Seismicity at the Castor gas reservoir driven by pore pressure diffusion and asperities loading

The 2013 seismic sequence at the Castor injection platform offshore Spain, including threeearthquakes of magnitude 4.1, occurred during the initial filling of a planned Underground GasStorage facility. The Castor sequence is one of the most important cases of induced seismicity in Europe and a rare example of seismicity induced by gas injection into a depleted oilfield. Here we use advanced seismological techniques applied to an enhanced waveformdataset, to resolve the geometry of the faults, develop a greatly enlarged seismicity catalogand record details of the rupture kinematics. The sequence occurred by progressive faultfailure and unlocking, with seismicity initially migrating away from the injection points,triggered by pore pressure diffusion, and then back again, breaking larger asperities loaded tohigher stress and producing the largest earthquakes. Seismicity occurred almost exclusivelyon a secondary fault, located below the reservoir, dipping opposite from the reservoirbounding fault.