POROELASTIC STRESSING AND INDUCED SEISMICITY NEAR THE LACQ GAS-FIELD,SOUTHWESTERN FRANCE

Hundreds of shallow, small to moderate earthquakes have occurred near the Lacq deep gas field in southwestern France since 1969. These earth quakes are clearly separated from tectonic seismicity occurring in the Pyrenees, 25 km to the southwest. The induced seismicity began when t he reservoir pressure had declined by approximately 30 MPa. Repeated l eveling over the field shows localized subsidence reaching a maximum o f 60 mm in 1989. Segall (1989) suggested that poroelastic stressing, a ssociated with volumetric contraction of the reservoir rocks, is respo nsible for induced seismicity associated with fluid extraction. To tes t this model, we compare the observed subsidence and hypocentral distr ibutions with the predicted displacement and stress fields. We find th at the relationship between average reservoir pressure drop and subsid ence is remarkably linear, lending support to the linear poroelastic m odel. Displacements and stresses are computed based on a priori knowle dge of the reservoir geometry, material properties, and reservoir pres sure changes. The computed vertical displacements are found to be in e xcellent agreement with the subsidence observed from leveling. Stress perturbations accompanying gas extraction, computed using the same par ameters, are found to be approximately 0.2 MPa or less. Changes in Cou lomb failure stress are computed assuming that slip occurs on optimall y oriented planes. The predicted failure zones correlate very well wit h the spatial distribution of earthquakes if the perturbing stresses a re small in comparison to the ambient regional deviatoric stresses and if the minimum regional compressive stress axis is vertical. Accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to importa nt inferences about the crustal stress state.