Subsurface electrical resistivity structure in the active transpeninsular strike-slip Province of Northern Baja California, México

The San Miguel Fault (SMF) zone in northern Baja California, México, consists of multiple right-lateral strike-slip faults formed by transtensional deformation along the Pacific-North American plate boundary. This area is characterized by elevated microseismicity and occasional larger earthquakes (M ≥ 6). In August 2020, two earthquakes (ML 4.7 and 5.1) occurred within 20 min of each other, initiating a prolonged aftershock sequence. The SMF is composed of discontinuous fault segments, most of which are aligned with the regional northwest-southeast plate motion, while some segments are obliquely oriented relative to the regional strike. This structural complexity, combined with seismic activity marked by persistent microseismicity, intermittent quiescence, and sporadic larger earthquakes, indicates intricate fault interactions during stress release. Characterizing the geoelectrical structure of the crust in this region is essential for understanding earthquake generation dynamics. This study uses a three-dimensional electrical resistivity model constructed from 38 magnetotelluric observation sites distributed along three profiles to elucidate subsurface electrical properties. The model reveals a resistive zone (>1000 Ohm-m) in the upper 8 km, overlying a highly conductive zone (<10 Ohm-m) between 10 and 30 km depth. Most relocated events were concentrated at depths of 5 and 15 km, predominantly at the boundary between resistive and conductive domains. The deep conductive anomaly is attributed to mantle-derived fluids produced by the transtensional tectonic regime in northern Baja California. The upward migration of these fluids reduces rock strength, promoting the clustering of seismic events above the conductive zone. The highly resistive rocks associated with plutons of the Peninsular Ranges Batholith may act as strength heterogeneities, allowing stress accumulation that can occasionally result in more destructive earthquakes.