Earthquake focal mechanisms of the induced seismicity in 2006 and 2007 below Basel (Switzerland)

To stimulate the reservoir for a "hot dry rock" geothermal project, that was initiated bu a private/public consortium in the city of Base, approximately 11500 m(3) of water were injected between december 2nd and 8th, 2006, at high pressures into a 5km deep well. more thatn 10500 seismic events were recorded during the injection phase, and minor sporadic seismic activity was still occurring more than tow years later. the present article documents the focal mechanisms of the 28 strongest events, with M-L between 1.7 and 3.4 that have been obtained by the swiss seismological service3 (SED) during and after the stimulation. The analysis is based on data that was recorded by a zix-station borehole network, operated by the project developers, as well as by several permanent and temporary surface networks. the hypocenters of the events are located inside the stimulated rock volume at depths between 4 and 5 km within the crystalline basement, of the 28 faultplane solutions two are normal faulting mechanisms and one is astrike-slip mechanism with a strong normal component. all others are typical strike-slip mechainsms with mostly ns and ew striking nodal planes, as a consequence, the t-axes are all nearly horizontal and oriented in a ne or sw direction (mean zimuth 46 +/- 11 degrees) and the P-axes of the strike-slip events point in a NW or SE direction (mean azimuth 138 +/- 13 degrees). Overall, the observed focal mechanisms agree with what would be expected from both the stress observation within the well and the stress field derived from the previously known natural seismicity.