Map: 7.0-Magnitude Earthquake Near Tonga Promps Brief Tsunami Alert

A major, 7.0-magnitude earthquake struck in the South Pacific Ocean on Monday, according to the United States Geological Survey. The quake prompted a brief “Tsunami Threat” for Tonga.

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.

Tsunamis are a series of long waves caused by a large and sudden displacement of water in the ocean, usually from a large earthquake on or below the ocean floor. Tsunamis radiate in all directions from the epicenter and can cause dangerous coastal flooding and powerful currents that can last for hours or days.

The temblor happened at 1:18 a.m. Tonga time about 49 miles southeast of Pangai, Tonga, data from the U.S.G.S shows.

U.S.G.S. data earlier reported that the magnitude was 7.1.

As seismologists review available data, they may revise the earthquake’s reported magnitude. Additional information collected about the earthquake may also prompt U.S.G.S. scientists to update the shake-severity map.

Aftershocks in the region

An aftershock is usually a smaller earthquake that follows a larger one in the same general area. Aftershocks are typically minor adjustments along the portion of a fault that slipped at the time of the initial earthquake.

Aftershocks can occur days, weeks or even years after the first earthquake. These events can be of equal or larger magnitude to the initial earthquake, and they can continue to affect already damaged locations.