Over the past seven years, regions with historically low seismicity in the Central U.S. have seen a remarkable increase in the frequency of earthquakes thanks to human activity. Although limited to a few areas—particularly northern Oklahoma and southern Kansas, and regions in Colorado, New Mexico, central Arkansas, and north-central Texas—the average instance of earthquakes of M3.0 and greater has risen from 24 per year from 1973–2008 to 193 per year from 2009–2014.
There have been several examples of relatively larger induced earthquakes that have caused property damage, including the 2011 sequence of M5.0–5.7 earthquakes in Prague, Oklahoma, the 2011 M5.3 earthquake in Trinidad, Colorado, the 2011 M4.7 earthquake in Guy-Greenbrier, Arkansas, and the 2012 M4.8 earthquake in Timpson, Texas. Mounting evidence relates these earthquakes to the injection of wastewater fluids for disposal or enhanced oil recovery methods at Class II injection wells. These are common practices in oil and gas production. "Fracking risk" is a common misnomer for this hazard; properly, the term "fracking" refers specifically to hydraulic fracturing, which does stimulate small earthquakes but has not been linked to larger magnitude, damaging earthquakes in the U.S. at this time. (Learn more about the impacts of fracking and the injection of waste water from oil extraction into disposal wells in the blog, "Does Fracking Cause Earthquakes?".)
The hazard posed by larger magnitude induced earthquakes is different from that of natural earthquakes, in part because induced seismicity acts on much shorter time scales. Although high-rate injection wells are more likely to be associated with earthquakes than low-rate wells, in general there is no simple predictor between influences caused by human activity (e.g., injection rate, cumulative injected volume, wellhead pressure, injection depth, etc.) and timing or magnitude of induced earthquakes; clear correlations have been noted in a few cases. Some states are making efforts to regulate or monitor the wastewater injection process to control the rate of induced seismicity. Regulations governing wastewater disposal appear to have had some impact on the rate of induced seismicity in Oklahoma since they were introduced in March 2015, although it is too soon to tell if the decline will be permanent or transient.
Challenge for modelers
The primary challenge for hazard and risk modelers is constructing realistic seismicity and ground motion models for induced earthquakes given the lack of data on—and thus uncertainty in—the ground motion and potential magnitude of these types of earthquakes.
For example, do seismicity models used for natural events apply to induced earthquakes? In other words, can the regional rates of small magnitude induced earthquakes be used to project the frequency of larger events? Given the relatively recent increase in seismicity rates in the U.S., what is the appropriate time period of earthquake catalog to consider for applying standard Probabilistic Seismic Hazard Analysis methods to forecast near-term hazard? And what is the maximum magnitude of earthquake that can be induced? Although the largest observed event in the U.S. so far is the 2011 M5.7 earthquake in Prague, Oklahoma, larger magnitude earthquakes have been induced by other human influences worldwide and there is potential for M7.0 or larger earthquakes in the Central and Eastern U.S.
Evaluating the risk
The Midwestern states of Colorado, Kansas, and New Mexico, and the Southern states of Arizona, Oklahoma, and Texas, are historically perceived as low seismicity areas. Most areas in Texas and Kansas do not require seismic resistance in building design, and most of the other states have only minimal requirements. A large proportion of homes in the Southern states (20-36%) consist of masonry veneer, for example. These weather-protecting veneers are not designed to withstand seismic shaking; they are weakly connected to wood studs and vulnerable when subjected to earthquake shaking.
Most of the areas experiencing larger magnitude induced earthquakes have very low earthquake insurance penetration, suggesting that awareness of earthquake risk is low. (Arkansas has a higher take-up rate, probably due to the well-known New Madrid seismic zone.) Nevertheless, damage has been experienced and claims have been made.
To help companies assess the risk, AIR is developing an Induced Seismicity Model for the Central and Eastern U.S. The model will be incorporated into the updated AIR Earthquake Model for the United States schedule for release in 2017. It integrates the latest data and scientific opinion put forth in the new USGS 2016 One-Year Seismic Hazard Forecast with research conducted by AIR scientists to address additional uncertainties beyond the USGS formulation. The AIR induced seismicity model will cover USGS-defined "induced seismicity source zones" in the Central and Eastern U.S., with a focus on Oklahoma, Kansas, Colorado, New Mexico, Arkansas, and Texas. The AIR model will provide a stochastic induced seismicity catalog of these regions, employ ground motion prediction equations for the Central and Eastern U.S., and use new research on the ground motion behavior of induced earthquakes.