Abstract

Observations show the appearance of abnormal b‐values prior to a mainshock. The precursor time T of b‐value anomalies prior to a forthcoming mainshock is related to the magnitude M of the event in an equation: log(T)=q+rM, in which q and r are two constants. In this study, implications of this equation will be explained. In addition, the mechanism causing b‐value anomalies prior to a mainshock will be explored. From numerical simulations based on the 1D Knopoff–Burridge spring‐slider model, Wang (1995) found a power‐law correlation between b and s, in which the parameter s is the ratio of the spring constant (K) between two sliders to that (L) between a slider and the moving plate. The power‐law correlations are bs−2/3 for the cumulative frequency and bs−1/2 for the discrete frequency. Because L of a source area is almost constant for a long‐time period, b directly relates to K. Smaller (larger) K results in a higher (lower) b‐value. Wang (2012) found forumla, in which ρA and VP are the areal density and P‐wave velocity of a fault zone, respectively. Experimental results show that VP is strongly influenced by the water saturation in rocks. The water saturation in the source area varies with time, thus leading to a temporal variation in VP as well as K. This results in the temporal variation in b‐values.

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