From Basin to Block: Integrated Magnetotelluric and Electrical Resistivity Imaging of Active Structures Beneath Mexico City

The Mexico City conurbation, hosting more than 20 million inhabitants, is exposed to significant geological hazards due to its location within the Trans-Mexican Volcanic Belt and the ongoing subduction of the Cocos Plate beneath the North American Plate. In addition to regional volcanic and tectonic activity, intense groundwater extraction has induced land subsidence and surface fracturing, exacerbating seismic risk and damage to critical urban infrastructure. Recent shallow microseismic events (<4.0 Mw) recorded in May and December 2023, with epicenters at depths of 1–2 km in the western sector of the city, highlighted the need for improved characterization of poorly known active or reactivated faults beneath densely urbanized areas. This contribution presents an integrated application of two complementary geophysical methodologies - magnetotellurics (MT) and two-dimensional electrical resistivity tomography (ERT-2D) - aimed at multiscale subsurface characterization in Mexico City. Deep MT surveys were designed to image electrical resistivity structures down to 2–3 km depth along one profile crossing the urban area, providing insight into regional fault systems and lithological contrasts potentially associated with earthquake generation. This MT profile demonstrates good data quality using robust impedance estimation and shows strong correlation with lithological information from deep boreholes drilled after the 1985 Mw 8.1 earthquake, validating the methodology for urban hazard studies. At the local scale, ERT-2D surveys were implemented to investigate an E–W oriented surface discontinuity exceeding 800 m in length, associated with recent microseismicity and severe infrastructure damage. Two orthogonal ERT profiles, acquired using a dipole–dipole array despite significant urban noise, resolved subsurface structures to depths of ~80–90 m. Inverted resistivity models reveal a heterogeneous shallow subsurface, showing low-resistivity saturated horizons, intermediate resistivity unconsolidated volcanic and sedimentary units, and high-resistivity zones interpreted as massive rocks or weak, unconsolidated tuffs. The combined MT–ERT approach demonstrates the value of integrating deep and shallow geophysical imaging to identify fragile geological structures in complex urban environments, providing a scientific basis for seismic hazard assessment, urban planning, and civil protection strategies in Mexico City.