Why Even Small Tsunamis — and 1-Foot Waves — Can Wreak Havoc in California

On Wednesday, I was supposed to be out on assignment on a kayak off the San Francisco shore, recording a radio interview on the waters of McCovey Cove as the San Francisco Giants played a day game.

But after an 8.8-magnitude earthquake occurred off the coast of Russia’s Kamchatka Peninsula on Tuesday afternoon, regions across the Pacific Ocean, including all of the Northern California coast, were given notice about potential tsunami waves.

Suddenly, being out on a kayak that day — in a location marked as “hazardous” on the state’s tsunami danger map — didn’t seem like such a good idea.

While “tsunami” can conjure images of towering waves, these swells caused by quakes are frequently much smaller. On Tuesday night, the National Weather Service predicted the waves that would hit San Francisco early Wednesday morning would be less than a foot high, and it has now measured them at 1.2 feet,  with no damage reported near the Bay Area so far.

So, how unsafe would my kayak trip really have been?

Experts say that even puny waves can have a big impact. Lori Dengler, a tsunami expert from Cal Poly Humboldt, said even if 1 foot sounds manageable, the dangers are hiding in the deep.

“It’s not how high the water is, it’s how strong that water is flowing in and out,” she said.

• Jump straight to: What should you do — and not do — when smaller tsunami waves hit?

How dangerous can these ‘small’ tsunami waves be?

As it turns out, even “small” waves can pose big hazards.

Dalton Behringer, a meteorologist with the National Weather Service’s Bay Area office, said that a 1-foot swell may sound small, but when you add that foot to normal tide behavior, the swings from high to low can give a tsunami that extra push and boost overall wave heights much higher.

Think of it as not so much a single wave, Behringer said, but “as going from a normal low tide to a normal high tide,” — and this happening fast. Such a shift would usually happen over six hours, he said, but in a tsunami, such a rise “happens over 10 to 15 minutes,” Behringer said.

Following the huge 2011 earthquake in Japan that sent 9.0-magnitude shocks across the Pacific Ocean, a 1-foot tsunami caused significant damage to boats docked all the way in Santa Cruz.

Jim Pruett, general manager of Pillar Point Harbor in Half Moon Bay, said the first, second or even third waves from a tsunami should not be taken lightly. That’s because tsunamis are more like powerful swells than waves and don’t “crash,” meaning each wave can do substantial damage.

“It may be only a 12-inch wave, but there is a lot of water behind that wave,” Pruett said. “It’s traveled across the Pacific Ocean, and when the swell hits, it doesn’t stop. All that water continues to come in, so it surprises people.”

In a place like Pillar Point, which is well protected due to being a natural harbor, small swells like these usually only mean the harbor fills and empties like a bathtub, Pruett said. Still, he advised always to heed warnings from local authorities if they advise staying out of the water, away from beaches or to evacuate.

Another thing to remember: sometimes the later tsunami swells are even worse than the initial ones, Behringer said.

“The highest [swell] that we’ve seen so far has come a few hours after the initial wave,” Behringer said. “With tsunami waves, we can see the wave energy actually build for several hours after the initial waves and then dissipate.”

Pamalah MacNeily, owner at Blue Waters Kayaking in Tomales Bay, said she canceled a kayak tour on Wednesday as a precaution,  just like I did. It isn’t the first time MacNeily’s done so after a tsunami warning, either, even when the weather looks nice and the bay seems fairly unaffected.

“We have to be cautious,” she said. If a tsunami came in from just the right direction, Tomales Bay “would be crushed,” MacNeily said.

“We have to be cautious with people’s lives, and we also obey the advisories,” she said.

What should you do — and not do — when smaller tsunami waves hit?

“Please don’t go and try to look at them,” Dengler said.

Instead, she and others advised going online to the National Oceanic and Atmospheric Administration’s website to watch the swells, since they are few and far between — and may not even look like much to the naked eye.

“This type of tsunami is actually arriving relatively slowly,” she said. “There is really nothing dramatic to see.”

A one-foot swell might not sound that bad in a kayak or near shore. But even if you assume you’d be able to handle it, the real dangers are lurking below and around you in the currents — and the dangerous objects, like boats, that might get tossed in your direction.

“Once the swell hits the shoreline, all that water has to retreat back to the ocean,” Pruett said. “So the normal currents will be extremely strong, ripping people or structures back out to sea.”

Swimmers and people in small crafts should get themselves and their boats out of the water. Conversely, “the safest place for a larger vessel is out at sea,” Pruett said, pointing to Tuesday night’s mass evacuation of boats off Honolulu’s shores into the ocean, where they can more adeptly handle large swells and won’t be tossed into the shoreline.

Pruett said during a volcano-triggered tsunami in 2022, the swells hit at high tide, when the water levels were already high — damaging the Santa Cruz Harbor, which is artificially constructed — and therefore less resilient to tsunami-like swells.

“Even though it was less than a foot, it still raised the harbor about 3 to 5 feet. Because when it hit the shore, it built up,” he said.

How can I prepare for a future tsunami?

The good news: Pruett said the San Francisco Bay is especially resilient to tsunami-like waves and that “the mouth of the Golden Gate will significantly knock down any large swells.” Nonetheless, in a future tsunami, “large amounts of water will still come into the Bay,” he said.

And if you needed another reason to take tsunamis seriously, this simulation shows the effect that just one tsunami wave could have on the bay.

Luckily, there’s a lot you can do to prepare for if and when the next tsunami advisory happens. First of all, make sure you sign up for alerts; know the difference between a tsunami warning, watch and advisory; and have an evacuation plan just in case.

Read our full guide on how to prepare for a possible tsunami.

And which coastal areas were more affected by Wednesday’s tsunami?

In California, Crescent City in Del Norte County was the most vulnerable, seeing waves of up to 4 feet.

Many residents were prepared and even pulled their boats out of the water on Tuesday night in anticipation of any large swells.

In 1964, Crescent City was the site of the worst tsunami recorded in the United States, which killed 11 people, injured 24 and wiped out 29 city blocks.

Cal Poly’s Dengler said so far on Wednesday, she’s seen up to five feet in Hilo, Hawaii, and 15 feet on the coast of Russia.

“It’s not just the surge coming across the Pacific and that first wave stopping,” she said. “A tsunami always generates a long train of waves, but then when it hits the coast, it reacts to the shape of the shoreline, the shape of the continental shelf and the shape of bays.

“All of those different shapes set up oscillations and vibrations,” she said. “You end up, especially at Crescent City, with amplification of waves.”

KQED’s Ted Goldberg and Juan Carlos Lara contributed to this report.