Government officials believe there's a direct link between the temblors and injections of fuel gas into the storage system in Castellon.
The geographical institute said Saturday there have been 139 minor earthquakes in 10 days—a substantial increase compared to the weeks before that. During that same period, only 27 small quakes were felt in other areas of Spain.
Some of the earthquakes have exceeded magnitude 4.0, prompting regional prosecutors to open an investigation into the plant, which will help supply the Valencia region. Inspectors were sent to the site Thursday.