Mechanism of the ML4.0 25th April 2016 Lacq (SW France) earthquakerelated to the gas extraction field

There were 11 earthquakes of magnitude larger than 3.8 in metropolitan France in 2016 according to the availablecatalogue EMSC. Among them, the ML4.0 25th April 2016 earthquake is suspected to be induced in gas exploitationfield of Lacq, south-western France, close to the Pyreneans. The area is briefly covered by French nationalbroad-band and acceleration networks and the data are available from ORFEUS/EIDA (European Integrated DataAchieves, http://www.orfeus-eu.org/data/eida/index.html ). In order to understand the mechanism of this earthquakeand the state-of-art of the reservoir, it is essential to study the reliability of the focal depth and mechanism.We carry out a moment-tensor inversion using Genetic Algorithm. We select three broad-band stations at distanceof about 50 km and applied a bandpass filter between 16 and 32 seconds. Green functions are calculated for the 1Dlayered structure.We fix the epicenter position determined by ReNaSS (Reseau National de Surveillance Sismique,http://www.renass.unistra.fr) and change the possible focal depths. The inversions show a good convergence to asolution of Mw3.8 indicating EW running normal faulting. This mechanism is consistent with the reservoir geometryof the gas field and may be interpreted as subsidence of the reservoir. However the obtained solution doesnot fit the nearest station, located a few hundred meters from the epicenter, probably due to lack of the preciseepicenter position and/or local structure.