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Microseismicity induced during fluid-injection: A case study from the geothermal site at Groß Schönebeck, North German Basin

Kwiatek G., Bohnhoff M., Dresen G., Schulze A., Schulte T., Zimmermann G., Huenges E.

Acta Geophysica

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2010

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Vol. 58, no. 6

995-1020

EN

The technical feasibility of geothermal power production in a low enthalpy environment will be investigated in the geothermal site at Groß Schönebeck, North German Basin, where a borehole doublet was completed in 2007. In order to complete the Enhanced Geothermal System, three massive hydraulic stimulations were performed. A seismic network was deployed including a single 3-component downhole seismic sensor at only 500 m distance to the injection point. Injection rates reached up to 9 m3/min and the maximum injection well-head pressure was as high as ∼60 MPa. A total of 80 very small (–1.8 < MW < –1.0) induced seismic events were detected. The hypocenters were determined for 29 events. The events show a strong spatial and temporal clustering and a maximum seismicity rate of 22 events per day. Spectral parameters were estimated from the downhole seismometer and related to those from other types of induced seismicity. The majority of events occurred towards the end of stimulation phases indicating a similar behavior as observed at similar treatments in crystalline environments but in our case at a smaller level of seismic activity and at lower magnitudes.

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A study of the interaction among mining-induced seismic events in the Legnica-Głogów Copper District, Poland

Orlecka-Sikora B., Papadimitriou E., Kwiatek G.

Acta Geophysica

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2009

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Vol. 57, no. 2

413-434

EN

We applied the Coulomb stress transfer technique to investigate interactions among seismic events induced by mining works in the Rudna mine in the Legnica-Glogów Copper District in Poland. We considered events with energy greater than 10⁵ J from the period 1993-1999. We examined the influence of the cumulative static stress changes (ΔCFF) due to previous events on the generation of subsequent ones. The results indicate that in many cases strong mining tremors produce changes in the state of stress of a sufficient magnitude to influence subsequent events. The location of over 60% of events is consistent with stress-enhanced areas where the values of ΔCFF were above 0.01 MPa. For most of the events located inside areas of a calculated negative ΔCFF, their modelled rupture zone was partially located inside stress enhanced area, providing thus additional evidence for possible triggering at the nucleation point.

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Focal mechanism of earthquakes from the June 1987 swarm in Aswan, Egypt, calculated by the moment tensor inversion

Awad H., Kwiatek G.

Acta Geophysica Polonica

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2005

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Vol. 53, nr 3

275-291

EN

We obtained seismic moment tensor solutions of ten events from the June 1987 earthquake swarm, which occurred along the Kalabsha fault zone in the northern part of the Lake Aswan area in Egypt. In addition, the composite fault plane solution of this sequence was also calculated. The waveform data were obtained from the Aswan seismological network, which consists of 13 field stations with short period seismometers GS-13. The June 1987 swarm was a sequence of microearthquakes (M ? 3.4), shallow (0-10 km) events forming two successive bursts, which took place on June 17 and 19. The moment tensor solutions indicate that the focal mechanisms of events from this swarm sequence are expressed by right-lateral strike slip faults. They represent also an effective east-west compressional stress field acting in the area. Geological and geophysical data demonstrate that the Kalabsha fault zone is a right-lateral strike slip fault that consists of several fault segments trending in the east-west direction, perpendicular to the axis of the main course of the Lake As-wan. Thus, the focal mechanisms of the 1987 events are consistent with the local tectonics of the area.

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A search for sequences of mining-induced seismic events at the Rudna Copper Mine in Poland

Kwiatek G.

Acta Geophysica Polonica

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2004

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Vol. 52, nr 2

155-171

EN

A detailed analysis of the Rudna copper mine's seismic catalogue for 1980-2003, with a threshold local magnitude ML> 1.3, allowed to distinguish several types of event sequences related to 345 stronger mining events of magnitude greater than 3.0. Events without aftershocks and foreshocks were observed as well as those where the number of aftershocks numbered more than ten, while foreshocks - several. Foreshocks were recorded for 58 events, nearly all of them several tens of minutes before the main seismic event. Aftershocks accompanied the 190 strongest events; the greatest number of aftershocks was 10 for an event of magnitude just over 3.0. For the two strongest events, of magnitudes ML = 4.1 and ML = 4.2, the number of aftershocks was radically different, 9 and 1, respec-tively. Different numbers of aftershocks were observed depending on the loca-tion of the main event in various mining areas, which is related to the differences in the seismicity of individual zones within the mine. Non-parametric probability distribution functions of magnitude have been determined for the different seismogenic zones. In addition, the value of the b coefficient of the Gutenberg-Richter relation has been analyzed for each zone. These studies confirmed that seismicity varies within the mine.

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A search for the slow initial phase generated by seismic events at a copper mine in Poland

Kwiatek G.

Acta Geophysica Polonica

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2003

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Vol. 51, nr 4

369-385

EN

Observation of seismic events induced by mining activity at the Rudna copper mine allowed to identify the occurrence of the Slow Initial Phase on the seismograms. A total of 4384 seismograms from 137 seismic events were analyzed. The seismic moment of the strongest event was 3.36x10(14) N.m and that of the smallest was 2.66x10(11) N.m. the Slow Initial Phase was noted on 182 seismograms, visiblein order to remove the influence as a gradual and extremely slow increase of the signal amplitude directly before the P-wave first arrival. This increase could be related to the attenuation of the signal propagating from the source to the receiver. In order to remove the influence of inelastic signal attenuation on the path from the source to the receiver, of the recording equipment and of other factors on the initial part of the seismograms of the analyzed events, numerous simulationswere carried out, allowing for appropriate corrections to the duration time of the Slow Initial Phase. These corrections did not have any significant impact on the final results, which leads to the conclusion that the Slow Initial Phase originates in the seismic source and is not effect related to the propagation of the wave from the source to the station.