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How dangerous are tsunamis and what should I do if one is headed this way?

A crowd of people and first responders monitor the lower end of Santa Cruz Harbor after a tsunami surge last Saturday.
Jerimiah Oetting
A crowd of people and first responders monitor the lower end of Santa Cruz Harbor after a tsunami surge last Saturday.

Last weekend’s tsunami raised as many questions as it did boats. While the Santa Cruz Harbor suffered $6.5 million damage, it’s nowhere near the $20 million done by the last tsunami 10 years ago. There were no deaths locally and for much of the Monterey Bay area, Saturday’s tsunami was little more than king tides with a lot of promotion.

The tsunami, however, did bring a flood of questions. How vulnerable is Monterey Bay to a catastrophic tsunami? What are the areas most likely to be affected? And when they say “get back from the beach,” how far back do you need to go?

For answers, KAZU's Doug McKnight spoke with Dr. Gary Griggs. He is a Distinguished Professor of Earth and Planetary Sciences at the University of California Santa Cruz. He told me the risk of a catastrophic tsunami hitting Monterey Bay is very low.

This conversation has been edited for length and clarity.

Gary Griggs (GG): California as a state in the last 200 years has had 19 tsunami deaths. That's like less than dog bites and bees and wasps. So it's a really low hazard for, you know, dying in a tsunami. The biggest was in Crescent City in 1964. Santa Cruz had one death in a 1946 tsunami. It was an older guy standing in the intertidal zone when the wave came in.

So we have to be smart. We have a warning system in place now that says, you know, don't stand down on the waterfront. I'm going to say that our risk here versus almost anything else is much, much lower.

Doug McKnight (DM):  Where do tsunamis come from?

GG:  The ones that have hit us most often have been these big earthquakes in Japan and Alaska. And it's sort of like a tide coming in all at once rather than every six hours. The normal wind waves we see along the beach have links between crests of, you know, several hundred to several thousand feet. A tsunami is 90 to 100 miles. And these wind waves, we see the surfers are taking advantage of, may move at 10 or 20 miles an hour, maybe a little faster. Tsunamis move at four hundred and fifty to five hundred miles an hour or at the speed of an airliner.

DM: The two most recent tsunamis damaged the Santa Cruz Harbor. Why was it hit so hard?

GG: Well, because it's an enclosed channel it's sort of like a funnel in a sense and it just starts, you know, picking up the boats. Same thing happens in Crescent City. I think Monterey Harbor is protected by a breakwater and I don't think it's had any significant damage whereas in Santa Cruz it can keep going right up that channel. And also when the momentum of that wave hit the back end of the harbor, that's when it actually came up into the low areas on either side and did some flooding.

DM: The advisories on Saturday said over and over, if you are near the beach get back. How far back should people go?

GG: Well, you know, when the Japan tsunami came, they did a reverse 911 one call and it went out all over. I was on my way to Sacramento that morning and when we got to the crest of Highway 17, there were hundreds of cars. And I said, “what's going on?” Well, it said, “go to high ground,” so they went up to 1800 feet.

But when you look at where the tsunamis reached in most places, I think that's a pretty short distance. I think what you don't want to be doing necessarily is standing on the shoreline. I mean, if you're 10 feet above sea level, you're probably in pretty safe conditions.

DM: Is it maybe not a good time to go surfing?

GG: Well, there were a lot of people out in the water and there was this high school surfing contest I heard about at pleasure point.

I think people think you're going to see these beautiful 10 or 20 foot high waves, but that's not really what happens. You know, you just get this huge surge like the tide coming in very quickly. I think some of the kids over there in the high school surfing contest had trouble getting back out of the water because the tsunami hit at high tide, which moved the water up higher than it would have been had it been a low tide.

DM: Where do you not want to be during the next tsunami?

GG: I guess where I wouldn't want to be is on a little boat in the harbor. I don't know what happened in Capitola, which is very low lying and has been flooded [during] high wave and high tide conditions.

So I think the lowest areas of the river mouth, the harbor, Capitola, are the places that I would just be careful.

DM:  Should people who live on Monterey Bay prepare for the next tsunami like we do for earthquakes?

GG: I'd say we have a lot of other much higher risk things we do every day, like texting while we're driving. But people don't think about that. They just keep doing it.

I think in terms of tsunamis, the historical record is 200 years, I'd say it's just not a high risk. And if you know that if there's one that's on the way, get 10 feet higher, don't stand near the ocean and watch for it to come in. I think that's probably the best information or the best warning I could give.

Dr. Griggs added that is not to say the ocean isn’t dangerous. For example, while tsunamis are low risk, climbing on shoreline rocks during a storm can be very dangerous.

GG: In this last winter, eight people died falling off of rocks and getting carried off beaches by wave. So there's a lot of other things you can do that aren't very smart. Be careful. The ocean bats last.

Doug joined KAZU in 2004 as Development Director overseeing fundraising and grants. He was promoted to General Manager in 2009 and is currently retired and working part time in membership fundraising and news reporting at KAZU.