A 7.0 magnitude earthquake struck off the coast of Tonga early Monday. The island nation felt six aftershocks but did not immediately report damage.

Preliminary reports from the U.S. Tsunami Warning System said tsunami waves were possible for coastlines within about 190 miles of the earthquake’s epicenter, including those on Tonga. The system issued an all-clear notice about an hour later, after a tsunami would have struck had one materialized.

It subsequently revised the assessment, saying “the tsunami threat from this earthquake has now passed”, but authorities in Tonga issued further warnings.