Abstract: The great Hongtong M8 earthquake occurred in 1303 in Shanxi Province,was the earliest M8 earthquake in China, according to historical records. Though the earthquake has a history of more than 700 years, small earthquakes have been continually occurring till now. This study determines the fault geometry of this earthquake by inverting seismological data of present-day moderate-small earthquakes in the focal area. We select a strip-shaped zone from the relocated earthquake catalog in the period from 1981 to 2013 to invert fault plane parameters of this earthquake. The inversion results of the Hongtong M8 earthquake are as follows: the strike is 19.3°, the dip angle is 88.5°, the slip angle is -170.0°, the fault length is about 75.5 km, the fault width is about 26.2 km, the lower-boundary depth is about 37.35 km and the buried depth of upper boundary is about 11.12 km. The relocated earthquakes are divided into two parts by the Hongtong fault. The earthquakes which located in the north of Hongtong fault are named as the north part, and the earthquakes which located in the south of Hongtong fault are named as the south part. We invert fault plane parameters for the two parts, the inversion results of the north part are as follows: the strike is 13.7°, the dip angle is 76.6°, the slip angle is -157.6°, the fault length is about 32.7 km, the fault width is about 21.7 km, the lower-boundary depth is about 32.86 km and the buried depth of upper boundary is about 11.97 km; the inversion results of the south part are as follows: the strike is 20.3°, the dip angle is 87.1°, the slip angle is -154.6°, the fault length is about 45.9 km, the fault width is about 16.6km, the lower-boundary depth is about 25.50 km and the buried depth of upper boundary is about 9.32 km. It shows that the seismogenic fault is a right lateral strike-slip fault. The seismogenic fault depth of the north part is deeper than the south one. The seismogenic fault depth and its dip of the north part are similar to the results of the speculative conclusion by deep seismic exploration. The projection of the seismogenic fault on the ground corresponds well with the high intensity region of the Hongtong earthquake.

Key words: The 1303 Hongtong M8 earthquake, Seismogenic fault, Moderate-small earthquakes, Tectonic stress field