Samples of Hot Dry Rock Soultz Injection Micro-earthquakes - Mechanism Using Alternative Source Models

Abstract

The geothermal reservoir at Soultz-sous-Forets is a valuable natural laboratory for understanding the mechanisms of the micro-earthquakes generated during stimulations and circulation tests. There is an ongoing effort towards the retrieval of the mechanisms though the indication of the type of fracturing of the rock massif still remains ambiguous. Recent studies indicate prevailingly shear slip but rarely there is also observed a non-shear pattern. Moment tensor – used today as a universal tool for description of the mechanism – captures general balanced dipole sources. However, for practice it may be too general as it may include also non-physical sources. Opening new fractures can be described in the first approximation by a tensile crack, optionally combined with a shear slip. We call this alternative model a shear-tensile/implosion source model. From the bulk of the seismicity recorded during stimulation in 2003, we concentrated on the first phase of the injection when only a single borehole in the site was stimulated. We processed thirteen micro-earthquakes with magnitudes bigger than 1.4 and not processed previously. We found that again their source mechanisms were dominantly pure shear on pre-existing faults just as the stronger micro-earthquakes. They are also in agreement with the stress pattern from in-situ measurements.