7 million Americans threatened by human-induced quakes

Around seven million people in the United States live in areas at risk of damaging, human-induced earthquakes that are often linked to fracking, a report out Monday said.

Because the US Geological Survey is including the human-triggered earthquakes on its forecast maps for the first time, the odds of damaging shaking in certain parts of the country is much increased for 2016, it said.

The seven million people who are at elevated risk live in the central and eastern United States, which has seen the most significant increase in human-triggered seismic activity, according to USGS.

While some western regions are also experiencing induced earthquakes, these don't significantly change the hazard level there because natural earthquakes are already prevalent in the area.

The primary cause for human-induced earthquakes in many parts of the central and eastern United States is wastewater disposal, the USGS reported.

Such wastewater is linked to hydraulic fracturing, or fracking, the process of shooting water mixed with sand and chemicals deep into the earth to crack rock formations and bring up oil and natural gas trapped inside.

The process has unlocked massive amounts of oil and gas in the United States over the past decade.

But along with the oil and gas comes plenty of brackish water, which is disposed of by injecting it into separate wells that are dug as deep as a mile (kilometer) below ground.

The unnatural addition of the water can change pressure along fault lines, causing slips that make the earth shake, experts say.

The USGS said the states at highest risk of induced seismic activity are, from highest to lowest: Oklahoma, Kansas, Texas, Colorado, New Mexico and Arkansas.

Oklahoma and Texas are the states with the largest population exposed to the effects of human-triggered earthquakes, it added.

"In the past five years, the USGS has documented high shaking and damage in areas of these six states, mostly from induced earthquakes," 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," he added.

Monday's report is a supplement to the USGS's normal 50-year forecast, a timeframe that was established to help inform engineering designers and those setting building codes, because it is based on the average lifetime of a building.

The USGS said it chose the short timeframe of one year for its 2016 supplement "because induced earthquake activity can increase or decrease with time and is subject to commercial and policy decisions that could change rapidly."