Mother Jones reports on recent earthquakes in regions not accustomed to much seismic activity, Oklahoma, Arkansas, and Ohio. Much of their story consists of anecdotal evidence, particularly the strongest earthquake in Oklahoma history at magnitude 5.6 in November 2011, which happened along a fault which a Univ. of Oklahoma geophysics professor referred to as “a dead fault that nobody ever worried about.” Since this quake “injured two people, destroyed 14 homes, toppled headstones, closed schools, and was felt in 17 states,” people are starting to worry.
The putative culprit is “injection wells,” the destination of wastewater from hydraulic fracking (and in some cases from dewatering operations). We looked at earthquake data a while ago, because of a study from the U.S. Geological Survey which reported a dramatic increase in earthquakes of magnitude 3.0 or greater over a large area of the U.S., stating that the increase is “almost certainly man-made,” and attributing it to oil and gas production.
We saw that there was indeed a recent, and rather large, increase in the number of earthquakes of magnitude 3 or greater in their study region (longitude 108W to 85W, latitude 25N to 50N). This new article made me wonder, did the increase also occur in Ohio (just east of their study region)? Also, what about the rest of the USA?
So I retrieved earthquake data from the U.S. Geological Survey for the time span January 1973 to the present, for the entire continental USA (specifically, longitude 125W to 65W, latitude 25N to 50N). Let’s look at the data from 1973 through the end of 2012.
Here are the locations of earthquakes with magnitude 3 or greater (click the graph for a larger, clearer view):
Clearly the majority of earthquakes happened along the “Pacific rim,” where continental plates border each other. But how has the number of quakes changed over time, and how are the changes distributed geographically?
I split the entire region into 5×10-degree latitude-longitude grids. Then I counted the number of earthquakes of magnitude 3 or greater in each grid. I then “normalized” the numbers to compensate for the fact that different regions have vastly different numbers of quakes. I didn’t normalize the usual way — subtract the mean and divide by the standard deviation — I just divided by the mean value.
Here’s the result, with the time series of normalized quake count for each grid superimposed on the map of the USA (click the graph for a larger, clearer view):
The largest “spike” happened in 1980 in the Pacific northwest. I strongly suspect this is related to the eruption of the Mt. St. Helens volcano, which was preceded by much seismic activity.
The next largest spikes are recent. The biggest is in the grid covering the northern half of Oklahoma and the grid just to the east of that. There’s also a recent spike in the Pacific southwest, covering the southern California/Mexico border.
There’s no sign of enhanced activity in the grids which cover Ohio. But it turns out that there have been so few quakes of magnitude 3 or greater, the counts are too low to have much statistical power. If we count the number of quakes of magnitude 1 or greater, we get this:
Now the grid which includes northern Ohio shows some sign of recent activity, and that which covers southern Ohio shows a distinct spike.
I also looked specifically at Oklahoma (the bulk of it, longitude 100W to 95W, latitude 33N to 37N) and Ohio (about longitude 85W to 80W, latitude 38N to 42N). At magnitude 3 and above, Oklahoma shows a distinct recent spike:
It’s also there when including all quakes of magnitude 1 or greater:
For Ohio, the count of quakes with magnitude 3 and above is just too small to tell very much, not more than 3 in any year:
But when counting quakes of magnitude 1 and above, there’s an unmistakeable recent spike:
So far, the fossil-fuel industry has denied any connection between recent earthquake activity and oil/gas production. The U.S. Geological Survey disagrees. Who you gonna believe?