Abstract: In this paper, the formulae for the theoretical earthquake displacement fields of all kinds of point and line sources (compressional, tensional, compressional and torsional, tensional and torsional, single-couple and doublecouple) are derived. The mechanical character of intraplate earthquake sources has been discussed. In comparing the observed earthquake displacement fields and some near field data such as the predominant directions of collapse of buildings with the results of theoretical computation, the present authors have the opinion that the sources mechanism of the Xingtai earthquake of 1966 accords with the model of a tensional fracture, while that of the Dongchuan earthquake of 1966 accords with the model of a compressional and torsional fracture. Based on the theoretical formulae and the actually observed displacement data, the seismic moment M0 and the stress drop △ of the Xingtai earthquake are given to be 6.51026 dynccm and 39 bars respectively.Finally, it is pointed out that the source mechanisms of great earthquakes can reflect the action of the tectonic stress in this region for a long time and the double-couple model (shear dislocation model) may not be the only suitable model for intraplate earthquake.