EarthquakesFracking-induced tremors lead to changes in building codes, insurance rates

For its upcoming National Seismic Hazard Map, used by engineers to update building and construction codes and by insurers to set policy rates, the U.S. Geological Survey (USGS) will take into account risks posed by induced or man-made earthquakes. For North Texas, where earthquakes are historically uncommon, an increase in earthquake risk is likely as the Dallas area has suffered more than 120 earthquakes since 2008. The increase will be small, said Mark Petersen, chief of the USGS’s National Seismic Hazard Project in Golden, Colorado. The hazard map predicts the location, the frequency, and the strength of earthquakes in a given region.

The Dallas Morning News reports that earthquakes are occurring in parts of the country where previous hazard maps did not predict a risk. Scientists have attributed these earthquakes to nearby fracking operations. USGS scientists have linked two Dallas-area earthquake clusters with wastewater injection wells, where oil and gas companies dispose fluids used in the fracking process.

“We’re putting a lot of effort into understanding this,” Petersen said.

Between 2010 and 2013, residents of central and eastern U.S. states felt an average of five times as many earthquakes per year as they did between 1970 and 2000. Most of these man-made earthquakes are small, but in 2011 a 5.7-magnitude earthquake struck near injection wells in Oklahoma.

The current USGS hazard map, released in 2014, predicts a region’s 50-year earthquake risk based on how many quakes the area has sustained over hundreds of years. Induced earthquakes, however, strike and disappear over a shorter timeframe. “It’s very difficult to assess where induced seismicity might occur,” Petersen said. “They’re based on economic and policy decisions which are difficult to forecast, especially over a 50-year time period.”

To reflect the uncertainty, the USGS has decided to publish the hazard map annually instead of every six years. The agency is also investigating the behavioral differences between natural and man-made earthquakes. “There really aren’t any distinguishing features on the seismogram that tell us where an earthquake came from,” said Bill Ellsworth, a USGS seismologist in Menlo Park, California, who studies induced earthquakes. “They’re releasing the stored energy in the earth whether they’re natural or induced.”

Susan Hough, a USGS researcher who has studied the intensities of both types of earthquakes, concludes that induced earthquakes create less shaking than natural earthquakes six miles from the epicenter and beyond. Closer to the epicenter, both types of earthquakes produce the same amount of shaking.

“These induced earthquakes are just systematically different,” she said.

Hough claims that induced earthquakes tend to be shallower than natural earthquakes and that they release less energy as the fault slips. There are two possible reasons for this. First, induced earthquakes have had less time to build up stress in the fault. “A natural fault will sit there and build up and build up and build up,” said Hough. “And when it finally breaks, there will be a certain stress level that it’s releasing. But if you nudge that fault to go sooner, it hasn’t built to the breaking point already.”

Second, fluid from an injection well may help push the fault apart so that it slips more smoothly with less friction. The USGS is factoring Hough’s findings into a new ground-shaking model that will take depth into account for the first time.

Once the USGS releases the new hazard map, some public safety officials and building engineers may have to update their building standards, but that may take years. “By the time we’ve made a map for a building code and that building code gets passed into law, the seismology may be significantly different than it was when we made the map,” Petersen said. “It’s difficult to know how to deal with that.”