Unraveling the Causes of the Seismicity Induced by Underground Gas Storage at Castor, Spain

The offshore Castor Underground Gas Storage (UGS) project had to be halted after gas injection triggered three M4 earthquakes, each larger than any ever induced by UGS. The mechanisms that induced seismicity in the crystalline basement at 5-10 km depth after gas injection at 1.7 km depth remain unknown. Here, we propose a combination of mechanisms to explain the observed seismicity. First, the critically stressed Amposta fault, bounding the storage formation, crept by the superposition of well-known overpressure effects and buoyancy of the relatively light injected gas. This aseismic slip brought an unmapped critically stressed fault in the hydraulically disconnected crystalline basement to failure. We attribute the delay between induced earthquakes to the pressure drop associated to expansion of areas where earthquakes slips cause further instabilities. Earthquakes occur only after these pressure drops have dissipated. Understanding triggering mechanisms is key to forecast induced seismicity and successfully design deep underground operations. Plain Language Summary Underground fluid injection for energy-related activities usually induces microseismicity (not felt earthquakes). But felt earthquakes are sometimes induced, which may cause public concern and project cancellation. This was the case of the offshore Castor underground gas storage (UGS), Spain. Gas injection induced numerous seismic events, including three with magnitudes around 4, larger than any other earthquake ever induced by UGS. These earthquakes occurred after the stop of injection in the crystalline basement, which is hydraulically disconnected from, and significantly deeper than the storage formation. To explain this seismicity, we propose a combination of mechanisms. First, the Amposta fault, bounding the site, crept (failed slowly, aseismically) because of pore pressure buildup induced by injection, which reduces the effective compression stabilizing faults, and stress variations caused by gas buoyancy. Aseismic slip of the Amposta fault provoked underground displacements, which reactivated a critically stressed unmapped fault in the crystalline basement. We attribute the delay between earthquakes of the sequence to rock expansion, and stabilizing pore pressure drop, in areas where slip displacement might cause new earthquakes. Earthquakes occur only after the pressure drop dissipates, which takes some time. Assessing fault stability prior to gas injection would have identified the risk of inducing seismicity.