In the early Autumn of 1303AD, a large earthquake with a tremendous impact occurred in the northeast of Hongtong County, Shanxi Province, and this earthquake was the first major earthquake of M8 identified by seismogeologists through the study of historical records. The magnitude of the earthquake was large, and the isoseismal line was distributed in the NNE direction. The meizoseismal area was mainly located in the densely populated Fenwei fault-depression zone, so it caused great economic and property losses and casualties at that time, and left a lot of historical data. Most scholars have identified the seismic rupture of this earthquake as the Huoshan piedmont fault, but the current research methods are focused on geological methods such as seismogeological surveys and trenching. At present, in addition to seismogeological investigation and research, there is an urgent need for detailed geophysical exploration of the fine structure and seismogenic environment of the 1303 Hongtong earthquake area and the deep structure of the Huoshan piedmont fault. The phase tensor decomposition techniques and NLCG three-dimensional inversion were used to process the data of a MT profile, which is 160km in length and across the 1303 M8 Hongtong earthquake area, combined with the present-day crustal vertical motion data(including GPS and leveling data)and the latest geological and geophysical survey results in and around the study area. The results show that the Huoshan piedmont fault is an obvious large electrical boundary zone in the study area. In the middle and deep part, it is a low resistivity belt, which runs through the whole scale of the crust. The fault is a NNE-trending dextral normal fault, which may be the basement fault dividing Ordos block and North China block, extending from the surface to 40km underground. The Lishi Fault also shows as an obvious electrical boundary zone, which may be a large-scale fault system in the study area. With the Huoshan piedmont fault as the boundary, the Ordos block and North China block on the east and west sides of the fault show different electrical structural characteristics. The Ordos block in the west shows a stable tectonic environment, while the lithosphere in the North China block in the east is seriously damaged and has a trend of thinning. The results of magnetotelluric survey support the point that the Huoshan piedmont fault is the seismogenic fault of Hongtong earthquake in 1303. The earthquake might occur in the low resistivity zone under the Huoshan piedmont fault, and the focal depth may be between 10~20km. We believe that the seismogenic environment of the 1303 Hongtong earthquake may be controlled by multiple factors, such as the northeastward extrusion of the Qinghai-Tibet Plateau and the possible overall counterclockwise movement and uplift of the Ordos block, which led to an obvious right-slip movement of the Huoshan piedmont fault near the Linfen Basin. The upwelling of soft fluvial material in the lower and middle crust of the eastern part of the Linfen Basin caused the regional extension of the North China craton, leading to dip slip of the Huoshan piedmont fault, which may be the main controlling factor for the generation of this earthquake.

The Tienshan orogenic belt is one of the most active intracontinental orogenic belts in the world. Studying the deep crust-mantle structure in this area is of great significance for understanding the deep dynamics of the Tienshan orogen. The distribution of fixed seismic stations in the Tianshan orogenic belt is sparse. The low resolution of the existing tomographic results in the Tienshan orogenic belt has affected the in-depth understanding of the deep dynamics of the Tienshan orogenic belt. In this paper, the observation data of 52 mobile seismic stations in the Xinjiang Seismic Network and the 11 new seismic stations in the Tienshan area for one-year observations are used. The seismic ambient noise tomography method is used to obtain the Rayleigh surface wave velocity distribution image in the range of 10~50s beneath the Chinese Tienshan and its adjacent areas (41°~48° N, 79°~91° E). The joint inversion of surface wave and receiver function reveals the S-wave velocity structure of the crust and uppermost mantle and the crustal thickness below the station beneath the Chinese Tienshan area(41°~46° N, 79°~91° E). The use of observation data from mobile stations and new fixed seismic stations has improved the resolution of surface wave phase velocity imaging and S-wave velocity structure models in the study area.
The results show that there are many obvious low-velocity layers in the crust near the basin-bearing zone in the northern Tienshan Mountains and the southern Tienshan Mountains. There are significant differences in the structural characteristics and distribution range of the low-velocity zone in the northern margin and the southern margin. Combining previous research results on artificial seismic profiles, receiver function profiles, teleseismic tomography, and continental subduction simulation experiments, it is speculated that the subduction of the Tarim Basin and the Junggar Basin to the Tienshan orogenic belt mainly occurs in the middle of the Chinese Tienshan orogenic belt, and the subduction of the southern margin of the Tienshan Mountains is larger than that of the northern margin, and the subduction of the eastern crust is not obvious or in the early subduction stage. There are many low-velocity layers in the inner crust of the Tienshan orogenic belt, and most of them correspond to the strong uplifting areas that are currently occurring. The thickness of the crust below the Tienshan orogenic belt is between 55km and 63km. The thickness of the crust(about 63km)is the largest near the BLT seismic station in the Bazhou region of Xinjiang. The average crustal thickness of the Tarim Basin is about 45km, and that of the Junggar Basin is 47km. The S-wave velocity structure obtained in this study can provide a new deep basis for the study of the segmentation of the Tienshan orogenic belt and the difference of the basin-mountain coupling type.