Nearly seven million Americans are vulnerable to man-made earthquakes in central and eastern US states in 2016, according to a US Geological Survey (USGS) report. It estimates the probability, strength and potential areas where man-made and natural earthquakes can occur.

"This research also shows that much more of the nation faces a significant chance of having damaging earthquakes over the next year, whether natural or human-induced," said Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project. "By including human-induced events, our assessment of earthquake hazards has significantly increased in parts of the US."

The study revealed that while western states like California run the highest risk of being hit by natural earthquakes, several central US areas are equally at risk of damage occurring from a man-made and natural earthquake. An earthquake hazard map, which was published as part of the report, shows the percentage of damage expected across states deemed to be most at risk of being hit by earthquakes.

The six states found to be at the highest risk of damage included Oklahoma, Kansas, Texas, Colorado, New Mexico and Arkansas. "In the past five years, the USGS has documented high shaking and damage in areas of these six states, mostly from induced earthquakes," said Petersen, according to a Sky News report. Central US is being carefully studied due to the increasing seismic activity in the region over the last six years.

Several factors can trigger a manmade earthquake, including waste water from oil and gas production being disposed off into deep underground wells. "Wastewater disposal is thought to be the primary reason for the recent increase in earthquakes in the CEUS [Central US]," the report says.

"While most injection wells are not associated with earthquakes, some other wells have been implicated in published scientific studies, and many states are now regulating wastewater injection in order to limit earthquake hazards."

Last year experts found that a series of earthquakes that hit Ohio in 2014 were caused by fracking — a process that involves drilling deep into the earth and injecting a high-pressure water mixture into the rocks. According to the report published in the Bulletin of the Seismological Society of America, researchers found hydraulic fracturing had led to the increased seismic activity.