Samples of HDR Soultz Injection Microearthquakes: Mechanism Using Alternative Source Models

Tectonic earthquakes are supposed to be mostly shear slip movements along faults, in their force equivalent represented by the double-couple (DC). Earthquakes where crustal fluids act within the rupturing process may, however, be non-shear. Moment tensor (MT) is a general dipole source allowing describe mechanisms of seismic sources ranging from tectonic earthquakes to various types of non-shear foci, which include both natural and induced events. Non-DC components retrieved in an inversion of seismic data may however be spurious, appearing as the artifact of low-quality data or failure in localization of the hypocenter and in modeling the velocity/attenuation structure. Then, the MT may be a too general description of the mechanism, and it is reasonable to use a priori a more constrained model. This problem is especially urgent when studying microearthquakes from hydro-fracturing, which are monitored from a single well. In this configuration, typical in oil industry, it is impossible to determine all six components of the MT. A clue is a simple shear-tensile/implosion (STI) source model, consisting of a slip along the fault with an off-plane slip component. Although the model is non-linear (described by 4 angles pointing the fault plane normal and non-orthogonal slip vector, and by a magnitude), it is useful by reducing the number of parameters and avoiding unphysical combinations of the MT components a priori. When seismic events are clustered, we can apply relative moment tensor inversion method which helps to reduce the demand for exact modeling of the Green's function. In this approach, they are substituted in the equations by geometrical angles of the ray between the source and the particular station. Hence only a minimum of structural information is needed, sufficient to perform the ray tracing only, yielding the azimuths and take-off angles. A great opportunity to examine the role of pressurized fluids in fracture opening and earthquake triggering is offered by the seismicity induced during the massive fluid injection experiments at the Hot Dry Rock (HDR) site Soultz-sous-Forêts. The site is located in Alsace (France) within the hottest geothermal surface anomaly of the Upper Rhine Graben. From the bulk of the seismicity recorded, we have processed several microearthquakes with magnitudes between M = 1.4 and 1.5 which occurred during the first phase of 2003 fluid injection, when only the borehole GPK3 was stimulated. Mechanisms resulting from the alternative approaches offer the clue to estimate the reliability of the shear vs. non-shear source components, thus indicating the type of the fracturing.