A report on United States earthquake risk areas reveals what seismologists have gradually come to suspect: The interior states are now more likely to experience earth tremors than the famously quake-prone cities of the West Coast. Most of the time, human activity is to blame, but the question remains—how much damage can human-induced earthquakes do? 

The U.S. Geological Survey (USGS) has for the first time attempted to estimate the scale of earth movement that human activities generate. It acknowledges that oil and gas drilling operations are setting off thousands of earthquakes. So far, however, these have done little damage, often being too small to even detect without sensitive equipment.

However, the USGS warns that there is a danger of human-induced quakes with magnitudes as large as 7, the size of the 1989 disaster that killed 63 San Franciscans and did more than $5 billion dollars in damage.

Oklahoma, a fracking center, has seen magnitude 3 and larger earthquakes increase from 1.5 a year to 2.5 a day, and the increase shows no sign of slackening.

^{Credit: USGS via Science. Not all the earthquake zones in the American interior coincide with fracking regions, but Oklahoma matches almost perfectly.}

The injection of water and other chemicals to break rocks and extract fossil fuels can trigger fault lines, but the USGS says it is not usually the fracking itself that causes the quake, but the disposal of wastewater into deep rocks afterward.

Fracking opponents have seized on the tremors as another reason to ban the practice. Defenders dispute the quakes are human-induced, but also argue they're too small to matter.

Confidence that all quakes triggered by the buildup of pressure along fault lines would be small was shaken in 2011. An earthquake in Prague, Oklahoma, right at the heart of one of America's largest fracking zones, registered 5.6 on the Richter Scale. This was the largest ever recorded in Oklahoma, and was accompanied by before and after shocks registering 4.7. Two people were injured and 14 homes destroyed. 

The causes of the Prague quake are being debated in court, but a paper in Geology lays the blame with fracking wastewater disposal. The Oklahoma government has refused to limit fracking or wastewater disposal.

The USGS fears larger quakes may be coming: “There are certainly faults large enough to produce a magnitude 7,” Justin Rubinstein, a USGS geophysicist, told Science. “We can’t rule this out.”

Predicting earthquakes in fracking zones remains a challenge. Seismologists are used to using past data to predict risk, looking for signs of large earth movements in pre-history where the our records do not stretch back far enough.

“This new report describes for the first time how injection-induced earthquakes can be incorporated into U.S. seismic hazard maps,” said USGS's Mark Petersen. “These earthquakes are occurring at a higher rate than ever before and pose a much greater risk to people living nearby.”

The report relies on a combination of recent events and knowledge of the deep fault lines being tampered with, but the authors admit it is only a first effort at quantifying a growing problem.