The National Weather Service’s National Tsunami Warning Center (NTWC) in Palmer, Alaska says there’s no threat of a Pacific-wide tsunami from a strong 7.4 earthquake that struck Tonga earlier today. According to USGS, a magnitude 7.4 quake struck near the Tonga Islands at a depth of 132 miles at 9:02 am Pacific Time today. “There is no tsunami danger for the U.S. West Coast, British Columbia, or Alaska,” the NTWC said in a bulletin released shortly after the earthquake. The earthquake was initially rated a magnitude 7.6 event before being corrected downward to 7.4.
In addition to issuing bulletins for Alaska, British Columbia, Washington, Oregon, and California, the NTWC also issued a separate bulletin to Hawaii announcing no tsunami threat. “Based on all available data, a destructive Pacific-wide tsunami is not expected and there is no tsunami threat to Hawaii,” NTWC’s Hawaii-specific bulletin read.
Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. Out in the depths of the ocean, tsunami waves do not dramatically increase in height. But as the waves travel inland, they build up to higher and higher heights as the depth of the ocean decreases. According to the National Ocean Service, the speed of tsunami waves depends on ocean depth rather than the distance from the source of the wave. Tsunami waves may travel as fast as jet planes over deep waters, only slowing down when reaching shallow waters. While tsunamis are often referred to as tidal waves, this name is discouraged by oceanographers because tides have little to do with these giant waves.