Promoting effects of 1975 Haicheng foreshocks on the occurrence of the mainshock
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Assuming that principal axis of the distribution of the 1975 Haicheng mainshocks and aftershocks means that there is already a fault (a 1) on N67°W, which together with Dayanghe fault (a 3) on its right side with the orientation of N50°W form a left step and left lateral fault system. The calculation indicates that the energy release ratesG 3 corresponding toa 3 is greater than that ofG 1 corresponding toa 1, which indicate thata 3 will extend first as compared toa 1, this results can illustrate qualitatively the observations of Haicheng foreshock distribution alonga 3’s orientation and in its northwest extending part; analyses also show that there may exist two types of secondary cracks in the step zone betweena 1 anda 3, tensile cracks and shearing cracks, which may provide space and channels for fluid transportation on one hand and may transform slip between the two faults on the other hand. Otherwise because the variations of foreshock locations with time have the spreading tendency to northwest direction alonga 3 and the fractals for the foreshocks are much less than for aftershocks, therefore the fractures for foreshocks may have relatively uniform orientations and may connect with each other as the evolution process of the foreshock series, the effects of which may be considered as that ofa 3’s growing to the northwest, for this case the variations of the mean stressesC m and the maximum shearing stressesS m neara 1 are then calculated, the results that tensileC m increase mean the local confining pressure or frictional stress decrease as the development of the foreshocks, and at the same period theS m increases, the combining effects of the two types of variations may help the occurrence of slip or initial fracture on the end part ofa 1, that is the occurrence and the development of foreshocks may have the promoting function to the occurrence of the main shock if the stress level in the source area has already been high enough, furthermore if another fault ofa 2 is then introduced further in the direction of NNE, the frictional stresses and maximum shearing stresses on the end part ofa 1 will be decreased further, this effects may be something like pouring more oil on the fire.
Key wordsenergy release rate mean stress maximum shearing stress
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