One of the most powerful earthquakes in modern history shook Russia’s remote east coast Wednesday, sending tsunami waves hurtling across the Pacific Ocean and putting nations from Japan to the United States to Chile on high alert with millions urged to evacuate.

Tsunami alerts were issued for Alaska and Hawaii on Tuesday after a major, 8.8 magnitude earthquake struck off the coast of Russia, according to officials.

Russia’s magnitude 8.8 earthquake spawned serious tsunami warnings, but waves have been moderate so far. Here’s the geological reason why