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.

Officials from Japan to California issued warnings after an 8.8-magnitude quake off Russia’s coast. Hours later, there were no immediate reports of major damage, although some areas remained on alert.

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