FLAC （Fast Lagrangian Analysis of Continua） can allow localized bands to develop and evolve partly because the code models the dynamic equations of motion and has been adopted to investigate some kinds of strain localization phenomena

though the numerical results concerning strain localization and strain softening are mesh-dependent. To obtain a further understanding of localized failure of rock under plane strain compression

some numerical investigations

such as the effect of specimen height and width

strain rate

shear dilatancy

pore pressure

and end constraint on shear bands patterns

were carried out by Wang et al. In addition

shear bands patterns of two and five seismic blocks models as well as thick-walled cylinder were also modeled by Wang et al. Shear strain rate field for seismic block model is numerically modeled by FLAC. The adopted failure criterion is a composite Mohr-Coulomb criterion w

ith tension cut-off and post-peak constitutive relation of rock is linear strain-softening. Numerical simulations of seismic block model are carried out in plane strain and small deformation mode. In the seismic block model

old fault lies between arbitrary two adjacent blocks; the numbers of old faults and blocks are 4 and 5

respectively. In addition

four rectangular surrounding blocks envelop a center square block. The seismic block model is loaded at constant velocity of 5×10

-6

m/s at top and base of the model. In addition

at left and right sides of the model the confining pressure remains a constant. Numerical results show that distributions of shear strain rate are highly non-uniform and conjugate shear fracture bands can be formed in seismic block model

especially for the higher normal stiffness and tangential stiffness of old fault. Anisotropy of the model leads to main shear fracture bands developing fully along a certain direction and at the same time the development of secondary shear fracture bands conjugate with the main bands are inhibited. Probably

complex geological structures are the reason for different spacings of shear fracture bands. The reason for earthquake occurring in the region where one shear fracture band intersects with another is that shear strain rate of the region is higher. Characteristics of strain localization can be revealed through investigation of shear strain rate fields. However

localization phenomenon cannot be observed from stress field due to the fact that in strain-softening stage stress will decrease

as is not similar to change in field of plastic shear strain that is increased sequentially though peak strength is reached. In the action of driving forces of plate tectonics

new shear fracture bands and old fault can unoverlap. Therefore

it is possible that the regions where arbitrary two shear fracture bands intersect are not in the old fault. The larger size of the center block results in the stability of the center block

which means that earthquake

does not occur easily in the center block

as is in agreement with many observations in field. However

earthquake probably takes place in the surrounding blocks or at the boundary between the center block and the surrounding blocks.