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. mechanisms that seismicity in the crystalline basement at 5–10 km depth 1.7 remain unknown. Here, we propose a combination of explain observed seismicity. First, critically stressed Amposta fault, bounding storage formation, crept superposition well-known overpressure effects and buoyancy relatively light injected gas. This aseismic slip brought an unmapped fault hydraulically disconnected failure. We attribute delay between earthquakes pressure drop associated expansion areas where slips cause further instabilities. Earthquakes occur only these drops have dissipated. Understanding triggering is key forecast successfully design deep underground operations.