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Laurie Dengler | How could the Cascadia earthquake be? – Standard Times

Laurie Dengler |  How could the Cascadia earthquake be?  – Standard Times


“Lori, you’re scaring me,” said an old friend after reading my column last week. He was probably speaking in jest as this was nothing new to him and he knew perfectly well that fear was never my intention.

Fear is a great way to sell newspapers and magazines and a terrible motivator to take preparedness action. Don’t take my word for it – my colleague Anna Marie Jones did a thorough study of the topic a decade ago ( and concluded that fear messages are pervasive in our culture, ineffective, and there are ways Better to promote action in our woefully unprepared audience.

If there is no fear of getting your attention, ignoring the elephant in the room won’t work either. And there are no two ways about it; Cascadia is the elephant of earthquakes and tsunamis in our North Coast communities. So, no hyperbole here. Reducing risk isn’t magic but we can do it. And sometimes an inflatable elephant helps.

When I talk about Cascadia, I mean an earthquake in the upper 80s to a low magnitude 9 scale ripped a rift from southern Humboldt County to Vancouver Island, Canada. Three things must be emphasized: you will almost certainly survive, the affected area will be massive, and it will take many years to reach the new normal after the disaster.

Start with the scale. I’ve lived on the North Coast since 1978, felt many earthquakes, and studied a lot. I’m known as an “earthquake expert” but for earthquakes the size of the Cascadia, to paraphrase Joni Mitchell, I don’t know earthquakes at all.

It is difficult to understand how large a region would be directly involved in the Cascadia earthquake. It was likely strong enough to cause damage in California, Oregon, Washington, and British Columbia and was felt in Nevada and Idaho. But the damage will not be uniform, and crumbling roads and bridges will isolate communities from one another.

The West Coast has experienced a number of devastating earthquakes in the past 50 years, but in all of them, the impact area has been determined, leaving nearby areas relatively unscathed. Firefighters, police and medical responders can reach most people who need help within hours. North Coast residents long used to being helpless and isolated for days at a time, but new residents are finding this year’s weather break a surprise. Convert outage days into weeks (add water and gas as well) to get an idea of ​​what might happen after Cascadia.

The 10-mile-wide fault in the December 20 earthquake increased energy in the high-frequency range. But the error was too short to produce longer periods. The Cascadia rupture will extend for several hundred miles, resulting in longer durations as well. And unlike the December 20 M6.4, it won’t expire in 10 seconds.

Frequency is important in earthquakes. A sharp short rupture in December produced vibrations that likely affected single-family homes and mobile homes. Longer periods affect larger structures such as tall buildings and bridges. The vibration spectrum is also important in liquefaction. The distance from the fault rip also modulates the vibrato, filtering out those high notes the farther you get. It can travel longer, too, which can cause damage in Redding, Sacramento, and San Francisco.

Let me take you back to 1964 for a taste of what it was like to be near the ground zero of a Cascadia-sized earthquake. Robert Pate worked at a radio station and used to carry a portable radio with him and whenever anything of interest happened, he would turn it on and start talking. He was at his home in Anchorage during the M 9.2 Alaska earthquake and had the presence of mind to convert for a few seconds to the earthquake (

Several highlights about Pat’s recording: he survived, he wasn’t hurt, he was scared, the shaking lasted a very long time, his house was standing but falling things put him to shame, the power went out, and he was able to make good decisions. What the recording didn’t include was the amount of time he spent alone after that. It took days to restore facilities and the area was dependent on airborne aid for weeks and it took years for Anchorage and other communities in southern Alaska to reach the new normal.

Anchorage had a population of 100,000 in 1964. There were only 15 deaths from vibration damage, and most were related to slope failure when permafrost liquefied. This was a time when many buildings were not built to earthquake standards. But wooden buildings are great at holding together in case of strong shaking, and even if the foundation is damaged, it provides the safety of life.

Fast forward 48 years to 2011. March 11th marks 12 years since the Great East Japan Earthquake and Tsunami. We know much more about this earthquake than we did in 1964; It is the most widely used and analyzed earthquake of the M9 scale in history. Powerful motion sensors, geodetic sensors on the shore, and a network of seismometers on the ocean floor provided minute details of the rupture process.

But like Anchorage in 1964, the shaking of the ground caused few casualties. The US Geological Survey estimates that more than 2 million people have experienced earthquakes similar to what caused devastation in Turkey and Syria. Non-tsunami deaths were less than 0.3% of that exposed group. The buildings did not collapse. Most of the deaths caused by vibration were due to fires and landslides.

Shaking was not the main cause in 1964 and 2011. What killed people was tsunamis, especially tsunamis that were larger than expected. Alaskan Natives were well aware of tsunamis and many remote communities recognized the shaking as a warning to head for higher ground. But the new residents didn’t have that mentality.

Everyone in Japan knows about the tsunami. I was at a tsunami meeting in Japan just six weeks before the 2011 earthquake where scientists, engineers, and public officials explained how the system of seawalls, vertical evacuation shelters, evacuation drills, and tsunami notifications set an example for the rest of the world. Unfortunately, their planning was based on a tsunami triggered by a magnitude 8 earthquake that flooded their 9.1 estimate.

The North Coast Tsunami maps are based on the Cascadia source and use the best information currently available on how high and how far tsunamis penetrate. They include an additional safety factor to be on the conservative side. The best way to remember 1964 and 2011 and face the elephant in our room is to look at maps and say if you live, work, or visit the tsunami zone. It’s easy to do at

Note: In order to generate interest without waving the flag of fear, we’ve made a series of PSAs with KEET, our public TV station. Three of the PSAs featured Ellie, an inflatable elephant. You can view them at, just scroll down to the “Elephant in the Room” links.

Laurie Dengler is Professor Emeritus of Geology at Cal Poly Humboldt, and an expert on tsunami and earthquake risk. Questions or comments about this column, or want a free copy of Living on Shaky Ground readiness magazine? Leave a message at 707-826-6019 or send an email to [email protected].




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