California may be more vulnerable to tsunamis than previously thought

Lessons learned in 2011

A magnitude 9.1 earthquake that struck Japan in 2011 tore apart an undersea boundary known as the subduction zone, where one tectonic plate sinks below the other. The resulting tsunami swept across the island, crossing the Pacific Ocean, causing damage thousands of miles into California.

In an email, Lori Dingler, emeritus professor of geophysics at Humboldt State University wrote: “We convinced us in 2011 that every subduction zone in the world is capable of producing an earthquake in the low 9-degree range and we should plan accordingly.”

To mark state tsunami preparedness week this week, the California Geological Survey is releasing new tsunami hazard maps that expand the danger zone inland in some areas. Emergency management officials will also work through major event response exercises, and test communication systems to ensure they are running smoothly.

Despite improvements in planning and awareness, challenges remain in preparing for a tsunami, particularly for more local “near-field” events, which can reach shore minutes after a major earthquake.

Tsunami expansion areas

When the March 2011 earthquake struck, Japan was well prepared for the tsunami, but it has underestimated the size and intensity of the floods on many evacuation maps.

“Japan has done more to reduce tsunami and earthquake risks than any other country in the world, but it has mainly relied on 400 years of written records to estimate the size and frequency of future seismic events,” Dingler said.

However, there was geological evidence for something bigger in the distant past of Japan: a magnitude 8.6 earthquake more than 1,000 years ago.

“This earthquake … is now known to some extent, but in 2011 it hadn’t gone into the planning process,” she said. “That has changed in both Japan and the United States, where using a 1,000-year event is now standard procedure.”

California, for example, is updating tsunami hazard maps, which were originally drawn in 2009, by modeling a range of realistic tsunami scenarios from near and far sources. Simulated sources include local marine faults, landslides, and distant earthquakes in Alaska, Chile, and Japan.

Additionally, the Cascadia Subduction Zone, which scientists fear may one day explode and unleash a major earthquake and tsunami, lurks nearby and stretches from Vancouver south to Cape Mendocino in northern California.

“Since 2009, we have had a lot of information, we have better tools for tsunami risk mapping, and we understand the sources of the tsunami a lot better,” said Rick Wilson, chief engineering geologist at the California Geological Survey, which produced the state maps. .

Wilson and colleagues have expanded the maximum inundation line in some communities to reflect plausible new scenarios for the worst: a magnitude 9.0 earthquake in the Cascadia subduction zone in the northwestern Pacific Ocean and a magnitude 9.3 earthquake in the subduction zone of the Alaska-Aleutian Islands. Both are plausible scenarios, but extremely rare, with a return period of 1,000 years, or a 5 percent chance of occurring in the next 50 years.

In other words, it likely won’t happen in our lives – but, like many low-probability events, it would have an enormous impact if it did.

While the most significant impacts of the Cascadia rupture would be felt in far northern California, Oregon and Washington, the Alaska earthquake scenario would direct ocean wave energy further south, inundating a large portion of California’s low coast and requiring the evacuation of 300,000 to 400,000 people. .

Half Moon Bay, south of San Francisco, can witness submersion as high as 50 feet, as it does in other parts of Northern California. Southern California beaches, such as Huntington Beach in Orange County, can experience floods from 15 to 20 feet.

Official warnings of distant tsunamis

If waves are from a distant source crossing the ocean, the NOAA Tsunami Warning Centers in Alaska and Hawaii provide state emergency managers with data on expected wave heights so that officials can determine which areas should be evacuated, then notify the public via alerts. Wireless emergency, local tsunami siren or other means.

Since the NOAA system relies on marine buoys to measure waves, there may be a significant delay in assessing the tsunami threat in real time.

Yvette Laddock, tsunami planning coordinator at the California Office of Emergency Services, said that during the time she’s spent waiting for information from the National Tsunami Warning Center, the state is in contact with local officials about the scale of the event, and whether they should put plans in place, such as Beaches and ports are closed.

“What we can do in the meantime is start taking preventative action and make sure the public is out of harm’s way,” she said.

However, for the public and other users, the tsunami.gov portal has a history of messaging issues, with important information often buried in a series of posts. Neither center provides a “bottom line” warning about exactly what threat exists and where either is.

“The large number of messages whenever an event occurs makes it difficult for us, even those who understand the system, to sort out the current state of all regions,” Dingler said.

Nature’s Warning: Earthquake

Waves traveling across the Pacific take several hours to arrive, but if tsunamis are generated close to the shore, there will not be much time to issue an official warning. So the audience should know how to move to higher ground or inland if there is a long or strong vibration on the coast.

That’s the purpose of this week’s tsunami awareness events in California, with many state and local educational efforts directed toward educating residents about the warning signs and encouraging them to practice evacuation routes.

Jason Palman, director of communications at the Southern California Earthquake Center, is coordinating TsunamiZone.org, which this year has registered more than 500,000 participants in evacuation drills and other activities in California, the Caribbean and other tsunami-prone regions.

“We try to build a culture of preparedness, where people can be inspired by seeing and hearing others in their communities take action to be safer from the tsunamis,” he said.

This training is especially important in the Cascadia region, where a devastating tsunami can arrive in as little as 10 minutes after a sea earthquake.

“Everyone who lives in my community and coastal areas of every other community from Humboldt County to British Columbia needs to be self-reliant and secure on their own,” Dingler said. “It’s a drum that many of us have been banging in the Cascadia region for nearly three decades.”

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