The Perugia (Italy) earthquake of April 29,1984: a seismic survey

International audienceA field study after the Perugia earthquake of 29 April 1984 provided more than 300 well-recorded events concentrated within two parallel clusters separated by 2 km and trending along the Apenninic direction. The length of the aftershock area is 14 km, focal depths being shallower than 8 km. Relocation of the main event places the epicenter at the southern end of the aftershock zone, suggesting a rupture propagation from SE to NW. Most focal mechanisms are consistent with normal faulting. The spatial distribution of seismicity suggests that the Gubbio normal fault was activated during the main shock. This earthquake, together with the Norcia 1979 and the Abruzzi 1984 shocks, is typical of the extension in the high Apennines generated by the flexure of the mountain chain in response to regional compression. The Parma 1983 event, a thrust, belongs to the compres- sion zone at the eastern flank of the chain. These results are consistent with the EW continental collision along the Apennines

Rapid response to the earthquake emergency of May 2012 in the Po Plain, northern Italy

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Injection tests at the EGS reservoir of Soultz-sous-Forêt. Seismic response of the GPK4 stimulations.

International audienceThe EuropeanEnhancedGeothermalSystem(EGS)programofSoultz-sous-Forêtsisorganizedaroundthree wells(GPK2,GPK3,andGPK4)drilledtoadepthofabout5000m.Hydraulicstimulationswereperformed inordertoincreasetheinjectivityofthereservoirandtheconnectivityamongthewells.Thestimulation ofGPK4wascarriedoutintwostages,inSeptember2004andinFebruary2005,followedby anacidificationtestperformedinMarch2005.Thestimulationsproducedfewerinducedeventsthanthose oftheotherwells,withinterpretationremainingdifficult.Inthisworkwepresentsomenewobservations ontheseismicityoftheGPK4stimulationsafteracompletereviewoftheseismicbulletinscollected in2004and2005.Furthermore,theeventswererelocatedusingthedoubledifferencemethod.The newimagesoftheseismicityarepresentedintemporalsequencesaccordingtothemainvariationsof theinjectionparameters.Theseismiceventsthatoccurredduringthe2004stimulationaregroupedin adensecloudandcenteredonthewellopen-holesection,whilein2005seismicitydepictsaspecificpattern suggestingthatthe“natural”stressfieldinthereservoirwasnotcompletelyrestored.Finally,theevents recordedduringtheacidifiedtestshowthatthereservoirbehaveddifferentlyfromthepreviousinjections, whichsuggeststhatadifferentmechanismhascontrolledtheinducedseismicity

Injection tests at the EGS reservoir of Soultz-sous-Forets. Seismic response of the GPK4 stimulations

The European Enhanced Geothermal System (EGS) program of Soultz-sous-Forets is organized around three wells (GPK2, GPK3, and GPK4) drilled to a depth of about 5000 m. Hydraulic stimulations were performed in order to increase the injectivity of the reservoir and the connectivity among the wells. The stimulation of GPK4 was carried out in two stages, in September 2004 and in February 2005, followed by an acidification test performed in March 2005. The stimulations produced fewer induced events than those of the other wells, with interpretation remaining difficult. In this work we present some new observations on the seismicity of the GPK4 stimulations after a complete review of the seismic bulletins collected in 2004 and 2005. Furthermore, the events were relocated using the double difference method. The new images of the seismicity are presented in temporal sequences according to the main variations of the injection parameters. The seismic events that occurred during the 2004 stimulation are grouped in a dense cloud and centered on the well open-hole section, while in 2005 seismicity depicts a specific pattern suggesting that the "natural" stress field in the reservoir was not completely restored. Finally, the events recorded during the acidified test show that the reservoir behaved differently from the previous injections, which suggests that a different mechanism has controlled the induced seismicity

Seismic response of the fractured and faulted granite of Soultz-sous-Forets (France) to 5 km deep massive water injections

The European Enhanced Geothermal System (EGS, formerly Hot Dry Rock, HDR) programme of Soultz-sous-Forets is organized around three wells drilled to a depth of about 5000 m. Hydraulic stimulations were performed in the wells in 2000 (GPK2 well), 2003 (GPK3 well) and 2004 and 2005 (GPK4 well). The stimulation of GPK2 induced more than 700 seismic events with a magnitude greater than 1.0. The seismicity depicts a dense, homogeneous cloud, without any apparent structure. Medium-size earthquakes represent more than 80 per cent of the cumulative seismic moment. The b-value of the Gutenberg and Richter law is larger than 1.2. The injectivity has been increased by a factor 20. These characteristics indicate that the stimulation reactivated a 3-D dense network of fractures. The stimulation of GPK3 induced only about 250 events with a magnitude greater than 1.0 but with a greater proportion of large events, up to 2.9. The hypocentres form clear structures identified as large faults, the b-value is about 0.9 and the large events (M > 2.0) account for the greater part of the cumulative seismic moment. The injectivity of the well, which was already high before the stimulation, remained unchanged. The stimulation of GPK4 was achieved in two stages. This stimulation produced even less induced events, making the interpretation difficult. The differences between the seismic response of GPK2 and GPK3 are due to the presence of large faults cut by GPK3 or in its close vicinity and reached by the injected water. Once a seismic event occurs on a fault, a sequence of earthquakes is triggered and the seismicity behaves, for a large part, independent of the injected flow rate. The stimulations also show some evidence that creeping could be a major source of deformation, if not the main one. The future EGS programme will have to drill wells in zones free of large faults to avoid poor hydraulic performance and inconvenience to the population