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Promoting　effects　of　1975　Haicheng　foreshocks　on　theoccurrence　of　the　mainshock

Acta Seismologica Sinica(English Edition) 1994年第3期7卷 409-414页

作者：李兴才 Institute of Geophysics State Seismological Bureau Beijing China

Assuming that principal axis of the distribution of the 1975 Haicheng mainshocks and aftershocks means that there is already a fault (a1 ) on N67°W, which together with Dayanghe fault (a3) on its right side with the orien tation of N50°W form a left step and left lateral fault system. The calculation indicates that the energy release rates G3 corresponding to a3 is greater than that of G1 corresponding to a1, which indicate that a3 will extend first as compared to a1, this results can illustrate qualitatively the observations of Haicheng foreshock distribution along a3′s orientation and in its northwest extending part; analyses also show that there may exist two types of secondary cracks in the step zone between a1 and a3, 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 north west direction along a3 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 of a3′s growing to the northwest,for this case the variations of the mean stresses C, and the maximum shearing stresses Sm, near a1 are then calculated, the results that tensile Cm, increase mean the local confining pressure or frictional stress decrease as the development of the for eshocks, and at the same period the Sm increases, the combining effects of the two types of variations may help the occurrence of slip or initial fracture on the end part of a1, 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 fau

关键词： energy release rate mean stress maximum shearing stress

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