Tsunami risk higher than expected in some coastal cities

The risk of powerful and destructive tsunamis in earthquake-prone zones like Los Angeles and Kingston is higher than previously thought, according to new research by geologists who studied this year's earthquake in Haiti.

The risk of powerful and destructive tsunamis in earthquake-prone zones like Los Angeles and Kingston is higher than previously thought, according to new research by geologists who studied this year's earthquake in Haiti.

A paper this month in the journal Nature Geoscience explains why these -- and other coastal cities -- should prepare for tsunamis. I spoke last week with Matt Hornbach, the study's lead author and a research associate at the University of Texas at Austin's Institute for Geophysics.

Did the earthquake in Haiti lead you to conduct this research or had you already been working on the project?

I'd just started working on the same fault system, but on the section further west where it intersects with Jamaica. In the past year, I conducted preliminary studies and looked through the literature on past earthquakes and tsunamis in the area. I started trying to understand why there were tsunamis in Jamaica. It was a real surprise to me. Then the [Haiti] earthquake occurred. The timing of my interest and that earthquake seemed to coincide.

Why were places like Haiti and Los Angeles not considered a high tsunami risk?

Traditionally the places that are considered the most at risk for tsunamis are near large subduction zones where one plate is pushed underneath another plate. They typically generate large earthquakes. With large earthquakes, the plates can cause a disruption in the water, which can generate the wave. We know from the Sumatra tsunami that these sorts of settings can generate these large waves. But you don't really think of tsunamis associated with places like the San Andreas Fault. Much of the San Andreas Fault runs on land. This is true of Haiti as well. You don't expect it to generate a very large wave. Anytime it shifts the ground or moves the surface, it's not pushing too much water around.

How did you determine that the tsunami risk in those areas is higher than expected?

It was a multi-faceted approach. This is really only for Haiti that I can say this, but I think it's probably a fair bet that similar situations apply in California. We took an approach in Haiti where we [first looked for] the most likely causes of tsunamis there. We collected data to see where the fault was and what the fault did. Did it have any vertical displacement to push up water? It had a little bit. There was clear evidence of near-shore slope-failure that correlated very well with the tsunamis. That was the more typical approach, actually going to look at data.

The real surprise in this study is if you look through the history books you see descriptions that are very clear, particularly in Jamaica, where they describe land sliding out. [That is] the exact same experience that occurred in Haiti where there was a landslide near the shore or on the shore. The water went out very quickly and came back in very quickly, as well. It turns out that these are real stories that are accurate. They're telling you that frequently landslides occur during earthquakes in this area that generate tsunamis.

What's the next step?

It's really critical that we educate people on the risk of tsunami in places like this. People might not know that when you see the water go out, you probably shouldn't stay near the shore. You need to head uphill. People need to try to avoid building in these places that are prone to failure. What we found in Haiti, as well as Jamaica, was the areas that fail are pretty consistent. Areas that fail are usually in river delta areas where there's recent, rapid accumulation of sediments that are not very cohesive. Because of this, when there's a relatively minor earthquake you can generate tsunamis. It's really important that countries near and on these faults pay close attention to where they build and how they build, so they can help minimize casualties from these sorts of events. Awareness and planning is really important.

Image: Matt Hornbach

This post was originally published on Smartplanet.com

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