State publishes new tsunami zone maps with far greater accuracy

See the new Mendocino County tsunami zone map here, and scroll down for details: https://www.conservation.ca.gov/cgs/tsunami/maps/mendocino

FORT BRAGG, 3/23/21 — This Wednesday, for the first time in three years, the mournful wail of the full tsunami siren will be heard in Point Arena and Fort Bragg, says Ryan Aylward, National Weather Service meteorologist and chair of the Redwood Coast Tsunami Work Group. While authorities are worried that some people who hear the full siren might think an actual tsunami is underway, this is also an opportunity to learn about brand new tsunami warning maps for the entire Mendocino Coast and about how tsunami warnings have changed since the 2011 Japanese tsunami hit.

Last week, just in time for Wednesday’s test of the Mendocino Coast tsunami warning system, the California Geological Survey (CGS) released their new tsunami hazard area​ maps, which cover Del Norte and Mendocino counties. Hazard maps for more counties will be released this week, which is officially California Tsunami Preparedness Week, March 22–26. Humboldt County’s map was released earlier.  The brand new maps produced by CGS show areas along California’s entire coast where a tsunami would inundate. The amount of detail in 2009 maps varied but these new maps are much more comprehensive with much more detail provided by new technologies used in making them.

Tsunami evacuation mapping is greatly expanded for the Mendocino Coast with the all-new 2021 maps which were done with an entirely new approach over the 2009 maps. The more detailed methodology revealed larger areas predicted to be inundated in a 1000 year tsunami event — that is, the rare kind of tsunami that is expected only once every millennia. They are designed to show places that might need to be evacuated in the case of the worst tsunami with the warning system under the guidance of local authorities the methodology to be used for lesser events.  Those new areas include properties further up local rivers and streams than previous mapping explored. Another place now partly in the maps that wasn’t before is the Ocean Lake Adult Mobile Home Park just north of Fort Bragg, said Jay Patton, who helped create the new maps as part of his job as engineering geologist at CGS. Several oceanfront campgrounds are also in yellow evacuation areas in the new maps.

The 2021 maps will soon cover the entire coast of California, much of which has never been mapped for tsunami risk. In Mendocino County, the last set of maps, which were completed in 2009, mostly mapped tsunami risk around places like Fort Bragg and Point arena — and even in such areas that were previously mapped, errors and omissions were found by the new high-tech mapping.

“Most of Mendocino county is not as susceptible to a tsunami as are places like Crescent City and Half Moon Bay,” said Patton. This is due mostly to the tall cliffs and bluffs that protect much of the Mendocino Coast, although underwater topography and other factors such as the angle of the coast also play a role.

The new maps show elevation changes over just one meter where the old maps showed elevation changes every 5 – 10 meters (16 – 33 feet), Patton explained. The greatly expanded and more accurate mapping of elevation pinpoints more risk areas, such as around the Garcia River north of Point Arena. The 2021 maps were created using lidar (a light mapping system, similar to radar, which is included in many modern phones), as opposed to the old method of having people interpret aerial photos. Patton said this has resulted in some large disparities between the old and new maps. For example, the area around the Point Arena lighthouse is actually 30+ feet higher in elevation than the old maps indicate, Patton said in the interview. 

Patton said there are two tsunami warning systems for those living in the yellow areas of the map, which indicate tsunami evacuation zones. The first is natural. “If you feel an earthquake for a long time, it’s time to get to higher ground,” said Patton. The second kind is the official warning system that will be tested on Wednesday.

How does the official test work?

COVID-19 canceled the 2020 tsunami warning drills, and in 2019, a federal government shutdown prevented a full test of the system. Tsunami sirens are turned on for short “growl” tests on a regular basis, Aylward said, however, authorities are concerned that people who hear the eerie wail of the siren for the full test on Wednesday may think an actual tsunami is coming.

“The key message we want to get out is this is only a test,” said Aylward.

First will be a reverse 911 call to go out at 10.a.m, only to those in impacted areas. Along with the siren, a fixed wing civil air patrol flight down the coast will be broadcasting the message that test is of the system underway.  The message blared from the air won’t mention tsunamis and has been simplified this year, Mendocino County Emergency Services Director Brentt Blaser said.  Blaser said the decision about whether the flight will be launched will be made this afternoon, based on weather and other factors.  The flight is planned to cover the area from Del Norte County to the Mendocino County line, where broadcasting will stop, with the plane to land in Sonoma county.  A second siren, to be activated from a dispatch center may also be heard, Blaser said.

Blaser said Mendocino County will not be part of the Emergency Alert System this time, although Del Norte and Humboldt are part of EAS this time, Blaser said.  That means there will be “bleed over” from the north in emergency messaging but it won’t be done on every local radio and television network. The Wireless Emergency Alert (WEA) system that comes across smart phones will not be activated for this test.

The public is asked to give feedback on Wednesday’s tsunami warning test to Aylward be emailing him at ryan.aylward@noaa.gov

See the Mendocino County map here:

https://www.conservation.ca.gov/cgs/tsunami/maps/mendocino

People who live along coastal rivers and streams may find a surprise in the new maps, which show a yellow tsunami evacuation zone, and a green safe zone. Enter any Mendocino County address in the search bar and it will come up in green or yellow, with the risk coding based on the 1000 year tsunami possibilities studied from different tsunami origin points, such as Japan, Chile, Alaska or the Pacific Northwest’s Cascadia Fault. The 1000 year tsunami would not have any consistent height or size, depending on factors ranging from the tides to the shape of the Coast. One can then scroll around and see where the yellow danger areas are located. Evacuation from a tsunami should be on foot when possible, as roads may be impassable by car. 

An example of an area that comes up on the new maps in yellow is parts of the Ocean Lake Adult Mobile Home Park North of Fort Bragg. Although Highway 1 provides protection to some extent, the walking tunnel under the road there negates it from being a true seawall, Aylward said.  

The maps are the start of a process to be led by the North Coast Tsunami Work Group and Mendocino County Emergency Services Director Brentt Blaser in which people living in hazard areas will be educated about risk and evacuation plans.
“This [senior living] park is one of those places where we will want to have education and outreach about the changes in the maps, “ Patton said.

A tsunami is a wave series created by an undersea disturbance, most commonly an earthquake. Although tsunamis usually come from the simple up and down motion of the sea floor in an earthquake, there is much about how they work that might seem counter intuitive. Different kinds of earthquakes have different effects. A strike and slip fault, like the San Andreas fault, which goes to sea at Point Arena, creates less tsunami risk than one in a subduction zone. The most likely destructive tsunami  for Northern California areas south of Cape Mendocino would likely come from a major earthquake in Alaska, Patton said. The Mendocino Coast would have just 3 to 4 hours to prepare for such a tsunami. Another possibility is that any earthquake could cause an undersea landslide, which would then create its own tsunami.

Rick Wilson, head of the CGS tsunami program, said in a press release that the yellow and green areas have a margin of error.

“There is some uncertainty when we draw the inundation lines,” Wilson said. “The maps are based on the best data we have, but there’s a margin of error when you’re trying to consider a thousand-year event, so we err on the side of caution. The [one meter] buffer zones included in the [yellow areas of the] new maps account for that uncertainty, “ Wilson wrote.

The Redwood Coast Tsunami Workgroup has developed evacuation brochures for Humboldt and Del Norte counties using the maps, and plans to release them for Mendocino County by April, Aylward said.

More than 150 tsunamis have hit California’s coast since 1800. Many are barely noticeable but nearly a dozen tsunamis have caused fatalities or significant damage, most recently during the March 11, 2011 tsunami generated by a magnitude 9.1 earthquake off Japan. After traveling 10 hours across the Pacific Ocean, the Tōhoku-oki tsunami caused $28 million in damage in Del Norte County, more than $4 million in damage in Mendocino County to Noyo Harbor, and $100 million statewide.

Noyo Harbor has one of the highest tsunami risks of anywhere along the North Coast, which does not change in the new maps. The 2011 tsunami featured after-midnight heroics by local boat owners and the Noyo Harbor Commission. The commission staff contacted all the boat owners in the marina who had not already showed up and most all of them got out of the harbor before the tsunami arrived from Japan the next morning. When boats are at sea, a tsunami will pass harmlessly beneath them, which was what happened here in 2011. If the 2011 tsunami were to happen today, a much more accurate prediction of the height of the wave would have come with it, Aylward said. This is because tidal information is now better incorporated at the time the prediction is issued. The height of a tsunami depends entirely on the state of the tide when it arrives at the shore. If the Japanese tsunami had arrived in Noyo Harbor at high tide, the destruction would have been much greater.

The most devastating tsunami to hit California in modern times occurred March 28, 1964. Several surges reaching 21 feet high swept into Crescent City four hours after a magnitude 9.2 earthquake in Alaska. Ten people were killed, and half of the waterfront business district was destroyed. Crescent City has the state’s highest tsunami risk.  An offshore ridge funnels tsunamis into the area.

Patton said each possible tsunami involves a variety of factors, which is why people are not encouraged to decide on tsunami risk based on factors like the predicted size of the surge.

“Every earthquake and every tsunami is different. We really don’t want people to decide on their own whether to leave based on the size of the tsunami, Patton said.

South of Cape Mendocino, the biggest tsunami threat to most of California would result from another massive earthquake in the Alaska and Aleutian Islands regions. The biggest threat to areas north of Cape Mendocino is a major earthquake on the Cascadia subduction zone — the 700-mile undersea boundary where tectonic plates are colliding. Scientists have evidence that the subduction zone generated a magnitude 9 earthquake and tsunami in 1700.

A similar event could send surges onshore up to 50 feet high toward Crescent City and 30 feet high along the outer coast of Humboldt Bay and the Eureka area. A big quake on the Cascadia megathrust subduction fault might cause five to six minutes of shaking and give people along the Del Norte and Humboldt coasts a maximum of 10 minutes to get inland or to high ground, the press release said

While the tsunami Hazard Area maps are informational, not regulatory, CGS also is working with an advisory panel to develop tsunami zone maps to guide local land-use planning under the Seismic Hazards Mapping Act. CGS currently creates regulatory zone maps for several earthquake-related hazards, helping to ensure public safety and minimize economic impacts.

Climate change is not yet a significant factor in the mapping, said Patton.

“Climate change has caused an increase in sea-level rise rates since at least the 1930s,” said Patton in the interview. “These increases are on the order of millimeters per year higher than the background sea-level rise,” he said. “This new mapping does not yet incorporate projections of sea level into the future, though the changes over the next decade are included in the uncertainty of the modeling. When we update these maps in the next 5 to 10 years, we plan on incorporating sea level rise into those models,” said Patton

How will that develop over the next 20 years?

“We have not yet done this analysis, so we don’t yet know how the models will change. However, the increase in sea level over the next 50 years may be on the order of a meter or so. The changes to the maps will be greater for flat and low-lying areas and less noticeable for places with steep slopes like sea cliffs,” Patton said.

One mistake this reporter made was trying to determine tsunami risk by elevation. Many maps and apps have out of date information about coastal elevations. The area around MacKerricher State Park shows more than 70 feet elevation, even though the area is virtually at sea level.

“The elevation of the tsunami hazard area varies across the landscape, so it is not a good idea to have the public bring into their minds an elevation criterion for being out of the Tsunami Hazard Area,” Patton said. “The example you provide, where the elevation appears off [70′ along the ocean] is a good example of why we may not want people to try to determine their elevation as a criterion. The best way for the public to learn about what the tsunami hazard is for a particular location is to use our online Tsunami Hazard Area Maps. It is as simple as entering an address or using the location services from one’s web browser or phone,” said Patton.

This week’s test is conducted by the National Weather Service, the California Office of Emergency Services, the Offices of Emergency Services for Del Norte, Humboldt, and Mendocino Counties, and Tribal Governments. For more information, contact: National Weather Service (707) 443-6484.

PREPARE: Find out more about preparing for earthquakes, tsunamis or any other disasters on the North Coast http://humboldt.edu/shakyground, or contact the American Red Cross: Humboldt, Del Norte, Lake, and Mendocino counties – (707) 832-5480. America’s PrepareAthon! (http://www.community.fema.gov/), the Great California Shakeout (https://www.shakeout.org/california/), and The tsunami Zone (https://www.tsunamiZone.org) are great places to get preparedness information for natural hazards.

For Mendocino Coast specific information on what to do before, during and after earthquakes and other disasters, sign up at Hubsandroutes.net, where groups of people have signed up to work together in hubs and in finding ways around after a disaster.

Natural warning signs that a tsunami may be on its way:

An earthquake that lasts for a long time – The longer the shaking, the more likely a tsunami could be on its way. The shaking could be weak or strong – the important signal is duration!

A sudden change in the character of the ocean – A quiet sea that suddenly becomes rough is a sign to move away from the beach.

The ocean receding unusually far – This is a sign that the water will rush back in much faster and higher than you expect. Head inland or to higher ground as soon as you notice.

A loud noise or roar from the ocean – This is an indication that the waves are building in size. Tsunami surges could arrive within minutes.

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