A nerdy scientist’s assessment of risk – Tornadoes vs. Earthquakes

Tornado Risk map - Dakotas

Tornado risk map from the National Weather Service for July 17th, 2015. Sioux Falls was squarely within the 2% zone, meaning there was a 2% chance of a tornado being seen somewhere within 25 miles of your location. Somehow…we all survived.

While at a conference for work in Portland last week, my colleague and I had a nice supper with two USGS colleagues who work in Menlo Park, California and Seattle, Washington, respectively.  We had relayed the news that the night before, there was storm damage in the area, with straight line winds and tornadoes both causing damage.  The West Coast colleagues…oh…let’s just call them “Ben” and “Tamara”…were flabbergasted that we’d want to live in an area so prone to devastating storms.

Tonight, the National Weather Service put out the map I’m showing here, noting that parts of South Dakota had either a 5% or 2% chance of a tornado hitting somewhere within 25 miles of your location. Let’s do the math!  What are the odds of a tornado hitting YOUR exact location on a night like this, when tornadoes were indeed spotted?

An area with a radius of 25 miles is over 54 billion square feet of territory. What are the odds of a tornado hitting your bit of territory, with a 2% chance of one being seen somewhere in this area? The average tornado path is 4 miles, or 21,120 feet. Average width is 500 feet. The average damage path is 10.5 million square feet. Even if you’re in the highest probability area, 5% chance of one being seen relatively near, the chances of one hitting YOUR chunk of territory…0.0010%. For the 2% chance of a tornado near you, the odds are 0.0004% it will strike your exact location.

Without Warning - Oregon Cascadia earthquake Comic

An actual comic-like booklet put out by the Oregon Office of Emergency Management. Yes, West Coasters…you’re in imminent danger of being swallowed whole by a massive earthquake. Tornadoes in the Plains? They’re nothing in comparison.

Just a guess…but we probably have chances similar to this only like 4-5 times a year. If you just use the same strike probability assumptions, you thus only have, likely at absolute most, a 0.0050% chance of a tornado hitting your location in a given year.

That’s an average of about one hit on your exact location every 20,000 years. Most tornadoes are weak F0 or F1, so even a hit might not be that bad.

Now, let’s reconsider the situation for my colleagues living in California and Seattle.  This week the New Yorker ran a story about the upcoming massive Cascadia subduction zone earthquake that could hit, The story said on average, something similar to the massive 1700 earthquake thought to have hit the area occurs about once every 250 years.

Put it all together, and Seattle is 80X as likely to get walloped with a 9.0 earthquake as Sioux Falls is to get a direct hit from a tornado, and even a direct tornado hit in Sioux Falls is likely to be far, far, less damaging than any earthquake.

You are welcome, West Coast storm-haters.  I hope I have reassured you that is once again safe to visit us in stormy Sioux Falls.  🙂

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