Abstract:The focal mechanism solutions of 1 700 ML ≥2.5 small to moderate earthquakes which occurred in the circum-Ordos block and its adjacent areas from Jan. 1, 2008 to June 21, 2014 are obtained using the HASH method. The stress tensors and the orientations of the maximum horizontal stress of the circum-Ordos block are determined by the MATLAB package for spatial and temporal stress inversion (MSATSI) using 1°×1° grids, with the damping parameter 1.2 at the 95% confidence level. The characteristics of the spatial distribution of the focal mechanisms are investigated and their tectonodynamic background is discussed. Main conclusions are drawn as follows. In the rift zone of Yinchuan-Jilantai, Hetao, Shanxi and Weihe, which are located around Ordos block, the main types of the composite focal mechanism are strike-slip and normal types, which reflect the Ordos block suffering shear and extension stress field. There are different stress regimes around Ordos. Yinchuan-Jilantai, which is located northwest of Ordos, is a normal stress state which matches well with the regional geodynamic mechanisms, but the nearly north-south active fault presents strike-slip regime in the southern part of the northwestern segment which is forced to right-lateral strike-slip shearing from the NE. Three types of the composite focal mechanism are thrust-strike-slip and normal types in the Haiyuan-Liupanshan fault zone located on the SW boundary of Ordos block, which are under the joint action of the NW and NE compressional stress field. The maximum horizontal principal stress orientations are dominanted by NE and NW orientation. That may derive from the NE-directional pushing force from the Qinghai-Tibetan plateau and NW-SE directional extension caused by the uplifting of the deep substance of the Qinghai-Tibetan plateau. The maximum horizontal principal stress orientation seems to rotate slightly anticlockwise in the Shanxi rift, their stress pattern: EW-NE-NEE from south to north, and most of them parallel to the main fault’s strike, which controls the boundary of the Shanxi rift belt. Datong basin, which is different from the entire grabens extension, is shear. These phenomena are related not only to the special geographical environment and tectonic conditions, but also to the volcano underground Datong basin. These are relatively independent of the local stress fields in transverse uplifts between basins, in which the focal mechanisms are dominated by reverse and uncertain types. The stress regime in the northern boundary of the circum-Ordos block is in a NE direction normal type stress state. There are two maximum horizontal principal stress orientations in southern area of Ordos’s block: Qinling tectonic belt is NW direction extension stress state, and Weihe graben is in EW direction extension stress state.
GUO Xiangyun,JIANG Changsheng,WANG Xiaoshan et al. Characteristics of Small to Moderate Focal Mechanism Solutions Stress Field of the Circum-Ordos Block[J]. jgg, 2017, 37(7): 675-685.