Is the Pacific coast prepared for a tsunami?

Southern California saw more seismic activity in 2024 than it had in the past 65 years. Farther north, a 7.0-magnitude earthquake off the coast triggered a tsunami warning for parts of California and Oregon in early December.

A tsunami didn’t come then, but the region’s proximity to the Mendicino fault means it could be hit by one in the future. Lori Dengler, professor emeritus of geology at Cal Poly Humboldt, says that the earthquake’s proximity to land caused concern about a tsunami forming.

“Because this earthquake was so close to the coast, there wasn't the usual time,” Dengler says. “Back in 2011 when the 9.1 occurred in Japan, there were nine and a half hours before the tsunami would have reached us. In this case, the first tsunami waves would have reached us in less than half an hour.”

“The Mendocino Fault is arguably the most active fault in the state of California. Since 1990, it has produced 200 earthquakes of magnitude four or larger. It produces earthquakes in the magnitude 5 and 6 range almost every year.

“It's a plate boundary, very similar to the San Andreas Fault. One side moves horizontally relative to the other like cars on a British freeway.

We call it a right-lateral strike-slip fault. So, we get lots of movement. As far as its tsunami potential is concerned, these magnitude 7 — low 7s, 7.2, 7.3 — have not produced significant tsunamis in the past, and strike-slip earthquakes that move the seafloor horizontally generally don't produce big tsunamis.”

“The Mendocino fault is not a big tsunami problem, but it is very, very close to major faults that do produce a tsunami. This region we call the Mendocino Triple Junction, where we have the Mendocino fault, the San Andreas from the south, and we have the Cascadia subduction zone to the north, all coming together.

“Now, the Cascadia subduction zone is our biggest tsunami threat on the U.S. West Coast. And we've got really good evidence that it last ruptured in 1700. And the magnitude was probably right around nine.”

“There was a tsunami then and we know about that tsunami partly from the oral history along the Pacific Northwest coast. We know it from geology studies of tsunami deposits, but we have the date from Japan because that tsunami was large enough to damage a number of coastal sites in Honshu.

‘So, our date of Jan. 26, 1700, is nailed down. In 1992, we had a magnitude 7.2 on the southern part of the Cascadia subduction zone right near the corner and that did produce about a three-foot-high tsunami.”

“If you're in the zone where the water hits, it has the capacity to destroy everything. And outside that zone, it doesn't affect anything.

“A tsunami is very different than a large storm wave. It's caused by deformation of the seafloor, where part of the seafloor is shoved up and over the other part of the seafloor.

So, once that bulge happens on the seafloor, and it can be a dropping down as well, and usually, both things happen, then the entire water column above it — and if it's in deep water, that's a lot of water — is deformed also, and then falls driven by gravity out in all directions. And we know a lot about how fast it travels. It travels about the speed of a jet plane. The deeper the water, the faster it travels. The shallower the water, it slows down."