Californians face far more dangers than they worry about a tsunami

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Our human minds and media often focus on or capture exciting events or those we have little control over, such as a tsunami or shark attacks. The recent release of revised tsunami inundation maps by the California Geological Survey, including for San Francisco Bay, has raised these concerns, but these need some context. What are these maps based on?

First of all, what are the biggest dangers we face along our coast, and what does history tell us? Sharks are something most of us instinctively fear, but what are the risks of becoming a shark lunch? In the last century of reasonably accurate calculations, the average annual number of deaths from shark attacks in all coastal states of the United States was one, and there are millions of people who visit the beaches each year. In the 200 or so years of record keeping, the total death toll from the California tsunami has been 17.17 in 200 years or one death every 12 years on average. Both are very low risk events.

For some comparisons, there are several bigger risks to worry about. Drug overdoses and poisons killed 93,000 people last year in the United States, car accidents kill more than 40,000 people annually, guns kill 39,000, fall over 37,000, and drown 3,700. You have a high chance of dying while driving to the beach or Drowning once you get there instead of becoming a tsunami victim. The risks of dying from a bee, wasp, or dog bite are much greater than dying from a tsunami.

New tsunami maps just released by the California Geological Survey extend projected areas of flood inland somewhat more than in previous maps. It is important to understand how these areas were identified as there are many coastal neighborhoods around San Francisco Bay located in these new areas.

As with many predictions today, it is based on mathematical models, in this case, a hypothetical 1,000-year earthquake and tsunami generated in the Aleutian Trench by a magnitude 9.3 earthquake. Again, for some important context, we saw a magnitude 9.2 earthquake in the Aleutian Trench in 1964, essentially identical to the event for which it was modeled. This triggered a tsunami that killed 12 people in Crescent City on California’s north coast and flooded 29 compounds in the city. Impacts in San Francisco Bay were minor except for some damage at Loch Lomond Yachting Harbor in San Rafael where damage was estimated at more than $1 million when the wharf ruptured.

It’s important to keep in mind that all models are wrong, but some can be useful. Trying to predict any future event that might happen once every 1,000 years based on 100 or 200 years of history is initially problematic and involves many significant uncertainties. During the 2011 Japan earthquake and 9.0-magnitude tsunami, which killed about 18,000 people in Japan, there was only one death from the tsunami along 1,100 miles of California’s coast—a photographer along the northern coast was standing on the coast when the waves were a success .

San Francisco Bay is isolated from tsunamis by the relatively narrow entrance at the Golden Gate so that the energy of waves entering the bay is minimal. The maximum increase in water height at the Golden Gate from the 2011 Japan tsunami was only five feet, and at Tiburon within the bay this was reduced to just three feet. For comparison, the highest astronomical tidal range in San Francisco is 9.2 feet.

The nearly 200-year historical record of the California tsunami is a much more accurate indicator of future losses and areas of inundation than the mathematical model. Only 13 or 14 devastating tsunamis reached the California coast during this period, killing 17 people. These tsunamis originated thousands of miles away in the trenches of Japan, Aleutian or Chile, and the effects along California’s central coast were relatively minor over the entire 200-year period.

Those areas that sustained the most damage during past tsunamis are those directly onshore areas of trenches or subduction zones where one large tectonic plate slides under another, resulting in the planet’s largest earthquakes and largest tsunamis. Recent examples include Alaska, Sumatra and Japan. These are exactly the same tectonic settings as the coast of Northern California from Cape Mendocino to the Oregon border. Geological conditions are suitable in this region (Cascadia subduction zone) for a very large earthquake and tsunami that would reach the shoreline in a matter of minutes. The rest of the California coast, including the San Francisco Bay Area, has an entirely different geological environment. It is very far from any major tsunami sources and has suffered very little damage and no tsunami deaths over the past 200 years. This shouldn’t be your main concern when you head to the beach.

Gary Griggs is Professor of Earth and Planetary Sciences at the University of California, Santa Cruz.

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