Alaska's earthquake late Monday night caused quite a scare to residents near the southern coastline -- awoken by tsunami alerts, urging them to evacuate, scrambling to pack and head to higher ground and emergency shelters. All alerts were canceled several hours later.
So why didn't a tsunami happen?
Tsunamis are caused from an abrupt displacement in a large displacement of water in the ocean. This displacement is caused by a sudden disturbance on the ocean floor, either by an earthquake or a submarine volcanic eruption or a rare meteorite impact. These waves can travel very quickly -- at speeds of up to 600 miles per hour, quickly becoming a matter of life or death for those who live on the coastline in its path.
Tsunamis are often caused by earthquakes, but not every earthquake causes a tsunami. That's because not every earthquake forces a large upward thrust in the ocean floor.
According to the USGS, today's earthquake was a "strike-slip" quake:
"The January 23, 2018 M 7.9 earthquake southeast of Kodiak Island in the Gulf of Alaska occurred as the result of strike slip faulting within the shallow lithosphere of the Pacific plate...That type of seismic event doesn't usually result in big tsunamis, unlike earthquake along subduction zones where one plate essentially rises off another abruptly, pushing up a lot of water."
The Great Alaska Earthquake of 1964, which led to tsunamis, landslides, and over 100 fatalities did occur at a subduction zone, which often leads to more of a vertical thrust and displacement of land and a much higher risk for tsunami waves.