Rupture Development in Fault Damage Zones Consisting of Sets of Preexisting Small-Scale Parallel Cracks

Earthquakes are modeled, most often with a single or a few large-scale geological fault planes that are rupturing, again quite frequently, in infinite or semi-infinite solid media. Such models can give an insight into the generation mechanism of low-frequency waves, but actual faults may have damage zones consisting of sets of much smaller-scale preexisting cracks that can expand, mechanically interact with each other in quasi-static and dynamic manners, and possibly produce higher-frequency waves. However, the mechanics of local interaction of developing multiple smaller-scale cracks and higher-frequency waves has not been fully clarified yet. Here, therefore, by using the experimental technique of dynamic photoelasticity and numerical finite different schemes, we investigate rupture development and wave evolution in two-dimensional linear elastic brittle specimens with sets of small-scale preexisting parallel cracks that model dip-slip faults. Each of the specimen is subjected to tension externally applied at a prescribed constant strain rate and acting parallel to its free surfaces. Our experimental observations for various distribution patterns of initial cracks with different dip angles exhibit that, at least for the case of sets of vertically dipping cracks, after surfacing of the primary rupture, i.e. a total split of the specimen into two by the primary rupture in this case of specimens of finite size, the rupture jumps on the top free surface and the secondary, delayed rupture propagates at a remote distance from and in the direction opposite to the primary one. The secondary and further ruptures, or a cluster of ruptures, are initiated even without additional external load and apparently caused by dynamic waves due to the primary rupture. It may be obvious but further examinations suggest that if the dip angle is sufficiently large, rupture propagates dynamically in a similar way while for smaller dip angles rupture tends to expand rather quasi-statically, connecting preexisting cracks.