We present an approach to study the rock failure mechanisms due to fracture growth or activation. Our approach includes a series of numerical geomechanic simulations of an incremental rock failure (fracture growth) accounting for elastic wavefield generation and propagation. We then record these wavefields and perform their seismic moment-tensor inversion. We then try to establish connections between seismic moment-tensor solutions and different geomechanic scenarios of the fracture growth with possible applications in monitoring hydraulic fracturing, reservoir development, and local tectonic stress analysis. Our results show that in most cases the amplitudes of generated P-and S-waves can be reasonably well approximated by a moment-tensor point source. When the fracture hits the pre-existing crack then we observe stronger seismic emission compared to the case of the fracture growth in continuous medium. Thus our geomechanic modeling confirms the concept that the most noticeable microseismicity may come from activating the existing natural fractures rather than from the main fracture growth. We also note that the S-wave radiation pattern may be asymmetric (does not correspond to any ideal moment tensor) radiating more energy forward when the fracture hits long pre-existing cracks. Finally, our examples show that the moment tensors may give misleading idea about the direction of the fracture growth (advancement). This result should be kept in mind when interpreting microseismic data in the hydrofrac monitoring applications.
