Half-moon events are a special type of microseismic source mechanism that is found at various hydraulic-fracturing sites, but hardly observed in natural seismicity. This event type can be explained either by a vertical slip on a nearly vertical fault plane or by a horizontal slip on a nearly horizontal fault plane. For this, special stress conditions are required, for instance, nearly equal horizontal and vertical compressional stresses and significant shear stresses. Such conditions are created during hydraulic stimulation as shown by our numerical simulations. By applying fracture pressure to the surface of the hydraulic fracture, the stress field in the vicinity of the hydraulic fracture can locally rotate and horizontal or vertical faults become optimally oriented. We found that such rotations can occur in locations where the elastic properties of rocks change (i.e., the fracture crosses a layer interface) or at the tips of the hydraulic fracture. Depending on the stress regime, our model explains half-moon events generated by slip on nearly horizontal fault planes in strike-slip environments and by slip on nearly vertical fault planes in normal faulting tectonics. Moreover, our models explain several common characteristics observed in multiple case studies. This includes the observation of the high portion of half-moon events and opposed shear senses in different depths and on opposite sides of the fracture.

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