A Civil Guard helicopter on Friday landed on the platform of an offshore gas-storage plant located off Spain’s Mediterranean coast, whose activity is believed to be the source of some unusual seismic activity in coastal towns in the area over the past few weeks, police sources and a spokesman for the company managing the plant, Escal UGS, confirmed.

The guards inspected the installations of the plant after the prosecutor’s office in Castellón opened a probe to clarify what is happening at the Castor Project gas plant, which is located 22 kilometers from the coastal town of Vinaròs in Castellón province.

The move by the prosecutor comes a day after it was revealed that the former Socialist government of José Luis Rodríguez Zapatero had failed to investigate the possible seismic impact of the plant’s operations could have despite a request to do so from a number of parties, including the Catalan regional government. Tarragona province, part of Catalonia, borders Castellón.

The central government’s representative in Castellón province, Javier Molina, said he would take action against former ministers in the Zapatero government over responsibility for sanctioning the plant’s activities if it is demonstrated that there was “negligence” involved in the administrative approval for operations.

Minister Soria accepts a “direct relation” between the injection of gas and the unusual seismic activity

The Industry Ministry and the Valencia regional government announced on Thursday that work at the plant would be halted indefinitely until the origin of the quakes in adjacent coastal towns has been established.

The tremors, the latest of which measuring 3.7 on the Richter scale occurred on Friday morning, have sparked protests by the inhabitants of the affected area.

Industry, Energy and Tourism Minister José Manuel Soria said Thursday that there “appears to be” a “direct relation” between the injection of gas into the storage plant and the unusual seismic activity.

Scientists from the National Geographic Institute and the Geological Mining Institute are investigating the causes of the tremors to gauge if they have been sparked by the giant storage plant, where gas is being injected into a depleted oilfield 1,800 meters below the seabed.

According to data from the National Geographic Institute, a total of 23 tremors were detected on Wednesday night. Two of these registered 3.9 and 3.8 on the Richter scale, while on Thursday one quake hit 4.2.

The plant, into which 1.2 billion euros is being invested, plans to store up to a third of Spain’s gas needs for 50 days.