ASEISMIC FAULT SLIP AND BLOCK DEFORMATION IN NORTH CHINA

In North China, the tectonic fault-block system enables us to use the Discontinuous Deformation Analysis (DDA) method to simulate the long-t erm cross-fault survey and other geodetic data related to aseismic tec tonic deformation. By the simulation we have found that: (1)Slips on f aults with different orientation are generally in agreement with the E NE-WSW tectonic stress field, but the slip pattern of faulting can var y from nearly orthogonal, to pure shear along the strike of the faults , this pattern cannot be explained by simple geometric relation betwee n the strike of the fault and the direction of the tectonic shortening . This phenomenon has been observed at many sites of cross-fault geode tic surveys, and might be caused by the interactions between different blocks and faults. (2) According to the DDA model, if the average ase ismic slip rate along major active faults is at the order of several t enths of millimeter per year as observed by the cross-fault geodetic s urveys, the typical strain rate inside a block is at the order of 10(- 8) year(-1) or less, so that the rate of 10(-6) year(-1), as reported by observations in smaller areas, cannot be the representative deforma tion rate in this region. (3) Between the slips caused by regional com pression and block rotation, there is a possibility that the sense of slip caused by rigid body rotation in two adjacent blocks is opposite to the slip caused by the tectonic compression. But the magnitude of s lip resulting from the tectonic compression is much larger than that d ue to the block rotation. Thus, in general, the slip pattern on faults as a whole agrees with the sense of tectonic compression in this regi on. That is to say, the slip caused by regional compression dominates the entire slip budget. (4) Based on (3), some observed slips in contr adiction to ENE tectonic stress field may be caused by more localized sources, and have no tectonic significance.