Abstract

Ten years of seismic recording allows us to locate more than 800 tremors with magnitudes ranging from 1.0 to 4.2, in a small (10 by 10 by 5 km) volume near a major gas field (Lacq, France). The seismic activity began 10 yr after gas extraction began. Nearly all of the epicenters are located within the lateral extent of the gas field. The only perturbation of the stress or strain fields, which has the same spatial and temporal scale as the seismicity, is the 500-bar drop in gas pressure due to 20 yr of gas exploitation, demonstrating that these events are induced. The network, established to monitor this activity, consists of eight telemetered stations including a well-bottom geophone located at 3200 m depth. Using three-dimensional relocation techniques, we computed new hypocenter positions for more than 280 events. These better-located hypocenters form a diffuse zone that mimics the local dome structure. Taking the regional tectonic stresses into account, we suggest that these ruptures may be the result of unfolding of the dome structure.

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