Coupled processes to explain induced seismicity: the case of the Underground Gas Storage of Castor, Spain

Abstract

Pressure buildup is the standard, but often insufficient, explanation for induced seismicity, , especially when it comes to delayed effects. Coupled processes may play relevant roles, so that they must be accounted for to find the mechanisms triggering induced seismicity, especially when it comes to delayed effects. We illustrate the importance of coupled processes in induced seismicity through the case of the Underground Gas Storage (UGS) project of Castor, Spain, where, cushion gas injection induced hundreds of events, with maximum magnitudes of 4.1, leading to the cancellation of the project. Gas injection lasted for 15 days, and the largest earthquakes occurred 17 days after the stop of injection. Injection overpressures had dissipated by the time of the largest events. The gas was injected at 1.7 km depth, while the induced seismicity occurred at depths ranging from 4 to 10 km. These characteristics of the induced seismicity at Castor pose a challenge on explaining its causes. Coupled processes provide a plausible explanation when combining poromechanical stresses, buoyancy, and shear-slip stress transfer. If coupled processes had been considered in the assessment of the induced seismicity at Castor, the induced earthquakes at Castor could have been anticipated and managed.