Dengler

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For those of you still with me, here is a more detailed explanation. Our biggest tsunami threat (big in terms of water height) is the Cascadia subduction zone. The CSZ is a fault system that extends from Cape Mendocino to Vancouver Island in Canada. There are multiple lines of evidence that the most recent earthquake occurred on Jan. 26, 1700 and ruptured the whole zone in an earthquake of about magnitude 9.

Humboldt County’s tsunami signs, maps and information materials are all based on this worst-case Cascadia earthquake. The reason is simple — with a Cascadia earthquake, you need to respond without any official notification or guidance. The earthquake will cause damage and disrupt communication networks in addition to roads and other infrastructure. Don’t count on cell phones, commercial media and the emergency alert system to work. You need to recognize that the long duration shaking is your warning and take action on your own.

Here’s where it may get confusing. Scientists approximate the CSZ rupture zone as a plane roughly 650 miles long and 40 to 70 miles wide. It breaks the seafloor near the continental slope and dips gently eastward. At Cape Mendocino, the edge is less than ten miles offshore. It’s about 40 miles offshore of Crescent City, 60 miles off of Central Oregon and nearly 80 miles west of the Washington coast. If the Cascadia rupture averages 50 miles in width, much of it is beneath land in Humboldt County. Coastal Humboldt is on top of it. Where I sit in McKinleyville, the CSZ fault plane is eight miles beneath me. In Willow Creek, the depth is closer to 15 miles.

A tsunami is caused by deformation of the sea floor. Short of asteroids, subduction zone earthquakes produce the most widespread tsunamis.

Most subduction zones are offshore and the slip during a great earthquake deforms a lot of sea floor. The greater the slip and the more water atop it, the bigger the tsunami. We are unique — perhaps a quarter to a third of the Cascadia rupture zone in Humboldt County is beneath land. That means not as much water volume to be displaced as in Oregon and Washington.

This has consequences for both tsunami generation and earthquake shaking. We WILL have a tsunami. That tsunami will consist of numerous surges lasting hours and could reach heights of twenty to thirty feet along much of the Humboldt County coast. While likely not as high as what will hit Oregon and Washington, this is still a major tsunami. Much of the Sendai coast in Japan experienced water heights in this range in 2011 and I personally saw the damage it caused.

Although our next CSZ tsunami may not be as high, it will arrive more quickly. The source zone is closer to us than further north. First surges could reach us in only ten minutes. Residents of coastal areas in Northern Oregon and Washington may have twice as much time. And, because we are atop the rupture surface, it means much stronger shaking.

We will be getting new tsunami maps in two months. The good news is that in most of Humboldt County, the hazard zone is roughly the same and in the few areas where it has been changed, the hazard zone has become smaller. If you have a family tsunami plan, bravo to you and it is still good to go.

Lori Dengler is an emeritus professor of geology at Humboldt State University, an expert in tsunami and earthquake hazards. All Not My Fault columns are archived at https://www2.humboldt. edu/kamome/resources and may be reused for educational purposes. Leave a message at 707-826-6019 or email Kamome@humboldt.edu for questions/comments about this column, or to request a free copy of the North Coast preparedness magazine “Living on Shaky Ground.”