Connection between earthquakes and deep fluids revealed by magnetotelluric imaging in Songyuan, China

Songyuan is the most earthquake prone area in northeast China. Since 2006, earthquakes have occurred in the area in the form of swarms, with a maximum magnitude of M L 5.8. There is much controversy about the cause of the Songyuan earthquakes. We attempted to determine the cause using a three-dimensional electrical conductivity structure inverted from a regional network of magnetotelluric data in the Songyuan area. The L-BFGS inversion method was applied, with a full-impedance tensor data set used as the inversion input. Combined with an evaluation of the earthquake locations, the resistivity model revealed a northeast-oriented hidden fault running through the Songyuan earthquake area (SEA), which was speculated to be the preexisting Fuyu-Zhaodong Fault (FZF). Our resistivity model also found an apparent lithospheric low-resistivity anomaly beneath the earthquake area, which breached the high-resistivity lithospheric mantle and stalled at the base of the crust. A petrophysical analysis showed that this lower crustal low-resistivity anomaly was most likely attributed to hydrated partial melting, which could release water into the lower crust during later magma emplacements. While weakening the strength of the FZF, these ascending fluids also increased the pore pressure in the fault, further reducing the shear strength of the fault. Shear stress action (a fault strike component of the east-west regional compress), together with possible near-surface disturbances, may drive the fault to slip and trigger the earthquakes in Songyuan. It is possible that the continuous replenishment of fluids from the deeper mantle forces the Songyuan earthquakes into the form of swarms. We infer that the Songyuan earthquakes could be attributed to a combination of preexisting faults, regional stress, and deep fluids associated with plate subduction, and near surface disturbances might induce the earthquakes in advance. The Songyuan earthquakes are inherently induced earthquakes, fed by deep fluids.