Using the complete core drilling method, the authors completed two boreholes on both sides of Xiadian active fault on which an M 8.0 earthquake occurred in 1679. The authors collected some samples from XD1 (on the upthrown plate)at a 0.5 meters interval roughly, and from XD6 (downthrown plate)at a 1.0 meter interval. Comparing the Fe²⁺ /Fe³⁺ ratio in same borehole core, the authors find that the Fe²⁺/Fe³⁺ ratio of gray-black sediment is greater than gray-yellow sediment in the same drilling, and gray-black sediment is in a strongly reducing environment. When the same sedimentary strata are gray-black sediments, Fe²⁺/Fe³⁺ ratio in the strata on the fault’s downthrown plate is larger than that on the upthrown plate. The reducing environment for the deposits on the fault’s downthrown plate is stronger than the upthrown plate.