Featured image: Karst rocks in Segovia, Spain. Photo by Luis Fernández García, CC-BY-SA 2.1.
Paper: Injection-induced earthquakes near Milan, Kansas controlled by karstic networks
Authors: Charlène Joubert, Reza Sohrabi, Justin L. Rubinstein, Gunnar Jansen, Stephen A. Miller
On November 12th, 2014, a magnitude 4.9 earthquake rattled the city of Milan, Kansas. This event was the largest earthquake ever recorded in Kansas, adding to a trend of increasing seismic activity in the state since 2012. What could cause this kind of tectonic excitement in the stable central US?
Most recent earthquakes in Kansas, and in other central states like Oklahoma, Arkansas, and Wyoming, are actually anthropogenic: by pumping water and other fluids in and out of the ground, humans cause pressure changes in the subsurface that can jack open pre-existing faults. This makes it easier for the faults to slip, generating what are known as “induced” earthquakes.
In some ways, the 2014 Milan, Kansas event is a textbook example of an induced earthquake, since it occurred close to the Dane wastewater injection well when injection volumes were rapidly increasing. But it also presented a puzzle for seismologists: though the prevailing regional stresses were aligned northeast-southwest, the fault responsible for this quake was at a north-northeast angle, misaligned by around 15 degrees. Earthquakes generally happen on pre-existing faults aligned with regional stresses because that configuration requires the minimum amount of stress to overcome friction and make the fault slip. Though 15 degrees may not sound like much, it actually represents a large increase in the amount of stress or pressure needed to set off the 2014 event on this particular fault.
Charlène Joubert and co-authors used computer models to simulate how pressure from the Dane injection well interacted with the local geology around Milan, Kansas. They found that the particular rocks that make up the region strongly influenced the pressure distribution underground and enabled the misaligned fault to slip.
The Dane well injects wastewater into a set of carbonate rocks known as the Arbuckle Group. These rocks formed in a sea that stretched across the midcontinental US during the Ordovician period roughly 450 million years ago. The carbonates later developed what’s known as a karstic texture: they’re riddled with holes and tunnels made by rainwater dissolving parts of the rocks to form a complex drainage system that is often compared to Swiss cheese.
Typical models for how changes in pressure propagate underground assume that rocks have uniform porosity, like the evenly distributed holes in a kitchen sponge, but karst rocks have a very non-uniform porosity. Joubert and colleagues’ models showed that the karstic texture and non-uniform porosity of the Arbuckle rocks could explain how fluid injection at the Dane well led to a sufficiently large and sudden increase in pressure to set off the Milan earthquake, even though the fault that hosted the earthquake was misaligned with the regional stresses.
Human activities like wastewater injection and hydraulic fracturing have clearly changed patterns of seismic activity in places like the central US, and these changes are reflected in the latest maps of seismic hazard: high-risk red patches have appeared in previously low-risk states like Kansas and Oklahoma. While we understand the basics of how underground pressure changes can affect seismicity, this new study shows that regional geology plays an important role in modulating those pressure changes, and can help us better estimate the risk of induced earthquakes.