Mechanism of the ML4.0 25 April 2016 earthquake in southwest of France in the vicinity of the Lacq gas field

Abstract

The source mechanism of the ML 4.0 25 April 2016 Lacq earthquake (Aquitaine Basin, South-West France) is analyzed from the available public data and discussed with respect to the geometry of the nearby Lacq gas field. It is one of the biggest earthquakes in the area in the past few decades of gas extraction and the biggest after the end of gas exploitation in 2013. The routinely obtained location shows its hypocenter position inside the gas reservoir. We first analyze its focal mechanism through regional broad-band seismograms recorded in a radius of about 50 km epicentral distances and obtain EW running normal faulting above the reservoir. While the solution is stable using regional data only, we observe a large discrepancy between the recorded data on nearby station URDF and the forward modeling up to 1 Hz. We then look for the best epicenter position through performing wave propagation simulations and constraining the potential source area by the peak ground velocity (PGV). The resulting epicentral position is a few to several km away to the north or south direction with respect to station URDF such that the simulated particle motions are consistent with the observation. The initial motion of the seismograms shows that the epicenter position in the north from URDF is preferable, indicating the north-east of the Lacq reservoir. This study is an application of full waveform simulations and characterization of near-field ground motion in terms of an engineering factor such as PGV. The finally obtained solution gives a moment magnitude of Mw 3.9 and the best focal depth of 4 km, which corresponds to the crust above the reservoir rather than its interior. This position is consistent with the tendency of Coulomb stress change due to a compaction at 5 km depth in the crust. Therefore, this earthquake can be interpreted as a relaxation of the shallow crust due to a deeper gas reservoir compaction so that the occurrence of similar events cannot be excluded in the near future. It would be necessary to continue monitoring such local induced seismicity in order to better understand the reservoir/overburden behavior and better assess the local seismic hazard even after the end of gas exploitation.