The California Geological Survey’s Tsunami Program released an updated version of the tsunami maps for Southern California.
LOS ANGELES COUNTY一 In March of this year, the California Geological Survey updated the Tsunami Hazard Area Maps for seven counties in California, including Los Angeles County.
This is the first time the maps have been updated since 2009 and a few areas saw a minor increase in hazard zones including Marina Del Rey, Port of Los Angeles, Long Beach, and Naples.
The changes to the maps were in part because of a recent tsunami modeling that reflected a large-scale disaster.
“One of the benefits of the new maps is that they incorporate a minimum baseline of flooding equal to a 1000-year event, which translates to a 5 percent probability of flood exceedance within the next 50 years,” said Rick Wilson senior engineering geologist and tsunami unit manager with the California Geological Survey. “In other words, we looked at extremely large but very rare tsunami events to create the maps.”
The 1000-year baseline and the extra buffer were added after research from the 2011 Great East Japan Earthquake and Tsunami, according to Wilson.
On March 11, 2011, Japan experienced a magnitude 9.0 earthquake near the Tōhoku region. The earthquake triggered a massive tsunami that took almost 20,000 lives and flooded more than 200 square miles, according to the World Vision website.
Japan had created evacuation plans based on historical records that went back thousands of years, but those records did not include a 1000-year event like the 2011 tsunami, according to Wilson.
“Early on we worked with colleagues in Japan about what happened there and how we can make it better on our side,” said Wilson. “I was a part of two different groups that went to Japan to look at deposits five months after the tsunami…the damage was hard to see and a very emotional experience, the second time was years later.”
Wilson went back to Japan to look at the recovery efforts and how they could apply to California. Working with colleagues from Japan and the U.S. Geological Survey, Wilson and his team used statewide tsunami analysis, prehistoric modeling and worked with consultants to create a map and model of what a 1,000-year event would look like for California.
The increases in the hazard areas are in part because of the addition of a 1000-year event as a baseline and in part because the original maps followed the landscape rather than roads and landmarks.
“Where the boundary followed the landscape instead of roads and landmarks, some people had difficulty finding their location and they did not know if they should evacuate,” said Wilson. “Moving the Tsunami Hazard Area to a nearby road, landmark, or some other known feature helped the public more easily identify if they were at risk.”
There are primarily two types of tsunamis, a local tsunami that can be generated offshore by events like a submarine landslide, or a distant tsunami that can be generated on the other side of the Pacific.
For the 1000-year scenario, Wilson said the most realistic worst-case scenario is a magnitude 9.3 earthquake off the coast of Alaska that would trigger a tsunami that would hit Southern California in five to six hours.
To prepare for this, Wilson and his team are working with the National Tsunami Warning Center to be able to send out a warning and communicating directly with communities and first responders to understand the risk and evacuation protocols.
“This is a great opportunity for the public to go to our website, and find out if they live, work, or visit areas where there is a tsunami hazard,” said Wilson. “If there is a tsunami threat, take the opportunity to develop or update a plan for you, your family, or your business for evacuating out of the Tsunami Hazard Area.”
The California Geological Society cautions communities to pay attention to the warning signs and to listen to local emergency officials for direction.
They also caution the boating community to try not to navigate their boats out of the harbor during tsunami activity, and if they are offshore, to go beyond a depth of 30 fathoms to avoid tsunami currents.
To learn more, see the California Geological Society’s website at www.tsunami.ca.gov.