地震前兆特征与岩样剪切应变率异常数值模拟

利用拉格朗日元法(FLAC)模拟了岩样的宏观力学行为、剪切应变率异常及演化。在峰值强度岩石的本构模型分别取线弹性模型及应变软化模型的条件下，得到如下结论：1)在弹性阶段之后，应力-时步曲线具有一定的周期性。与地震复发的周期性类似。在加载过程中，形变场由均匀逐渐向不均匀转变．最终形成交叉的剪切带网络。被该网络所包围的试样中部可视为背景空区．在空区之外，地震群比较活跃。在应变硬化阶段。可以从未来震源观测到前兆一剪切应变率异常条带。2)在中期阶段之后，空区内地震活动的开始增强明显要比空区外地震活动增强晚。地震的迁移是由外向内。3)短期阶段则相反：在硬化阶段．剪切带内外的剪切应变率的差别不很显著。试样处于相对平静状态。在软化阶段，若采用剪切应变率最大的位置作为震源．可确定震源在试样的中部。源兆和场兆都集中在老断层上。

CHARACTERS OF EARTHQUAKE PRECURSOR AND NUMERICAL SIMULATION OF ANOMALIES OF SHEAR STRAIN RATE OF ROCK

Anomaly in shear strain rate (SSR) and its evolution as well as mechanical behavior of rock in biaxial plane strain were modeled by use of FLAC. In elastic stage, the adopted constitutive relation was linear elastic; however, in strain-softening stage, a composite Mohr-Coulomb criterion with tension cuff-off and linear post-peak constitutive relation was used. Beyond the elastic stage, the periodicity in the stress-timestep curve is similar to that in earthquake activities. In the process of loading,deformation field was observed to transit from within specimen uniform to nonuniform and finally network of shear bands (SB) was formed.Part in the middle of specimen which is surrounded by SB, can be seen as a seismic gap. Outside the gap, earthquake swarm is relatively active. In the strain-hardening stage,an effective precursor can be observed from the future earthquake source. Earthquake activities in the gap are obviously later than those in SB. After middle-term stage, migration of earthquake activities is from the SB to the gap, while it is opposite in short-term stage. In strain-hardening stage, no obvious difference in SSR was found inside SB and outside. In strain-softening stage, if zone with highest SSR is considered to be the source, then it lies in the middle of specimen. The field and source precursors appear in old fault belts.