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| Titel |
Source mechanisms of micro-earthquakes induced in hydraulic fracturing experiment at the HDR site Soultz-sous-Forêts (Alsace) in 2003 and their temporal and spatial variations |
| VerfasserIn |
J. Horalek, L. Dorbath, Z. Jechumtálová, J. Sileny |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250031551
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| Zusammenfassung |
| We have investigated source mechanisms of microearthquakes induced in the 2003
hydrofracturing experiment in the GPK3 borehole of the Soultz-sous-Forêts Hot Dry Rock
facility (located in the Rhine Graben) and their time and space distribution. During this
massive hydrofatcturing a total of 34000m3 fluids with the wellhead pressure up to18 MPa
were injected and about 5000 microearthquakes in the magnitude range of -0.9 -¤ M -¤ 2.9
were recorded by the surface local network. Depths of induced events varied between 3.5 km
and 5.5 km. Seismic activity continued ten days after the injection shut in, during this period
the largest events occurred. A peculiarity of the 2003-seismicity was a relatively
high rate of larger microearthquakes which occurred during whole experiment. We
investigated set of 45 microearthquakes of magnitudes M - 1.6 to 2.9 covering the whole
injection both in time and space. Unconstrained moment tensor (MT) expression of the
mechanism was applied, allowing description of a general system of dipoles, i.e.
both double-couple (DC) and non-DC sources like tensile fractures. Each of the
resultant moment tensors was tested for stability to the reduction of the stations used,
incorrectness of the amplitude readings due to distortion by noise, and structure
mismodelling.
The resultant moment tensors indicate dominance of the double-couple components - mostly
larger than 90%, minor non-double-couple parts were found to be insignificant; this implies
that all the larger microearthquakes were nearly pure shear slips. The focal mechanisms show
fairly large variability and are of a normal dip-slip, oblique normal to strike-slip types. The
T-axes are fairly stable, being concentrated sub-horizontally roughly in the E-W direction. On
the contrary, P-axes plunge varies largely from horizontal to vertical being well constrained in
the N-S direction. This points out that the Soultz upper crust is the extension stress
regime (similar magnitudes of the maximum and the medium stresses, Ï1 and Ï2), in
which the normal dip-slip, oblique normal and strike slip faulting is possible. The
temporal and spatial distribution of events investigated indicates a link of induced
seismicity to two fault segments, I and II in our notation, showing different source
mechanism pattern. Segment I located close to the GPK3-borehole was active during
the whole experiments while segment II only after the GP3-shut in. The largest
events occurred within both segments, a few days after the GPK3 shut in. This
however shows that the 2003-Soultz seismic activity developed to a great extent
independently of the injection strategy. Injected fluids acts only to decrease the Coulomb
friction by reducing the normal stress on the fractures and to ‘lubricate’ the fault
walls, enhancing a shear slip along the pre-existing fractures. In this respect the
2003-Soultz seismic activity resembles tectonic foreshock-mainshock-aftershock
earthquake sequence. Furthermore we infer that general assumption that highly
pressurized fluids generate tensile fracturing of the rock mass need not be correct. It holds
only if the injection pressure is higher than the tensile strength of the material. |
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