Greek authorities have warned of an increase in seismic activity around the Aegean tourist island of Santorini, advising people to avoid four ports, empty their pools, refrain from gathering in indoor spaces and shut schools on Monday.

The strongest earthquake recorded was magnitude 4.6 at 3:55 p.m. Sunday, at a depth of 14 kilometers (9 miles), the Athens Geodynamic Institute said.

Santorini is on high alert following over 200 undersea earthquakes, prompting authorities to close schools and deploy emergency crews. Though the quakes are not linked to its volcano, the increasing seismic activity is concerning,