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.

Schools on the Greek tourist island of Santorini will remain closed on Monday after more than 200 small earthquakes shook the region, civil protection authorities announced. Santorini is one of the most popular Greek islands for tourists.