public safety

Updated Tsunami Hazard Maps Show West Side’s Vulnerabilities

By Thomas K. Pendergast

Someday, deep in an ocean trench off the coast of the Aleutian Islands near Alaska, a tectonic plate will suddenly slip under its neighbor and a massive wall of water will be launched at the California coast, scientists predict. 

The San Francisco Bay Area can expect this tsunami to arrive about four or five hours later. If proper warnings go out and action is taken quickly, many lives could be saved. 

The California Geological Survey (CGS) released updated Tsunami Hazard Area Maps recently that show the City will be more affected on its northeast shoreline but of course the west side of town will be affected as well. 

The maps are based on a worst-case scenario of a 9.3 magnitude earthquake off the Aleutians – which has a subduction zone where tectonic plates meet; meaning one plate is slipping beneath another and pushing that other plate up – causing a significant and sudden change in the sea floor and thus producing a tsunami. 

Special attention was paid to that fault line because a tsunami produced there would be aimed right at California. 

The San Andreas, on the other hand, is a “strike-slip” fault, meaning the plates are sliding past each other. When they do, an earthquake results. But the seafloor doesn’t change enough to produce a significant tsunami. 

A 2019 report by the United States Geological Survey (USGS) says recent geological studies suggest Aleutian-generated tsunamis may occur more frequently than previously understood.

The new findings indicate that the recurrence interval for large tsunamis generated in the eastern Aleutians ranges from 164 to 257 years. Comparing new research at Driftwood Bay, in the Fox Islands, with previous research at Stardust Bay near Dutch Harbor, about 124 miles to the east, scientists found long geologic records of earthquakes and tsunamis spanning more than 1,000 years.

The map from the California Geological Survey was recently revised. It shows areas (highlighted in yellow) that are at the highest risk for being directly affected by a tsunami hitting the California coast. Image courtesy of the California Geological Survey.

The most recent tsunamis from the Aleutian ocean trenches occurred in 1946 and 1957, resulting in devastation among certain communities along the Pacific rim. 

 “That’s a subduction zone where it’s pointing right down at the California coast,” the tsunami unit manager of the CGS, Rick Wilson said. 

With this worst-case scenario, there might be a tsunami hitting the beach in the Richmond and Sunset districts measuring six to 10 feet tall, but then rising up to 20 or 30 feet as it goes inland up a slope. 

“The good news in both districts is that there’s a subtle slope to it going inland and that slope is what prevents water from going further inland. It looks flat but if you get out there you can see that there’s a slope that’s going up. It’s very subtle, but it’s there,” Wilson said. “And that helps to reduce the number of blocks that it would go inland, although it still goes in three-to-four blocks and that’s a lot of area. We don’t want to minimize that but it changes up and down the coast…. Every tsunami is different and every location reacts differently to a tsunami.”

The new map shows the tsunami wave going inland as far as 48th Avenue in the Richmond and 46th Avenue in the Sunset. 

“The tsunami is pushing water on shore and so what would start at the coast, which would be probably more like maybe 10, 15, 20 feet, could get up to 30 feet on land. Basically, it’s called ‘tsunami run-up.’ It pushes the water column up and onto the surface and it ramps up the topography as it comes on shore.”

The CGS produced its last set of such maps in 2009, but then in 2011 came the devastating Tohoku Earthquake in Japan, which measured at least 9.0 magnitude and created a devastating tsunami. 

Tragic as it was, it also provided a good opportunity for the CGS to study what happens after what is described as a once-in-a-1,000-years event like an earthquake that strong. 

Also, technology improvements since 2009 mean that research data points, each of which formerly represented an area the size of a football field, now represent the size of a house or a closet, thus making for greater accuracy overall. 

Wilson said he visited the impacted area in Japan about six months after this event.

“It was a very devastating situation there,” he said. “They were still knee-deep in recovery and trying to process how to get back to normal and what to learn from that.… The Japanese had told us that they had planned for their 100- and 500-year events, which is their historical events of flooding in certain areas.”

So, for events that have happened within the last 500 years, they had written records and created maps that had identified hazard areas to evacuate. 

“And they ended up getting what was considered a 1,000-year event. They had a geologic history in that … there were these large earthquakes and tsunamis occurring on about a 1,000-year basis, give or take a few hundred years but on average it was about 1,000 years,” Wilson said.

“The good news was despite the fact that they underestimated the size of the tsunami, they still saved 90% of the people in their zone. And that goes to another lesson we learned, was that they made their maps simple…. They educated their public. Their public was pretty well prepared for tsunamis that if they see it coming, get out of the way, even if you think you’re safe keep going further inland to be safe, just in case.”

On the other side of the Pacific is the California Office of Emergency Services (COES), which is responsible for coordinating a response to tsunamis among local authorities along the state’s coast. 

“The inundation maps inform different aspects of our overall statewide program,” Yvette LaDuke, manager for the COES tsunami and volcano program, said. “That’s used in local planning efforts to develop their tsunami response plans, including evacuation plans for tsunamis…. That helps us as far as our education and outreach for our local populations…. We know those zones ahead of time so that’s the areas we focus on. 

“It also informs our response planning,” LaDuke elaborated. “We can take specific data that we receive from the National Tsunami Warning Center, we get data for each event … and determine what the run-up is going to be for that event at the various points along our coast. Having those maps helps us package all that together and identify who needs to evacuate, how far inland they need to go to be in a safe area.”

One area added to the new maps covers about half of the San Francisco Zoo. 

“That was something new,” Wilson said. “In 2009 we did not see the flooding would be large enough to influence the zoo. The areas along the waterfront protected a lot of the zoo area but it’s unfortunate now we see that our new modeling shows that the zoo could experience flooding in this extreme event.”

“That would be up to those officials at the zoo to determine how best to implement any evacuations,” LaDuke said. “Some of our partners at the Geological Survey did go out and meet with some of the people at the zoo … to make them aware of what that new hazard area looked like so that they can make sure that they have their plans in place.” 

Interactive hazard map showing the earthquake hazards:

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