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2014-08-19 17:35:00



When it comes to making earthquakes, Mother Nature still packs the most powerful punch, a new study says.

Quakes believed to be caused by human activity, such as those associated with oil and gas extraction, don't produce as much shaking as naturally occurring quakes of similar magnitudes, according to research released Monday by Susan Hough, a seismologist with the U.S. Geological Survey.

Using data collected from the public following 11 moderate earthquakes between 2011 and 2013, Ms. Hough's study says that shaking felt from so-called induced quakes is less intense for distances beyond 6 miles from the epicenter. But shaking at the epicenter was as intense as naturally occurring quakes. That means damage from an induced quake is likely to be more concentrated at its epicenter.

"It turns out there's a big difference between these induced earthquakes and natural earthquakes," Ms. Hough said. Compared with naturally occurring quakes, "these induced quakes are essentially wimpy in terms of the shaking they're creating" further from the event.

It is unclear why the induced quakes seemed to weaken more quickly than natural quakes, Ms. Hough said. One possibility is such quakes may release less energy because the fault involved ruptures before it reaches its natural breaking point, she said.

The study, to be published Tuesday in the Bulletin of the Seismological Society of America, comes as scientists are struggling to better understand so-called induced quakes and better prepare the public for them.

Many scientists believe that earthquakes can be caused by activities associated with oil and gas extraction, usually by the injection of wastewater produced by the extraction back into the ground. Research says an increase in quakes in the central and eastern U.S. in recent years coincides with the rise in extraction activities there.

An average rate of more than 100 earthquakes a year above a magnitude 3.0 occurred from 2010 to 2013 in the central and eastern U.S., compared with an average rate of 20 events a year from 1970 to 2000, according to the USGS.

The USGS for the first time plans to release an earthquake hazard map for regions with induced quakes.

The issue is a politically sensitive one, and there is still much debate on the causes and impacts of induced quakes among the scientific community, politicians and industry representatives. The American Petroleum Institute has said there is not enough data to confirm a link between tremors and injection wells.

Such quakes, often in places not considered at high risk for earthquakes, have alarmed some public officials and residents.

In Oklahoma, where an induced quake is believed to have produced a magnitude 5.6 quake in 2011, the state's insurance commissioner has urged residents to purchase earthquake insurance.

Rob Williams, a USGS geophysicist in Golden, Colo., said Ms. Hough's research "may be a way to identify induced quakes from natural events." But he said it is too early to use the research to implement any community planning.

Scientists cautioned that while Ms. Hough's study should be incorporated into research about induced quakes, it isn't definitive, and needs to be backed by more seismological data based on ground motion measurements taken with scientific instruments, as opposed to reporting from the public.

"This is a good piece of information for us to be working from, but she's looking at [11] different earthquakes and that's really not enough earthquakes to make any decisions upon," said Justin Rubinstein a USGS geophysicist working on the induced quake hazard map.

Mr. Hough said "there's always going to be further work" on the topic. She said seismological data is helpful but quakes as experienced by people "is what we care about."

Russell Gold contributed to this article.





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