Analysing 50 yr of the Lacq induced seismicity (Southwestern, France) highlights the role of fluid injection

Abstract

SUMMARY The Lacq area in southwest France has been associated with continuous moderate induced seismic activity since 1969. However, the mechanisms driving this induced seismicity are not fully understood: reservoir depletion has been proposed as the main factor, and more recently wastewater injection has been suggested to play a more important role. The interpretation of these mechanisms relies heavily on the quality of earthquake locations, which we prove to be weak due to a lack of local instrumentation for several years. In order to provide the most complete and reliable induced event catalogue for the studies of the Lacq induced seismicity mechanisms and seismic hazard, we made an exhaustive compilation, analysis and improvement of all available catalogues. We also provided new earthquake detections and relocations in a 3-D velocity model from past and present temporary deployments never used for studying the Lacq area. Important remaining location uncertainties lead us to also carefully sort the events according to their location confidence, defining 3 classes of events (unconstrained location, location constrained within 2–3 km and 1–2 km, respectively). This new harmonized catalogue and the identification of well-constrained events, covering 50 yr of induced seismicity, allow us to propose that wastewater injection is almost certainly the main mechanism driving the seismicity, with (i) most of the constrained events located within the reservoir boundaries and (ii) the released seismic energy variations following variations in injection operations at different scales. In particular, we have also highlighted a change in the injection–seismicity relationship around 2010–2013. From 2013, despite lower injection volumes, seismicity remained persistent and some clusters of earthquakes were detected predominantly in spring, summer and early autumn, except in winter periods. From 2016, we observed a strong temporal relationship between days with higher rate/volume injections (approximately above 400 m3 d−1) and both clustered events and higher magnitude earthquakes (greater than 2.4).