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.

A powerful earthquake off Russia’s coast triggered tsunami waves that reached across the Pacific, prompting evacuations as far as Chile and Peru. Experts explain how sudden shifts in the ocean can send waves traveling more than 500 mph thousands of miles away.

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