Earthquake focal mechanisms of the induced seismicity in 2006 and 2007 below Basel (Switzerland) |
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Authors: | Nicholas Deichmann Jacques Ernst |
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Institution: | (1) Department of Geophysics, Stanford University, 94305-2215 Stanford, CA, USA;(2) Helmholtz-Zentrum Potsdam, Deutsches GeoforschungsZentrum (GFZ), Potsdam, Germany;(3) United States Geological Survey, 94025 Menlo Park, CA, USA;(4) Center for Deep Earth Exploration, Japan Agency for Marine-Earth Science and Technology, 236-0001 Yokohama Kanagawa, Japan |
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Abstract: | To stimulate the reservoir for a “hot dry rock" geothermal project, that was initiated by a private/public consortium in the
city of Basel, approximately 11500 m3 of water were injected between December 2nd and 8th, 2006, at high pressures into a 5 km deep well. More than 10500 seismic
events were recorded during the injection phase, and minor sporadic seismic activity was still occurring more than two years
later. The present article documents the focal mechanisms of the 28 strongest events, with ML between 1.7 and 3.4, that have been obtained by the Swiss Seismological Service (SED) during and after the stimulation. The
analysis is based on data that was recorded by a six-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 a strike-slip mechanism with a strong normal component. All others are typical strike-slip mechanisms
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 azimuth 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 observations
within the well and the stress field derived from the previously known natural seismicity. |
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