The last mega-quake struck the PNW 300 years ago: When will the next Big One strike?

The last known megathrust earthquake rocked the Pacific Northwest 321 years ago.

And as seismologists have warned, we are due for another one like it.

The state Department of Natural Resources notes these megathrust earthquakes - coming in at a magnitude-9.0 - happen every 300 to 600 years.

The last Big One also whipped up a devastating tsunami along the Pacific coast of Japan. We know as much as we do about the quake that struck in 1700 thanks, in part, to unfortunate witnesses there.

The devastation was enormous.

"People in Japanese communities along the coast noticed the water rise up. They called it the orphan tsunami - January 26, 1700," says Sandi Doughton, who covers science for The Seattle Times. "When that fault ruptured, the coastline dropped several feet.

Doughton is also author of the book “Full Rip 9.0: The Next Big Earthquake in the Pacific Northwest.”

She says we have "absolutely no idea" when the next mega quake will happen - and that has made it hard to convince people to invest in infrastructure for a calamity that may not hit even in their children's lifetimes.