Crust Structure And Occurrence Background In Linfen Seismic Region By Gravity Anomaly

The Linfen strongly seismic region is located in the north part of the Shanxi Rift Zone, which is the eastern margin of Ordos Basin. It is one of regions in China continental area with a frequent occurrence of earthquakes. In 1303 and 1695, two great earthquakes, which magnitude is over 7 and the distance of two macroscopic epicenter is less than 40Km, occurred one after another in this area. This phenomenon is really rare in the history of intraplate earthquake in our country. At the same time, the research of deep structure and seismic tectonic background in Linfen area has important significance. In view of the above, this paper researched the deep structure in Linfen strongly seismic region by combined the latest seismic, gravity exploration profile and plane gravity anomaly, and discussed the seismic tectonic background on this basis.In the part of profile, using the velocity profile of the Zhucheng-Yichuan deep seismic sounding profile (DSS) as a constraint, obtained the density structure of the profile by fitting high precision gravity data measured on this measuring line. The crust of Linfen Basin is uplift from west to east as a whole. The structure of crust can be divided into upper layer, middle layer and lower layer, the vertical distribution of density is biggest in upper layer among the three layers, and this distribution forms a low-density structure which is like a half graben, and its depth is about 18Km. In the underground of Linfen area, the Moho surface is uplift, because of this, the crust of Linfen area is thinner than surrounding areas, and the mantle material upwelling maybe result in high geothermal anomaly in Linfen area.In the part of plane, we obtained plane gravity anomaly in deferent depths of this area by processing the plane bouguer gravity anomaly data. The results show that, there is a negative anomaly with a large range in the middle of Linfen Basin, and its depths is consistent with the results obtained by the profile and low-speed structure in velocity structure. Moreover, there are some positive anomaly, and the transform boundary between negative anomaly and positive anomaly is significant gravity gradient zone, which gradient values is larger in north part than in south part. The distribution area of gravity gradient zone is consistent with the main-controlled fault, it can be considered that, the gradient zone is the conversion boundary of different physical properties medium, which is the boundary of the half graben in the profile result.In contrast to the previous velocity structure in this area, we considered that, the above low-density structure is expand to the north in the depth of 10-14km, it caused a isolated layered structure in the velocity structure of the northern Basin; and a low-velocity/density half graben structure in the middle of the Linfen Depression.At last, we discussed the seismic tectonic background by combined the above conclusions. We considered that, the northern and southern part of Linfen Depression is different in local tectonic environment, even though the both part is in the same tectonic unit. In northern part, squeezing action is dominant, while in the southern part, strike-slip movement is more intensive. That is why Hongtong earthquake and Linfen earthquake which occurred under the same regional stress filed are different in seismic elements such as occurrence time, epicenter location and magnitude.