Initiation of unstable slips–microearthquakes by elastic impulses

Resumo

A series of laboratory experiments have been carried out with a model of two granite blocks under biaxial compression loading. The experiments are mainly intended for assessing the possibilities of partially releasing the accumulated potential energy. The model was subjected to calibrated mechanical impacts (strokes) which induced elastic impulses. The mechanical stresses, strains, and acoustic emission were recorded. The strokes caused both large slips releasing the stresses down to their initial level and small slips which reduced the stresses by 5–8%. The small slips mostly occurred after the precursory emergence of the low frequency oscillations having low amplitudes. Before the large slips, the stages of speeding-up of the relative motion of the sides of the block contact was observed, similar to those emerging before the natural slips unrelated to the strokes. This feature was not universal: in some cases, the model recovered to the stationary state of the block contact without a large slip. All the slips occurred with a time delay after the stroke. The time delay was shorter when the energy of the blow was higher. With the shorter time delays, the small slip is more likely to occur. The energy of the impacts was by three orders of magnitude lower than the energy accumulated by the model, which points to the triggering mechanism of slip initiation. The series of strokes resulting in the small displacements partially reduced the accumulated energy and prevented the emergence of large motions such as the stick-slip events. If after a series of such blows a large sliding event still occurred, its energy was higher than in the slips unrelated to the impacts. The experiments revealed the difficulties in solving the problem of earthquake hazard reduction by elastic impacts.