| 摘 要： | Based on fracture mechanics it has been found that earthquake precursors related to volumetric strain e are characterized by quadrants, blind zone and uneven propagation, and the approximate formulae for spreading range of precursor have been obtained. These results are coincident with the information of resistivity ρ_x observed before the Tangshan earthquake, and then mechanical explanations have been made for some precursors of ρ_x, such as precursors appearing concentratedly within the period of 1—2 years before the instability of the fault (i.e. subcritical period in which the fault spreads acceleratingly). Before instability of the spreading of fault, the range of e=10~(-6) reaches about 150 km, its propagating velocity is about 100 km/yr, the amplification coefficient of ρ_x is about 10~4, etc. The half length of fault before instability is about 35km. It seems that in propagating process the spreading of the fault is delayed by barrier.
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