The controversial practice of hydraulic fracturing to extract natural gas does not pose a high risk for triggering earthquakes large enough to feel, but other types of energy-related drilling can make the ground noticeably shake, a major U.S. government science report concludes.

Even those man-made tremors large enough to be an issue are very rare, says a special report by the National Research Council. In more than 90 years of monitoring, human activity has been shown to trigger only 154 quakes, most of them moderate or small, and only 60 of them in the United States. That's compared to a global average of about 14,450 earthquakes of magnitude 4.0 or greater every year, said the report, released Friday.

Most of those are caused by gas and oil drilling the conventional way, damming rivers, deep injections of wastewater and purposeful flooding. Only two worldwide instances of shaking — a magnitude 2.8 tremor in the state of Oklahoma and a 2.3 magnitude shaking in England — can be attributed to hydraulic fracturing, a specific method of extracting gas by injection of fluids sometimes called "fracking," the report said. Both were last year.

"There's a whole bunch of wells that have been drilled, let's say for wastewater and the number of events have been pretty small," said report chairman Murray Hitzman, a professor of economic geology at the Colorado School of Mines. "Is it a huge problem? The report says basically no. Is it something we should look at and think about? Yes."

With increased drilling to satisfy the U.S. thirst for energy, it is important to watch injection and other wells better and consider potential repercussions before starting, the report said. No one has been killed, nor has there been major damage, from man-made quakes in the United States, said the report by the council, which is part of the National Academy of Sciences, a private non-profit institution that provides expert advice to the government.

Potential for 'significant seismic events'

"There is potential to produce significant seismic events that can be felt and cause damage and public concern," the report said.

The research council report shows that most of the tremors that can be blamed on humans occurred in the states of California, Texas, Colorado, Oklahoma, and Ohio. California and Oklahoma had the biggest man-made shakes as byproducts of conventional oil and gas drilling. Colorado has one of the most documented cases of three 5.0 to 5.5 man-induced quakes because of an injection well. Northern California also has 300 to 400 tiny quakes a year since 2005 because of geothermal energy extraction.

Man-made drilling — usually injections of fluids deep and at high pressure — can trigger shaking because it changes the crucial balance of fluid into and out of the subsurface. That can then affect the pore pressure of the soil and that's what helps keep faults from moving, Hitzman said.

The report makes sense as far as it goes, said U.S. Geological Survey seismologist William Ellsworth, but since the research council started its study, government geologists have noticed a strange increase in earthquakes that seem man-made. At a professional seismology conference in April, Ellsworth presented a USGS report on a six-fold increase in man-made quakes.

He pointed to induced quakes of magnitude 4 or larger in the past year in Texas, Oklahoma, Arkansas, Colorado, New Mexico, and Ohio, but said much of this happened too late for the research council to include in its study.

Low risk for quakes from hydraulic fracturing

Hitzman said it's still too early to tell whether those recent quakes would have changed the report's conclusions.

Another study — also too recent for the research council report — says a 4.7 magnitude quake in central Arkansas in 2011 was man-made and scientists are still looking at a 2011 quake in Oklahoma that measured 5.6 as a potential but not proven induced tremor, Ellsworth said.

The man-made quakes that Ellsworth has been seeing are almost all related to wastewater injection, he said. Ellsworth said he agreed with the research council that "hydraulic fracturing does not seem to pose much risk for earthquake activity."

If the country starts capturing the global warming gas carbon dioxide from coal power plants and injecting it underground, there is a potential for a larger quakes given the amount of the heat-trapping gas that would have to be buried, the council's report said. That's an issue that needs more study, it said.

Congress and the Department of Energy requested the 240-page report.