Dive deeper into the new Humboldt County tsunami maps – Times-Standard

Last Tuesday, the Humboldt County Board of Supervisors unanimously adopted the newly released California Geological Survey (CGS) tsunami maps for Humboldt County. It’s worth spending more time learning what these maps do, what they don’t show, and how to use them.

Tsunamis occur in a number of different ways, but whether they are caused by earthquakes, landslides, volcanic eruptions, or massive storm systems, they all involve displacing all the water over the ocean floor (or lake) and traveling the same way. It is the entire column of water raised or lowered that causes a tsunami and it is gravity that pushes the water out. Tsunami velocity is affected only by the depth of the water. It makes no difference whether displacement of two inches or 20 feet of water, a tsunami travels outward at about 450 miles per hour at average depths in the ocean, accelerating in deeper waters and slowing down as it approaches the coast.

Knowing when the first waves of a tsunami will arrive is straightforward. All that is needed is the time at which the earthquake or other source occurred and the depth of the sea floor along the path that the tsunami travels. The estimated arrival times of the first wave are usually accurate within a few minutes even for sources on the other side of the ocean. But knowing how high and strong spikes are is a more difficult problem.

The first tsunami models appeared in the 1980s when seismologists discovered how to combine earthquake / fault source displacement and water wave generation. There are three main parts to all tsunami models: source dynamics, deep ocean reproduction, and the interaction between tsunamis and coastal terrain. Each tsunami was an opportunity to calibrate and revise the models.

All parts of the model are essential to obtain reasonable results, and like all digital modeling, they rely on good data. There is no more important place than trying to estimate water height and ground penetration. People often ask me how high the next tsunami could be. It’s easy to fall into the trap of thinking a tsunami is a large, uniform wall of water that can be described as a single height. incorrect. The size and characteristics of the source, the amount of water displaced, the shape of the seabed, and the shape of the land are all factors that influence the inundation.

Hilo, Hawaii, and Crescent City, California are sometimes called a “tsunami magnet,” and for good reason. They both suffered more than their fair share of losses from the tsunamis. The sea floor off Hilo Bay is a bit like a large V-shaped hole and it’s easy to see how the incoming tsunami is focused and amplified from afar. Crescent City is a little more complex – the shape of the harbor certainly contributes, but the characteristics of the sea and continental shelf also increase tsunami heights.

The result of all of these variables is that a tsunami wave height can exceed 30 or 40 feet in a single area and it may be less than half that value just a few miles away. I’ve seen this many times after real tsunamis – in Peru, Chile, Papua New Guinea and Japan. The new CGS maps use better topographic data and depth measurements (water depth), resulting in more detailed and accurate maps.

People also ask what specific tsunamis does the Country Maps Model use? The answer is nothing. It is a combination of all the sources that can produce tsunamis along our coast. Designers studied dozens of potential earthquake sources, from the great earthquakes around the Pacific Rim to the Cascadia earthquake below our feet. After running the models, they are combined to produce the worst possible condition that produces the maximum potential internal penetration. This is what appears on the new Humboldt map of water flooding. I prefer to use the new evacuation maps that are more discreet and include an extra safety factor. Don’t worry about the source – the maps just released are meant to help you and your community plan to be tsunami-safe in the worst case.

The only thing that tsunami maps do not show is when the next great tsunami will happen. The CGS team took into account a 1,000-year time window in their planning, so these maps include rare events such as the 2011 Japan Tsunami that older maps did not include. It’s the best information we have right now for developing personal and community evacuation plans.

How about climate change or coastal subsidence during a major local earthquake? The new CGS maps don’t explicitly include sea level rise. However, the current rate of a few inches per decade is well within the added safety factor that evacuation maps include and the maps will be reviewed again in ten years. Landings are included, such as that seen in Japan 2011 and 1964 Alaska. Humboldt County differs from the subduction zone for each of these earthquakes. We are closer to the rupture surface and some models indicate that much of the Humboldt Coast is likely to rise slightly during the next earthquake. Lift not included in the form. Changes in ground level, such as what was observed in Japan in 2011, are also within the map safety factor.

These maps are only a starting point for planning if you need to evacuate the next time you feel an earthquake and how to plan an evacuation route. If you feel more comfortable going into some interior than the map suggests, that’s fine. Just remember to take a walk, make sure everyone in your family knows what to do, and start planning today.

Note: See the new maps at https://www.conservation.ca.gov/cgs/tsunami; Jason Patton’s presentation to the Humboldt County Board of Supervisors is at https://youtu.be/hayNz7j8O7s.

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