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.

Hours after a massive 8.8 magnitude earthquake struck near the Russian coast, becoming one of the strongest tremors ever recorded, tsunami waves arrived in Hawaii and the U.S. West Coast following efforts to get hundreds of thousands away from coastlines and to higher ground.

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