There are two types of rock failure: tensile and shear. Both failures release deformation energy in one form or another. But a big amount of deformation energy cannot be released by tensile failure. There are two reasons. Firstly, the tensile strength is normally much smaller than shear strength of rocks. Therefore the stored deformation energy before tensile failure is also much smaller than before shear failure. Secondly, the most energy rbeleasing by tensile failure happens at the front of the fracture propagation (Fig. 4a) because there is almost no residual tensile strength left after the initial tensile failure. So the second tensile failure behind the fracture front contributes hardly to energy releasing. On the contrary certain residual shear strength always exists in the rock after the initial shear failure. There is probably a big shear failed volume (e.g. 1,000,000 m3) behind the fracture front during the injection (Fig. 4b),
which could be the mechanism for the large amount of energy release in deep geothermy.