Examples

ModEM became available as EarthScope USArray MT data was being collected. Many of the early results on conductivity structure of the CONUS were obtained with ModEM.

Meqbel et al (2014) is an early, and highy cited, publication using ModEM. At left is a 3D view of deep conductivity variations beneath the Northwestern US. Major resistive features (cool colors) include the subducting oceanic lithosphere (Juan de Fuca Plate) on the western edge, the Wyoming Craton (WYC), Colorado Plateau (CP) and Medicine Hat Block (MHB). Individual cratonic blocks are separated by conductive sutures, extending through the lithosphere. The lithosphere in the Basin and Range (BR) is very thin. This is shown more clearly in the E-W cross-section through the center of the 3D model (lower panel). This section cuts the resistive subducting plate and WYC, and reveals a high conductivity layer near the moho (white dotted line) beneath the BR. Resistivity of 100 ohm-m below this is consistent with a relative dry asthenosphere beneath this conducting layer.

ModEM has been used continuously for interpretation as more USArray MT data has been collected, filling in our picture of lithospheric conductivity in the CONUS (e.g., Bedrosian and Feucht (2014); Yang et al. (2015), Bedrosian (2016); Murphy and Egbert (2017); Bedrosian and Finn (2021)). Inversions from various blocks have been synthesized to a construct lithospheric conductivity model for all of CONUS--as required for assessing power-grid hazards associated with geomagnetically induced currents (GIC). Below are the most recent synthesis results, from Murphy et al. (2022).

ModEM has also been successfully used for inversion of marine MT data. At right is a 3D image (Egbert et al. 2022) of resisitivity of the Cascadia forearc, imaged by an amphibous MT dataset, consistiting of nearly 400 sites (64 marine). Resisivity is shown here in cross-sections at one degree latitude increments, with superposed isosurface for conductive material (rho < 16 ohm-m). Gray surface: top of subducting plate (imposed a priori). Green line: outline of Siletzia (R1-R2), a thick section of mafic rocks erupted on the seafloor, and accreted to North America in the Eocene. C1 underthrust accretionary complex; C2 fluids accumulating at outer edge of Siletzia. C3 arc conductor.