Thursday, October 8, 2009

Three magnitude 7's in one day

According to the list of earthquakes from the USGS, there were three magnitude 7 quakes on Oct. 7.

These all occurred near Vanuatu, so are probably not independent. But if we take the annual global magnitude statistics, there are typically 17 quakes per year with magnitudes above 7. What are the chances of getting three independent magnitude 7's on the same day?

The chance of getting one is 17/365 = 4.6%. I think (correct my joint probabilities, please) that the chances of three in one day would then be this probability cubed, or 1 chnce in 10,000,

If you are more energetic than I am, go back and check when was the last time this happened.

Or, would it make more sense to simply consider the probability of a magnitude 8 quake, which would release more energy than three magnitude 7's anyway?

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