According to the US Geological Survey (USGS), the quake occurred 94 kilometers southeast of Sand Point, Alaska, and was initially recorded as a magnitude 7.4 quake.
— The Weather Channel (@weatherchannel) October 19, 2020
A second 5.8 magnitude earthquake 115 kilometers south-southeast of Sand Point, Alaska, was recorded shortly after the initial quake, followed by a series of strong earthquakes ranging between magnitude 3.6 and 5.8.
A tsunami warning effect is also in effect for South Alaska and the Alaska Peninsula from Kennedy Entrance, Alaska, to Unimak Pass, Alaska.
Video footage shared on social media captured the moment in which tsunami sirens were triggered near Sand Point, Alaska. The National Weather Service's Pacific Tsunami Warning Center reported late Monday that a 2-foot-high tsunami was observed at Sand Point.
— SV News 🚨 (@SVNewsAlerts) October 19, 2020
Large earthquakes can cause tsunamis or long high sea waves when slabs of rock on the seafloor abruptly move past each other, vertically displacing the overlying water. According to USGS, earthquakes with magnitudes between 6.5 and 7.5 do not typically cause "destructive tsunamis."
"However, small sea level changes might be observed in the vicinity of the epicenter. Tsunamis capable of producing damage or casualties are rare in this magnitude range but have occurred due to secondary effects such as landslides or submarine slumps," the USGS explains.