The M 4.5 Vallorcine (French Alps) earthquake of 8 September 2005 and its complex aftershock sequence

Julien Fréchet (1) , François Thouvenot (2) , Michel Frogneux (1) , Nicolas Deichmann (3) , Michel Cara (1)

1 IPGS - Institut de physique du globe de Strasbourg
2 LGIT - Laboratoire de Géophysique Interne et Tectonophysique
3 Schweizerischer Erdbebendienst

On 8 September 2005 a moderate M 4.5 earthquake occurred in the north-western Alps midway between Chamonix (France) and Martigny (Switzerland). The focal mechanism corresponds to a right-lateral strike-slip on a N60°E fault plane. The foreshock-mainshock-aftershock sequence is investigated on the basis of data recorded by a temporary network of 28 stations deployed for 1 month just after the mainshock, and data from permanent, regional seismic networks. Absolute and relative locations of more than 400 events are obtained with a mean uncertainty of approximately 0.2 km. Small foreshocks, the mainshock, and early and late aftershocks are located relative to the main aftershock set. The seismic sequence exhibits a surprisingly complex structure, with at least five clusters on distinct fault planes. The main elongated cluster agrees with the location of the mainshock, its hypocenter being 4.3 km below sea level. We discuss the relationship between the right-lateral fault beneath the Loriaz peak (the source of the Vallorcine event), the nearby normal Remuaz fault, and the regional seismotectonic stress field.

Mots clés: 2005 Vallorcine earthquake; Aftershocks; Hypocenter relocation; Similar waveforms; Western Alps seismicity; Remuaz fault

Domaines: Planète et Univers [physics]

Julien Fréchet, François Thouvenot, Michel Frogneux, Nicolas Deichmann, Michel Cara. The M 4.5 Vallorcine (French Alps) earthquake of 8 September 2005 and its complex aftershock sequence. Journal of Seismology, 2010, 15 (1), pp.43-58. ⟨10.1007/s10950-010-9205-8⟩. ⟨hal-00617322⟩