Earthquakes rattled Greece’s volcanic island of Santorini every few minutes overnight and into Wednesday as authorities bolstered emergency plans in case the hundreds of temblors over the past few days are a harbinger of a larger quake to come.

Crescent-shaped Santorini is a premier tourism destination with daily arrivals via commercial flights, ferries, and cruise ships. The island draws more than three million visitors annually to its whitewashed villages, built along dramatic cliffs formed by a massive volcanic eruption more than 3,500 years ago.