Oklahoma has been the epicenter of a new environmental problem related to modern times: an increase in the number of earthquakes. Usually the state has 0 – 3 earthquakes per year above Magnitude 3.0, with the strongest being a M5.5 in 1952. In 2009, however, the number of M3.0+ earthquakes each year began increasing. In 2013 the Oklahoma Geological Survey registered 2 of them each week, and since then the rate has continued to increase. As of 4/21/15, the current rate was 2.5 per day! The trend is shown in the chart at right.
Then, in 2011, a Magnitude 5.6 earthquake struck east of Oklahoma City. That’s big enough to cause damage.
According to the Oklahoma Geological Survey, it is
very likely that the majority of recent earthquakes, particularly those in central and north-central Oklahoma, are triggered by the injection of produced water in disposal wells.
The primary suspected source of triggered seismicity is not from hydraulic fracturing, but from the injection/disposal of water associated with oil and gas production. Produced water is naturally occurring water within the Earth that is often high in salinity and co- exists with oil and gas in the subsurface. As the oil and gas is extracted/produced, so is the water. This water is then separated from the oil and gas and re-injected into disposal wells, often at greater depth from which it was produced. (OGS, 4/21/15)
Be sure to catch the distinction here: the problem they identify is not the fracking itself, but rather the water that comes out as the oil and gas are extracted. That water is contaminated, and they dispose of it by injecting it into disposal wells at high pressure. These wells are the suspected culprits.
The area of increased seismic activity stretches from central Oklahoma north into Kansas. It is not the only area experiencing such increased activity. A recent U.S. Geological Survey report identifies 17 areas that have. Five are in Colorado, one in New Mexico, five in Texas, one in Arkansas, one in Mississippi, and two in Ohio. But by far the largest ones are the two involving Oklahoma and Kansas. The map at right shows their locations.
To understand the map, a little background is needed. The USGS studies earthquakes primarily to understand earthquake risk. Their knowledge is used by building officials to determine how earthquake-proof buildings in a given region need to be, and by the insurance industry to understand risk. Earthquakes tend to come in clusters; when there is an earthquake, it results in aftershocks, and sometimes it sets off additional earthquakes nearby. It does not add much to the understanding of risk to say that once there has been an earthquake, others are likely to follow. Thus, the USGS keeps catalogs of earthquakes in two ways. One way is to list an initial earthquake, its aftershocks and its subsidiary quakes as a single cluster. These catalogs are called nondeclustered. I don’t know why they use this awkward double negative, but the point is to try to identify the risk of when a cluster will occur. The other way is to list all earthquakes as individual, separate events. These catalogs are called declustered.
The map at right combines data from both kinds of databases. The dots represent the epicenters of earthquakes of Magnitude 2.7 or larger. The blue dots represent earthquakes prior to 2012 using a declustered catalog. The green dots represent 2013 earthquakes from a nondeclustered catalog. And the red dots represent 2014 earthquakes from a nondeclustered catalog. In other words, the red and green represent single years, and in those years aftershocks and subsidiary quakes have been grouped as a single cluster with their initial earthquake. The blue dots represent all the years before 2012, and each aftershock and subsidiary quake is counted as a separate event. Obviously, the recent outbreak of earthquakes has been quite severe. The majority of these earthquakes have been small or moderate. However, there has been a trend towards increased magnitude, and a few of them have been sufficiently large to cause some property damage.
The kind of waste injection well identified as the culprit in the Oklahoma earthquake swam is permitted in Missouri, but it does not appear to be common. Missouri has little oil and gas extraction compared to some other states. We have not been identified as one of the regions experiencing a swarm of human-induced earthquakes. Our earthquakes center around the New Madrid Fault, a known area of seismic activity.
Oklahoma Geological Survey. 4/21/2015. Statement on Oklahoma Seismicity. http://wichita.ogs.ou.edu/documents/OGS_Statement-Earthquakes-4-21-15.pdf.
U.S. Geological Survey. 2015. Earthquakes in Oklahoma of M3+. http://earthquake.usgs.gov/earthquakes/states/oklahoma/images/OklahomaEQsBarGraph.png.
U.S. Geological survey. Oklahoma Earthquake History. Web page accessed 4/25/15. http://earthquake.usgs.gov/earthquakes/states/oklahoma/history.php.
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.