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Strain localization and fluid migration from earthquake relocation and seismicity analysis in the western Vosges (France)

Abstract : To understand the mechanisms of intraplate earthquakes, well-documented seismological and geodynamical studies are needed, to provide information for adequate mechanical modelling. This paper describes a detailed analysis of the earthquake sequence associated with the Rambervillers (France) M∼ 5.4 earthquake (2003 February 22), which occurred in the western Vosges massif (France), some tens of kilometres to the north of the sites of the Epinal (1973-1974) and Remiremont (1984-1985) earthquake sequences. We computed the location of the mainshock and focal mechanism, together with the double-difference locations of 419 aftershocks of the subsequent earthquake sequence, which included 195 well-recorded similar events (earthquakes with similar waveforms). We combined pP phases recorded by remote dense seismic networks with waveform modelling, to accurately determine the mainshock depth (12.5 ± 1.5 km). Computations of focal mechanisms from regional waveform inversions showed a normal fault plane with a N315°± 10° strike and a 45°± 15° dip. A detailed space-time analysis allowed two earthquake sequences to be identified: a classic mainshock-aftershock sequence and a secondary sequence that began 250 days after the mainshock. More events were recorded during the secondary sequence than during the main sequence. Very few similar events were recorded during the first 2 days after the mainshock, although data were recorded continuously. Double-difference locations from traveltime differences show that these immediate, non-similar aftershocks occurred in a 4 km × 2 km subhorizontal area at a depth of about 12 km. Similar events were far more numerous during the secondary earthquake sequence. Double-difference relocation of these similar events revealed a N315° striking, 65° dipping fault plane that is compatible with the mainshock source mechanism. During this later sequence, seismicity oscillated over a vertical range of 2-3 km around a mean depth of approximately 12 km. This observation strongly suggests fluid migration. Normal faulting, subhorizontal faulting and fluid transfer at about 12 km depth can be interpreted in terms of the crust's response (mainshock and post-seismic relaxation) to flexural stresses induced by the alpine compression. This relaxation involves weakening and collapse of the crust, and fluid transfer.
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Submitted on : Monday, March 19, 2012 - 12:17:34 PM
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Jean-Luc Got, Vadim Monteiller, Jocelyn Guilbert, David Marsan, Yves Cansi, et al.. Strain localization and fluid migration from earthquake relocation and seismicity analysis in the western Vosges (France). Geophysical Journal International, Oxford University Press (OUP), 2011, 185 (1), pp.365-384. ⟨10.1111/j.1365-246X.2011.04944.x⟩. ⟨insu-00680364⟩



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