A massive 8.9 magnitude earthquake off the coast of Japan triggered the tsunami that reached the West Coast Friday morning. It was caused by a shift in the sea floor where two plates of the Earth’s crust meet. One section of the so-called subduction zone rose suddenly, pushing a massive column of water upward.

“And that energy has to go somewhere,” said Solomon Yim, a professor of civil engineering at Oregon State University and the acting director of the school’s Hinsdale Wave Research Laboratory.

The resulting wave rushed away from the earthquake at speeds approaching 500 mph, he said. Flying from Tokyo to the West Coast takes about 10 hours, Yim noted. The first tsunami surges hit the coast about 91⁄2 hours after the quake.

As a tsunami crosses the ocean, the visible wave itself is pretty small — six inches, maybe two feet at most, Yim said. But when it gets to shallower waters, the length of the wave is squashed, forcing the wave height up.

And how a tsunami hits the shore is dependent on the area’s bathymetry — the ridges and bumps and dips in the sea floor off the coast.

“Crescent City is always getting the brunt of these distant events,” said Pat Corcoran, hazards outreach specialist with the Oregon State University Extension Service and Sea Grant. “It’s the topography of the sea floor.”

And just south of Crescent City, an underwater ridge called the Mendocino Fracture Zone stretches for thousands of miles west of Cape Mendocino. The ridge acts as a guide for the incoming tsunami, concentrating the wave’s energy and directing it to the northern California coast.

“That feature of the sea floor got it on a track,” Corcoran said. “And so for these distant events, Crescent City always gets it worst.”

The sea floor north of the Mendocino ridge is shallower than the south side, said Harry Yeh, a professor of civil and construction engineering at OSU who studies tsunamis. That can trap the wave around the ridge, possibly focusing energy toward Mendocino and Crescent City.

And with the tsunami rushing to shore, Crescent City is hit repeatedly as waves reflect off of the headlands to the north and south, Yim said.

“Every time there’s a wave that hits directly, it will bounce to Crescent City,” he said. “Other places get hit with one wave; Crescent City gets double, triple, quadruple.”

While most of the Oregon Coast avoided major damage, that doesn’t mean coastal communities can be lulled into thinking they are prepared for a local earthquake and tsunami, Corcoran said.

The Cascadia Subduction Zone, which runs off the coast from Northern California to British Columbia, has the potential to cause a magnitude 9 earthquake, which would cause shaking for three to five minutes followed by a tsunami 15 to 30 minutes later, he said.

Friday’s tsunami warnings and evacuations “is a scenario that really is insignificant compared to the scenario we have to prepare for,” Corcoran said.