Abstract: As a north-westward fault zone located at the Western side of Yishu fault (in Shandong section of Tan-Lu fault zone) controlling Changqing-Linyi moderately strong seismic belt, the in-situ stress state presented in Mengshan fault zone plays an important role in the hazard research on the seismic activities in central Shandong. The hydraulic fracturing stress measurements of two deep boreholes located in the vicinity of Mengshan fault zone have been carried out; resulting in the maximum horizontal principal stress SH as 7.0-17.0 MPa and the minimum horizontal principal stress Sh as 5.0-11.0 MPa. The measurement results show that the relative size of triaxial principal stress at the depth over 300 m tends to SH＞Sv≥Sh, and that tends to SH＞Sv＞Sh in the range of the depth between 400 to 450 m. The direction of the maximum horizontal principal stress presents NE-NEE, which is consistent with results acquired from other data. The average values of the lateral pressure coefficient obtained by measured data are maximum lateral pressure coefficient KHmax=1.54 at Borehole ZK134 and KHmax=1.38 at Borehole ZK8. According to Byerlee’s law, Mengshan fault zone lies in a relatively stable in-situ stress state, ? m (the ratio of the maximum shear stress to the average principal stress) presented in two boreholes shows little change between the years of 2010 and 2011 indicating that the fault zone is in a steady state at the moment.

Key words: Mengshan fault zone, in-situ stress sate, Byerlee's law, ratio of the maximum shear stress to average principal stress