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Keywords:

  • source mechanisms;
  • tensile events;
  • stress;
  • hydraulic fracture;
  • seismic moment tensor

[1] We apply rock mechanics concepts to the seismological observations in order to explain why during hydraulic injection some events display tensile and some shear deformation. The presence of non-shear components depends on the differential stress and the fracture orientation with respect to the σ1 direction. Provided the slip vector is parallel to the traction we define four types of earthquakes according to the ratio of the shear and tensile components. Assuming a Griffith failure envelope, hybrid events containing both shear and tensile components can occur for fractures striking within 22.5° of σ1. We argue that pure tensile fractures striking parallel to σ1 are unlikely in the presence of natural fractures. The low shear traction of tensile events also implies their small stress drops. By applying the analysis to two different data sets, Soultz-sous-Forets and Cotton Valley, we show that different orientations of natural fractures and differential stress in the targeted formations made each region favorable for different non-DC components in the injection-induced seismicity.