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[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 s 1 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 s 1 . We argue that pure tensile fractures striking parallel to s 1 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, Soultzsous-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. Citation: Fischer, T., and A. Guest (2011), Shear and tensile earthquakes caused by fluid injection, Geophys. Res. Lett., 38, L05307,

[1] We present a numerical model of the subducting lithosphere that provides an alternative explanation for stresses causing deep earthquakes. Our model lithosphere is composed of a olivine, b spinel, g spinel, and perovskite + magnesiowüstite. The heat conduction equation is solved to determine temperature conditions in the slab and to locate the equilibrium phase transformations in pressure-temperature space. Volumetric strains in the subducting lithosphere are calculated from the density of individual phases and from the heat released or consumed in the phase changes. These strains are used as sources of stress in the subducting lithosphere. Dislocation creep and Peierls stress creep laws are included in the viscoelastic rheology. Volumetric reductions due to equilibrium phase transformations cause high shear stress in the transition zone because of the variable viscosity inside the subducting slab. Aspects of the model shear stresses are in agreement with observations of high seismic activity in the Tonga-Wadati Benioff zone. Compression is oriented along the dip of the slab, and extension is oriented in the plane perpendicular to the compression axis. Since our model stresses agree with the seismic observations, and because the model stresses are larger than those caused by buoyancy forces, our model provides a possible explanation of stresses causing deep earthquakes. Also, our model does not need metastability of olivine to explain the occurrence of high shear stress in the transition zone.

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