Movement characteristics and present seismic activity of Ordos Block

The present horizontal movement of the Ordos Block is the result of the movement interaction between the Ordos Block and the mainland of China.The main dynamic sources are the northeastern pushing by Qinghaie Tibet Block,and the northwestern extension of the northern margin of the block.The latest GPS velocity field observational evidences shows that the velocity field of Ordos Block and around has a significant change from 1999 to 2015,and the change is significant in the northeast margin,the northwest margin,the southwest margin,and the southeast margin of the block.The velocity of left lateral movement of the northsouthern margin of Ordos Block is much larger than the right lateral movement of its eastsouthern margin.By using the seismic data of Ordos Block since 1970,the analysis of strain accumulation,strain release,stress distribution,and stress field show that the northern margin of Ordos Block is the main active area not only in the block edge,but also in North China.The strain around Ordos Block has been accumulating continuously since 2000,and this is the longest period of strain accumulation since 1970.Some signs of strain relief appeared after Alashanzuoqi Ms5.8 earthquake in 2015.There are some areas with extremely large strain anomalies appear in the northern margin of Ordos Block.