Bifurcated Crustal Channel Flow and Seismogenic Structures of Intraplate Earthquakes in Western Yunnan, China as Revealed by Three-Dimensional Magnetotelluric Imaging

Based on broadband magnetotelluric data collected in western Yunnan, we obtain a three-dimensional crustal resistivity model, which reveals seismogenic structures of Yingjiang and Longling earthquakes. Our model suggests a bifurcation of the crustal flow in western Yunnan, with a southwestern branch running into the Tengchong Block north of the Dayingjiang Fault and a southeastern branch flowing into the Baoshan Block. The Yingjiang region features an upper and midcrustal resistive body above 20-km depth, surrounded by conductors in the midcrust. Six moderate earthquakes in Yingjiang occurred in the transition zone between the resistive and conductive structures. These earthquakes occurred in the context of a right-lateral accommodation zone between the Nabang and Binglangjiang Faults and in the presence of the southwestern branch of the crustal flow, which provides a mechanical decoupling under the rigid high-resistive upper crust. Beneath the Longling area, a high-resistive structure extends through the entire crust, but a zone of low resistivity is embedded in the depth range of 10-25 km where the 1976 M7.3 earthquake occurred. The high-resistive body provides a blockage to the previously proposed crustal channel flow along the Gaoligong shear zone and may accumulate sufficient stress. The Longling strong earthquakes may result from the sinistral strike-slip motion of the Ruili-Luxi Fault in the framework of the north to northeast trending compressive stress. The blockage to the crustal channel flow along the Gaoligong shear zone by the high-resistivity zone in Longling could be an important factor to produce the north to northeast trending compressive stress field in Longling area.