By David Jacobson, Temblor

Check your hazard rank

Yesterday, twin M=4.2 earthquakes struck near the town of Enid, Oklahoma, resulting in at least one damaged home, and shaking across much of Oklahoma and Kansas. Other than this minor reported damage, there is unlikely to be much else according to the USGS. However, because buildings in this part of the country are predominantly brick and masonry, it is possible that there will be more reports of cracking. It should be noted that the owner of the house that sustained damage said that they have earthquake insurance.

Both of yesterday’s earthquakes approximately 15 km from Enid, which is home to approximately 50,000 people, were strike-slip in nature. However, due to the lack of mapped features, it is unclear whether these quakes were right or left-lateral. Furthermore, according to Rall and Zoback, 2016 the sense of slip in earthquakes in this part of Oklahoma does not always match what would be expected given the regional compression. Therefore, all we can say is that these were strike-slip quakes.

When most people think of earthquakes in Oklahoma, their mind is drawn to induced seismicity. Last year, the USGS published a study where the effects of induced seismicity were incorporated into hazard. This study showed that parts of Oklahoma have a higher seismic hazard than California. Having said that, the developers of the model, which now appears in Temblor, stress that the uncertainties involved in forecasting induced events are extremely high. Furthermore, following the M=5.8 Pawnee earthquake in September 2016, there has been a reduction in fluid injection, which has already led to a decrease in the number of earthquakes across the state. Therefore, it is possible that the seismic hazard in Oklahoma is now lower than it was last year.

While we cannot say for sure if these earthquakes were induced, they are consistent with the depth and area where there has been a significant increase in the rate of earthquakes associated with water disposal. Even though there are no large-volume disposal wells nearby, Dr. Matthew Weingarten, a postdoctoral fellow at Stanford told us that “in adjacent regions, we’ve observed long-range effects of wastewater disposal wells.” This is the result of water being injected into the highly-permeable Arbuckle Formation. By adding significant pressure over long, lateral distances, critically-stressed faults are found, and rupture. Similar sentiment was echoed by Dr. Rall Walsh, a scientists at Design Geomechanics, who said that “What we are seeing is where the faults are critically stressed, not where the disposal wells are.”

What all of this means for residents of the Midwest is that they should still be prepared for potentially-damaging earthquakes. Even though the rate of M=3+ earthquakes has significantly decreased in the last few years does not necessarily mean that a large earthquake will not occur. Additionally, should there be a large quake, it will be felt across a very wide area. This is because the eastern U.S. is predominantly made up of sediments, uninterrupted by crystalline rock. Furthermore, due to lower building codes east of the Rockies, smaller magnitude earthquakes can cause more damage than a comparable quake in California.

References

USGS

Mark D. Petersen, Charles S. Mueller, Morgan P. Moschetti, Susan M. Hoover, Allison M. Shumway, Daniel E. McNamara, Robert A. Williams, Andrea L. Llenos, William L. Ellsworth, Andrew J. Michael, Justin L. Rubinstein, Arthur F. McGarr, and Kenneth S. Rukstales, 2017 One-Year Seismic-Hazard Forecast for the Central and Eastern United States from Induced and Natural Earthquakes, Seismological Research Letters, March 2017; 88 (2A), DOI: 10.1785/0220170005

F. Rall Walsh, III, and Mark D. Zoback, Probabilistic assessment of potential fault slip related to injection-induced earthquakes: Application to north-central Oklahoma, USA, 2016, Geology, doi:10.1130/G38275.1