Santa Monica tsunami forecast: everything you should know

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Tonight, I read a report outside of my area of expertise and will attempt to summarize what I learned in a scientist's version of a book report.

The topic is tsunamis and the risk they pose. #9 in NOAA/Pacific Marine Environmental Laboratory (PMEL) Tsunami Forecast Series deals with Santa Monica! (The entire series is available as free pdf downloads.) Santa Monica's report isn't as dramatic as #1 Hilo or #2 Crescent City, but Angelenos can take comfort in being considered one of the top 10 most-at-risk areas in the U.S. (and the one with the highest population).

Who can resist graphics like these? It's catnip for a scientist.

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Studies such as these require large teams of people working with large quantities of software and data. However, each report has a different lead author within the larger team. Even though they use substantially the same methodology and software, the lead authors' personality peeks through in the individual reports. Diego Arcas, a former Angeleno, wrote #9.

Like many nonlinear fluid models, high-resolution very realistic tsunami models take a long time to run — even on large computers. So, how can NOAA provide tsunami forecasts instead of hindcasts? They can take two approaches: run high-resolution simulations of hypothetical cases that span the range of possible scenarios, or run lower-resolution simulations that approach fidelity as much as time and computational resources allow. In these reports, they do both. (The 'reference' model is the high-res one.)

Arcas et al ran tsunami simulations using input from a database of past seismic events. They used both a high-resolution model and the faster, simpler model and then compared the outputs of both to data from a network of tide gauges. They then simulated 19 hypothetical (synthetic) cases along the perimeter of the Pacific Ring of Fire and reported those results.

Important point #1

The Santa Monica Bay is not at its highest tsunami risk from nearby earthquakes. It is most vulnerable to earthquakes in the south Pacific, synthetic cases #16 and #14, the catnip pictured above. This is good news because it gives us more time to prepare and move out of the way.

Important point #2

Past tsunamis have hit Santa Monica during low tide. If the tsunamis had hit at high tide, water levels would have been 1-2 meters higher and inland inundation would have been more severe. In other words, do not count on our luck holding up. See NOAA Tide Tables for Santa Monica Bay.

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Imagine adding 1-2 meters to these water levels.

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Important point #3

The Channel Islands are a wild card. Under certain circumstances, the tsunami's energy can reflect off the mainland coast, ricochet off one of the Channel islands and then head back towards the mainland again. If the waves set up to reinforce one another, the water levels could go even higher.

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Recall that waves can interfere with one another constructively, or destructively. Your noise-canceling headphones use destructive interference. Remember the aftermath of the Northridge earthquake in which one block could be devastated and one block could remain intact? The seismic waves reflected off the nearby granite mountains and interfered with one another. Look at the interference pattern below. Do you want to bet on where the waves will hit?

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Important point #4

Local landslides, such as ones that have occurred off the Palos Verdes Peninsula, can generate tsunamis. Because the source of the wave energy would be close, we would have little or no warning AND the energy would be coherent (not have time to dissipate).

Heavy rain and/or earthquakes precipitate landslides. Add some flooding from El Nino-fueled rain and we have the makings of a disaster movie.

Important point #5

If you live more than 15 meters above mean sea level, and sea level doesn't rise more than 4-5 meters in your lifetime due to global warming, you can probably sleep soundly. But don't let plausibility get in the way of your disaster movie plot. This could be the next Sharknado.

Aside

I now divide my time between LA and Boulder, Colorado, where I work as a geophysical data specialist. The National Science Foundation provides not only free data, but also a team of science-trained data consultants to help you make sense of data. We serve university, industry and government researchers. I'm puzzled why more journalists don't use our services.


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