Earthquakes threaten people’s lives and property, especially when cities are affected. Thus, detecting the crustal structure and seismogenic background beneath cities is important if we are to lessen losses. Because the city of Zhumadian in Henan Province, China, is a well-developed area with a large population, more attention should be paid to earthquake detection. We therefore collected data from 15 magnetotelluric (MT) stations along a 55-km survey line across Zhumadian. Then, subsequent to acquisition, processing, and data inversion, we developed a preferred electrical resistivity model. We accurately relocated original earthquakes during the period between 1981 and 2019 within the study area using the double-difference earthquake location algorithm. The relocated earthquakes were projected onto our preferred MT model. The new resistivity model presented here reveals that saturated rocks, including rocks with porosities of 4.31–19.69% and moderate salinities of 10 g/L pore fluid and/or graphite, form conductive features in the upper crust. We also show that aqueous fluids derived from metamorphic dehydration within the lower crust and mantle, not melts, constitute mid-crustal conductors and facilitate detachment structures. The data show that the relocated earthquakes are located adjacent to the boundaries between conductive and resistive areas as well as at the bottom of fault fracture zones. These outcomes imply that the combination of rock failure and fault slipping triggers earthquakes.