Last May I reported on the startling increase in the number of earthquakes in the Central United States in places not known for earthquake activity. Oklahoma and Arkansas had seen an increase of 16X in moderate earthquakes. I asked whether Missouri had also experienced an increase in earthquakes. It had, although nothing like the outbreaks in Arkansas and Oklahoma. This post updates that information. In the next post, I’ll look at some information identifying earthquake swarms that have been induced by human activities.
At right, I’ve updated a couple of graphics from the post last May. The first is a frequency chart of earthquakes above Magnitude 2.0 in a square region that approximates Missouri. (The USGS data tool does not permit selection of a state. The area I used this time is close, but not precisely the same as the one from 2014. The precise area is given in the Source List below.)
(Click on chart for larger view.)
The chart shows that Missouri had a small, temporary increase in earthquakes in the early 1990s. In 2008, however, the number of earthquakes began increasing, and it skyrocketed in 2013 and 2014. The average number of earthquakes for 1980-2012 was 8.3, while the average number for 2013-2014 was 47, an increase of 464%.
Most of them were small earthquakes. The largest was a Magnitude 4.7, which occurred in 1990. In 2013-14, the largest was a Magnitude 3.3, which occurred in Tennessee. Such an earthquake may be felt, but it rarely causes damage.
The second graphic at right is a map showing where earthquakes have occurred. The map shows earthquakes from 2008-2014, with a magnitude above 2.0, that occurred within the same geographical limits used above. Missouri’s earthquakes center on the New Madrid Fault, in the southeastern corner of the state.
A new USGS report identifies 17 regions that have been the site of induced earthquake activity. I’ll look at that report in the next post. Missouri is not one of 17 regions. It is perplexing, then, to wonder what might be causing such a large increase.
The data on Missouri earthquake frequency and the map of Missouri earthquakes both come from the USGS Earthquake Archives Data Portal, http://earthquake.usgs.gov/earthquakes/search.
To create the frequency charts, I downloaded data as a CSV file and processed it in Microsoft Excel. On the data portal page, I set the north limit as 40.964, the south limit as 35.729, the west limit as -95.999, and the east limit as -89.099. I set the starting date at 1980-01-01 00:00:00, and the ending date at 2014-12-31 23:59:59. I set the minimum magnitude to 2, and the maximum magnitude to 8.
To create the map, I used the same geographic limits, same ending date, and same magnitude limits. But I set the starting date to 2008-01-01 00:00:00.
Peterson, Mark, Charles Mueller, Morgan Moschetti, Susan Hoover, Justin Rubinstein, Andrea Llenos, Andrew Michael, William Ellsworth, Arthur McGarr, Austin Holland, and John Anderson. 2015. Incorporating Induced Seismicity in the 2014 United States National Seismic Hazard Model – Results of 2014 Workshop and Sensitivity Studies. USGS Open-file Report 2015-1070. http://pubs.usgs.gov/of/2015/1070.