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 M 4 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...