Authorities have declared a state of emergency on a second Greek island as a series of earthquakes continued to rattle the south-eastern Aegean Sea region.

The emergency measure announced on the island of Amorgos follows a similar decision for nearby Santorini on February 7 after thousands of unusual tremors were registered in an undersea area

The Greek island has a long history of earthquakes, but what is causing the recent series of quakes has scientists perplexed.