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对适用于加州帕克菲尔德附近的圣安德烈斯断层上中强地震(M～6)的地震不稳定模型,计算了作为时间和地面位置函数的理论压磁场。不稳定模型对一个32a地震轮回的所有时段模拟断层滑动和应力,其中包括失稳(主震)滑动。当该断层的一个近乎闭锁的断块受周围非地震滑动的作用变得充分承载以致于达到快速破裂的条件时,就会发生主震滑动。使用基于类似于应变核的压磁核的解析法,直接由断层滑动计算了压磁场。在地震间隔期间,闭锁断块上应力集中的增强将伴随着磁异常场的缓慢演变,在临近震中地区导致约±2nT的最大积累变化。主震滑动大约消除地震间隔期间所积累变化的一半,快速震后松驰消除剩余的大部分。地震间隔期间在闭锁断层附近一些测点上,场的变化速率逐渐增加,而另一些测点则逐渐减小。这种效应在主震前最后3～4a最显著,并且在适当选取的一些成对测点上,造成差值场时序趋势反向。如果这样的反向可与其他源所引起的地磁变化相区别的话,那么这种反向将构成临近破裂的前兆指标。
For the unsteady model of the earthquakes applicable to moderate to moderate earthquakes (M ~ 6) near the San Andreas Fault near Parkfield, California, the theoretical piezomagnetic field is calculated as a function of time and ground position. The unstable model simulates fault slip and stress for all periods of a 32a seismic cycle, including a destabilizing (mainshock) slip. A mainshock slip occurs when a near-latched block of the fault becomes sufficiently loaded by the surrounding non-seismic slip to reach the conditions of rapid rupture. The compressive magnetic field is calculated directly from fault slip using an analytical method based on a crushing core similar to a strained core. During seismic intervals, the increase in stress concentration on the block is accompanied by a gradual evolution of the magnetic anomaly, resulting in a maximum cumulative change of about ± 2nT in the immediate epicenter. The mainshock slip approximately halves the accumulated half of the change during the seismic interval and relaxes most of the rest by the quick post-earthquake maneuver. During some intervals near the latticed fault during the interval of the earthquake, the rate of change of the field gradually increases while the number of other measuring points gradually decreases. This effect is most pronounced in the last 3 to 4 years before the main shock and results in a reversal of the timing trend of the difference field over some appropriately selected pairs of measuring points. If this reversal can be distinguished from the geomagnetic changes caused by other sources, then this reversal will constitute a precursor indicator of the proximity of the rupture.