New tsunami hazard maps show evacuation areas in SLO County

A new tsunami hazard map for San Luis Obispo County shows that a significant portion of low-lying communities, from Cayucos to Oceano, could be at risk in the event of a worst-case catastrophic event hitting the California coast.

The California Gelogic Survey (CGS) has released the latest tsunami hazard map, as part of a review and update to the state’s tsunami risk assessment.

The maps are a joint effort of CGS, the California Governor’s Office of Emergency Services (Cal OES), and local agencies.

Previous tsunami risk maps were finalized in 2009.

“A lot has happened in the past 12 years,” said Rick Wilson, the head of the inquiry’s tsunami unit. “We have learned a lot of lessons.

In particular, the 9.1-magnitude Tohoku earthquake in Japan in 2011 and the tsunami that followed had a devastating impact, he said, and was larger than the region had anticipated.

Violent tremors, followed by a 9-meter-high water wave, destroyed many coastal communities along the country’s northeast coast and caused the meltdown of three reactors at the Fukushima Daiichi nuclear power plant.

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FILE – In this file photo from March 15, 2011, a Japanese earthquake and tsunami survivor rides his bicycle through the razed town of Minamisanriku in Miyagi Prefecture, northeastern Japan. March 11, 2021 marks the 10th anniversary of the earthquake, tsunami and nuclear disaster that hit the northeast coast of Japan. (AP Photo / David Guttenfelder, file) David Guttenfelder PA

The tsunami waves crossed the Pacific, reaching the coast of California about 10 hours after the earthquake. Heavy swells caused one death and $ 100 million in damage to 27 ports across the state. Repairs to Morro Bay’s docks, ships and infrastructure totaled approximately $ 500,000.

It was a thousand-year-old event for Japan, said Wilson, a senior geological engineer, and he taught scientists how to better plan for similar disasters along the California coast.

Planning for the worst case

By developing this latest edition of the map, scientists were both better informed and more conservative in their approach.

They looked at every major tsunami-generating event imaginable that could affect the California coast, including a once-in-a-millennium earthquake like the one in Japan.

Using high-resolution elevation data, scientists calculated how far inland tsunami waves could reach under 12 worst-case scenarios for SLO County. The resulting map draws a “flood line” across the county., indicating how much water could flood.

For the most part, the new map is not much different from the previous version, said Kelly Van Buren, emergency services coordinator for SLO County, although there are substantial changes in particular locations.

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A tsunami tidal wave pushed the Blue Horizon into the wharf near Tidelands Park in Morro Bay following a magnitude 9.1 earthquake off Japan in March 2011. Jayson Mellom The gallery

Cayucos saw the biggest increase in his danger zone. Morro Bay, Los Osos and San Simeon have also seen their evacuation areas expand, while at Avila Beach and Oceano the risk areas have shrunk.

California Geological Survey scientists and Cal OES officials verified their data by heading to the coast and surveying the landscape. They focused their efforts on populated and low-lying areas, including Cayucos, Pismo Beach, and Morro Bay.

Scientists worked with local emergency officials to extend the flood line to track streets and landmarks “to make the maps as simple as possible for people to understand,” Wilson said. The final map shows a jagged line almost parallel to the coast. Anything off the line falls in the tsunami evacuation zone.

Low areas are the most at risk

The biggest change in local tsunami risk is at Cayucos, where a tsunami could reach 50 feet above sea level at its greatest extent inland – 20 feet higher than the previous map shown , according to the CGS.

Almost half of the buildings in the community of about 2,600 people are in the new tsunami-prone area, and waves could flood the main business district. The danger extends beyond the first block of clifftop houses, all the way inland to Ocean Avenue.

This moderate expansion of the danger zone is due to scientists considering a larger earthquake in their calculations and a more accurate elevation map along the coast, Wilson said.

In Los Osos, the danger zone is limited to low-lying areas near the estuary, including the majority of Morro Bay State Park.

In South County, nearly two-thirds of Avila Beach is at risk if a major tsunami hits. But this is a reduction from the 2009 estimate.

Also in the bay of San Luis Obispo, the main tourist area of ​​Pismo Beach and the town pier are in the danger zone, as well as the beaches and low-lying streets to the south near Highway 1.

In Oceano, a major tsunami could inundate the airport and neighboring neighborhoods, as well as low-lying agricultural lands to the south. The danger zone has moved towards the sea compared to the 2009 map.

Fortunately, the Diablo Canyon nuclear power plant sits on a bluff 85 feet above the ocean. The site is out of reach of the worst tsunami counted in the new map.

Anyone can check if their home or business is in the Tsunami Risk Zone by entering their address on the CGS Tsunami Map website (also available at prepareslo.org/tsunami).

Tsunamis in SLO County

SLO County should be aware of two different types of tsunami threats, Wilson said. The first is an earthquake on a nearby fault that causes landslides off the coast. The moving rock mass could generate a big wave.

Offshore faults such as the San Gregorio Fault and the Hosgri Fault could themselves generate small tsunamis, due to the displacement of water as the fault slides during an earthquake, he said. Such a local tsunami could reach the shore in a matter of minutes.

In these cases, shaking is the first warning to reach higher ground as quickly as possible.

The other type of threat comes from distant earthquakes, such as the 2011 earthquake in Japan.

In this category, “the greatest danger comes from the Alaska-Aleutian region,” Wilson said. The new maps account for a 9.3 magnitude earthquake off Alaska as one of the worst-case scenarios. An earthquake of this magnitude in this region has a 5% chance of occurring over a 50-year period. This is the equivalent of an event over a thousand years.

A resulting tsunami would take about five hours to reach SLO County – “essentially the speed of a jet plane,” Wilson said.

This much more advanced warning would give emergency officials time to evacuate people from low-lying areas.

Wilson notes that “these cards are not meant to really scare anyone. … (They) are based on something that is unlikely to happen.

“It’s really very helpful to foresee the worst case scenario and hope for the best,” he said.

County to review security plans

SLO County is using the new map to update its tsunami plans.

The county emergency services office identifies particularly vulnerable facilities in the tsunami-prone area that may require special attention during an evacuation, such as schools or other recreational facilities, Van Buren said.

All emergency response partners are updated to make sure everyone is on the same page, she said. In addition, the county is also working on phased evacuation maps for smaller tsunamis.

The recently launched website, prepareslo.org, contains information on how the public can protect themselves from tsunamis.

The most important thing to know is that if you feel an earthquake, go to higher ground, Van Buren said.

This story was originally published July 19, 2021 10:50 a.m.

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Jen Schmidt is AAAS Mass Media Fellow covering natural hazards and scientific news. She completed her doctorate. in Earth Sciences at Lehigh University in 2018. She is an NSF-sponsored postdoctoral fellow at Temblor Inc., where she directs Temblor Earthquake News.


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