Heartland danger zones emerge on new U.S. earthquake hazard map
By Eric Hand | Sciencemag.org
The gentle landscape of southern Kansas doesn't exactly shout "earthquake country." Until recently, the notoriously flat state had just two of the seismic stations used for recording and locating earthquakes. Now, 21 are in place. They have been sorely needed. Since 2013, 192 earthquakes bigger than magnitude 2 have hit Harper and Sumner counties, on the border with Oklahoma, up from just two in the previous 35 years. "It feels like we're on the front lines of this thing," says Rex Buchanan, the state geologist for the Kansas Geological Survey in Lawrence.
Across the U.S. heartland, an oil and gas boom has driven a surge of small to moderate earthquakes. Scientists say that deep underground injection of wastewater from oil and gas operations is triggering the tremors by pushing critically stressed faults past the snapping point. On 23 April, the U.S. Geological Survey (USGS) released a report that, for the first time, accounts for these human-caused, or induced, earthquakes in a map of seismic hazards across the country. The new map highlights 17 areas in eight states with frequent induced earthquakes (see boxed areas on map). The probability of dangerous levels of ground shaking in some of these areas, such as the one that bleeds from central Oklahoma into southern Kansas, rivals that of California, the traditional earthquake king. "It was kind of a surprise," says Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project in Golden, Colorado.
So far, most induced earthquakes have done no more than rattle windows. But a few have been big enough to damage buildings, and now USGS says that it can't rule out the possibility of a magnitude-7 temblor, which would cause widespread damage.
USGS researchers had to develop new methods to make the map. Typically, in predicting future earthquake behavior, they assume that past is prologue. In places like California, where quakes are set off by well-understood forces that cause tectonic plates to grind past each other, seismologists can invoke centuries of earthquake statistics. But for the new, induced earthquake regions, the researchers modeled the future hazard based on tremors only in the past year. They also predicted the hazard just 1 year into the future, rather than offering the usual 50-year prediction.
That short time window is a challenge for engineers who design bridges and buildings meant to last decades, as it says nothing about the hazards a structure will face over its lifetime. Peterson isn't sure how engineers will use the new information, but he says the agency couldn't be confident in longer term predictions when so many factors—the price of oil, the actions of regulators—could influence earthquake rates.
Some of these feedback loops may already be having an effect. The price of oil has dropped drastically in the past year, and many operations are slowing down or shuttering. Some states are also beginning to crack down. On 19 March, Kansas's regulator, the Kansas Corporation Commission, issued an order that would reduce saltwater injection in Harper and Sumner counties by up to 60% for some wells—its first response to specific wells that seem to be triggering quakes. Buchanan says that April has been the quietest month for his state since August 2014, with just six earthquakes. "It's been a little bit of a roller coaster ride over the last 18 months," he says.
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