the short profiles was also acquired to improve the existing USGS regional gravity observations. Early in 2023 CERI acquired six telemetry systems for the Phoenix MTU5-C wideband recorders that are available from EPIC. This proposal is to extend the pilot study to test the feasibility of extending MT observations to depths of 400 km using longer recording periods of a month with the aid of the new MT telemetry system. Previous MT recording within the embayment used too short of observation periods of a few days to two weeks that only permit MT imaging down to 200 km at best with limited observational frequencies of 10-3 to 103 Hz. Recent seismic velocity tomography studies indicate low velocity anomalies in the mantle between 200 and 400 km related to past tectonic processes. Confirmation of the extent of these mantle features and their properties using MT is needed to better characterize them. The MT data can be combined with existing gravity data to constrain the physical properties of the mantle. This project is feasible because we will again borrow MT instruments provided by the ERIC available to member institutions. MT data will provide a new way to investigate geological structure associated with the New Madrid fault system that we expect will give us important clues to the origin of the crust and mantle structures and their physical properties. We are particularly interested in the role of conductive fluids that may exist in the mantle which can be detected by MT methods. Also, demonstrating the feasibility of MT monitoring down to 400 km will directly support a NSF Geophysics proposal for deep MT observations beneath the Mississippi embayment. The budget requested in this proposal will primarily go towards shipping the MT instruments to and from the EPIC in Soccoro, New Mexico, and expenses associated with the field campaign in Arkansas, Missouri and Tennessee.

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