Wednesday, May 11, 2011

Forecasting the shakes

First off, thanks for reading and thanks to those who've commented. To better quantify some of the earthquake hazards alluded to in the last post, I've been wading into the relevant literature for those who live near SLC. While there are a few papers relevant to the topic at hand (and apparently the Wasatch Fault is one of the most studied faults in the world), I'll briefly mention the predictions from a 1996 paper by McCalpin and Nishenko. The numbers here will still be centered around 1996.

Using a chronology of magnitude 7 or greater earthquakes along the 5 segments of the Wasatch Fault, they calculate (using some methods beyond the scope of this blog) the likelihood of similar magnitude quakes occurring over the next 50 and 100 years. First, over the last 5600 years there have been 16 earthquakes greater than a 7 magnitude; the average repeat time for these was 350 years. Of the 5 segments of the Wasatch Fault, they find that 4 of them ruptured between 620 to 1230 years ago. The Brigham City segment, however, has not ruptured for ~2120 years. This is somewhat disconcerting as they say it has an average repeat time (how often it ruptures) of 1275 to 2346 years (depending on the method used). The SLC segment is also suspect in that it hasn't ruptured for 1230 years and it has a repeat time of ~1350 years (see Table 6).

While that may seem abstract, the paper's abstract (oops) bluntly states that over the next 50 and 100 years, there is a 16 and 30% chance, respectively, of a magnitude 7+ earthquake occurring somewhere along the Wasatch Front. While these things cannot be predicted with anything close to pinpoint accuracy, these statistics come from the Journal of Geophysical Research (i.e., peer-reviewed science). Perhaps instead of springing for that extra-large-capacity ipad, one would be better served buying bags of wheat, rice, beans, and oats. For more perspective on these risks across the US, the USGS has some attractive figures showing the likelihood of similar events around the country.

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