A seismologist is warning people along parts of the California and Oregon coast that current tsunami warnings should be taken “very seriously.”

Maureen Long, who is also a professor of earth and planetary sciences at Yale University, said the most important thing is to “be aware.”

The National Tsunami Warning Center issued a warning after a powerful 7.0 magnitude earthquake struck off the coast of Northern California on Thursday. Nearly 5 million people are subject to this warning from Davenport, California, to Douglas/Lane Line, Oregon, the agency said. These include the San Francisco Bay Area, Eureka, Fort Bragg and Crescent City, California.

Forecasts indicate tsunami activity is now likely to reach the coast. Long said the reason for this delay – between the earthquake and possible tsunami impacts – is because earthquake waves travel faster.

A tsunami wave “typically travels about as fast as a jet plane in the open ocean, whereas earthquake waves travel much faster,” she said. This delay means it is easier for experts to issue tsunami warnings, Long said.