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.

A warning of increased seismic activity in Santorini has been issued in a Foreign Office travel advisory. The alert reflected advice from the Greek Ministry of Civil Protection for the Aegean island and its smaller neighbours of Anafi, Ios and Amorgos, following more than 200 tremors registered in the area over 48 hours.

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.