On March 11, 2011, a massive earthquake shook Japan, triggering a deadly tsunami that wreaked havoc both along the Japanese coastline and an ocean away.

Now, a decade later, scientists are using lessons learned from that disaster to reevaluate the risk posed by such rare but high-impact tsunamis.

Scientists in California are striving to better understand the range of possible tsunami threats posed by earthquakes, and are reaching deeper into the geologic record to estimate how coastal communities in the United States might flood if another “great” earthquake materializes.

Lessons learned in 2011

The magnitude 9.1 earthquake that struck Japan in 2011 ruptured an undersea boundary known as a subduction zone, along which one tectonic plate dives beneath another. The resulting tsunami waves ravaged the island nation, and traversed the Pacific to cause damage thousands of miles away in California.

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

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

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

Expanding tsunami zones

When the March 2011 earthquake struck, Japan was well prepared for a tsunami, but had underestimated the extent and severity of inundation in many of its evacuation maps.

“Japan has put more effort into tsunami and earthquake hazard reduction than any other country in the world, but they had primarily relied upon the 400 years of written records to estimate the size and frequency of future seismic events,” Dengler said.

Yet there was geologic evidence of something bigger in Japan’s distant past: a magnitude 8.6 tremor more than 1,000 years earlier.

“That earthquake … is now fairly well known, but in 2011 hadn’t made it into the planning process,” she said. “That has changed in both Japan and the U.S., where using a 1,000-year event has now become standard procedure.”

California, for example, is updating its tsunami hazard maps, which were originally drawn in 2009, by modeling a suite of realistic tsunami scenarios from both nearby and faraway sources. Simulated sources include local offshore faults, landslides and distant earthquakes in Alaska, Chile and Japan.

In addition, the Cascadia subduction zone, which scientists fear could someday rupture and unleash a significant quake and tsunami, lurks nearby and extends from Vancouver south to Cape Mendocino in Northern California.

“Since 2009, we have a great deal more information, we have better tools in mapping tsunami hazards, and we understand tsunami sources a lot better,” said Rick Wilson, a senior engineering geologist with the California Geological Survey, which produced the state maps.

Wilson and his colleagues have expanded the maximum inundation line in some communities to reflect new reasonable worst-case scenarios: a magnitude 9.0 earthquake on the Pacific Northwest’s Cascadia subduction zone and a magnitude 9.3 earthquake on the Alaska-Aleutian Islands subduction zone. Both are plausible, yet very rare, scenarios, with a 1,000-year return interval, or a 5 percent chance of happening in the next 50 years.

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

While the most significant impacts of a Cascadia rupture would be felt in far Northern California, Oregon and Washington, the Alaska earthquake scenario would direct ocean wave energy farther south, flooding much of California’s low-lying coastline and requiring the evacuation of 300,000 to 400,000 people.

Half Moon Bay, south of San Francisco, could see inundation up to 50 feet high, as would other parts of Northern California. Southern California beaches, such as Huntington Beach in Orange County, could see 15 to 20 feet of flooding.

Official warnings for distant tsunamis

If waves from a distant source are traversing the ocean, NOAA’s tsunami warning centers in Alaska and Hawaii provide state emergency managers with data on expected wave heights so officials can decide which areas to evacuate, and then notify the public via wireless emergency alerts, local tsunami sirens or other means.

Because NOAA’s system relies on offshore buoys to measure waves, there can be considerable delay in assessing the tsunami threat in real time.

Yvette LaDuke, the tsunami planning coordinator with the California Office of Emergency Services, said that during the time spent waiting for information from the National Tsunami Warning Center, the state is in contact with local officials about how big the event might be, and whether they should set plans in motion, like closing beaches and harbors.

“What we can do in the interim is start taking protective 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 problems, with critical information often buried in a string of bulletins. Neither warning center provides a “bottom line” on exactly what threat exists and where either.

“The current plethora of messages whenever an event occurs makes it difficult for even those of us who understand the system to sort through the current status for all regions,” Dengler said.

Nature’s warning: the earthquake

Waves traveling across the Pacific take several hours to arrive, but if a tsunami is generated close to shore, there will be little time to muster an official warning. The public should therefore know to move to high ground or inland if there is long or strong shaking at the coast.

That’s the purpose of this week’s tsunami outreach events in California, with many education efforts at the state and local levels geared toward making residents aware of the warning signs and encouraging them to practice evacuation routes.

Jason Ballmann, communications manager for the Southern California Earthquake Center, coordinates the website TsunamiZone.org, which this year registered more than 500,000 participants in evacuation drills and other activities in California, the Caribbean and other tsunami-prone regions.

“We are trying to build a culture of preparedness, in which people can be inspired by seeing and hearing others in their community taking action to be safer from tsunamis,” he said.

This training is particularly important in the Cascadia region, where a devastating tsunami could arrive in as little as 10 minutes after an offshore quake.

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