The US Geological Survey (USGS) regularly puts out maps assessing earthquake hazards across the United States. Up until now, they've focused on natural earthquakes, with the West Coast as the focal point.
But this year's map is different. For the first time, the USGS is including the risk of seismic activity caused by humans — particularly earthquakes set off by the injection of wastewater from oil and gas operations in states like Oklahoma and Kansas.
That changes the picture considerably. The USGS found that a bunch of states east of the Rocky Mountains are at risk of potentially "damaging" man-made earthquakes that could affect buildings, pipelines, and other infrastructure. All told, some 7 million people live in these areas. Here's the agency's forecast for 2016:
California, which sits right on the boundary between the Pacific and North American tectonic plates, still faces the biggest risk from naturally occurring earthquakes. The USGS calculates a 5 to 12 percent chance of a "damaging" quake for parts of the Bay Area in 2016.
But then there's Oklahoma. A huge chunk of the state has a 5 to 10 percent chance of a "damaging" earthquake this year. Oklahoma isn't naturally a hotbed of seismic activity. But since 2008, as the state's oil and gas industry has expanded, the region has experienced hundreds of small quakes, usually magnitude 3.0 or smaller. USGS is now warning that bigger shocks that could damage buildings are possible.
Kansas, Texas, Colorado, New Mexico, and Arkansas also face increased risk from man-made earthquakes.
Now, an important caveat: the agency doesn't predict how big any of these quakes might be. It just pegs the odds of some sort of "damaging" earthquake in each region. But size matters. A worst-case scenario for Oklahoma might be a magnitude 6.0 quake. That's bad and worth preparing for, but it's thousands of times weaker than the worst-case scenario along California's San Andreas Fault or for a catastrophic Cascadia earthquake near Seattle. Not all dangers are equal.
As part of its assessment, the USGS also identified more than 21 specific areas east of the Rockies where human activity has induced earthquakes since 1980. They are circled on the map below.
How oil and gas operations cause earthquakes in Oklahoma
Oklahoma has unexpectedly become the earthquake capital of the US in recent years, with 857 earthquakes of magnitude 3.0 in 2015. And scientists who have studied the issue usually point to the expansion of oil and gas operations.
In particular, Oklahoma has a number of "de-watering" operations that separate out the saltwater from oil that's pulled up from conventional wells. This leftover liquid then gets injected back underground into separate disposal wells so that it doesn't contaminate local freshwater.
But there's a catch: In some places, injecting water back underground can push the nearby crust downward and increase pressure in cracks along key underground faults. That can make those faults more prone to slippages and earthquakes. This doesn't happen with every injection well, but it happens with some.
Many people have also wondered if fracking is to blame. This is a bit more complicated.
There's little evidence that fracking itself — in which water, chemicals, and sand are injected underground to crack open shale rock and extract oil and gas — is causing many earthquakes in the Midwest. But after wells are fracked, the resulting wastewater does need to be pumped out and disposed of. When that wastewater is injected back underground into disposal wells, tremors can sometimes result.
Still, situations vary. In Oklahoma, used hydraulic fracturing fluid makes up less than 10 percent of the wastewater injected into disposal wells. By contrast, in other areas that have seen man-made earthquakes, such as Guy, Arkansas, and Youngstown, Ohio, fracking fluid makes up a much bigger portion of the injected wastewater.
In Oklahoma, a handful of wells seem to be causing most of the earthquakes
Most of Oklahoma's 4,000-plus injection wells don't seem to be causing any earthquakes. But a 2014 Science study, led by Katie Keranen of Cornell, found that a handful of high-activity wells appear to be driving the uptick in seismic activity in central Oklahoma:
In particular, four injection wells southeast of Oklahoma City appear to have been responsible for fully one-fifth of earthquakes since 2008. These wells are part of a de-watering operation by New Dominion and inject 4 million barrels of wastewater underground each month.
The Science study also noted that pressure from the wastewater wells can travel through the subsurface farther than anyone thought. The four high-activity dewatering wells are capable of triggering seismic activity in a 772-square-mile vicinity.
That could explain why, for example, the town of Jones, Oklahoma, has seen thousands of small earthquakes since 2008 despite the fact that there are no wastewater wells in the immediate vicinity.
So far, these earthquakes have mostly been too small to do serious damage or endanger lives. Still, they have stirred up concern among Oklahoma's residents, and regulators have been debating (arguably too sluggishly) whether further safeguards are needed.
-- Last year, Buzzfeed's Dan Vergano did an excellent deep dive into man-made earthquakes in the Midwest.
-- What are the odds that a giant earthquake will devastate Seattle? We took a closer look here.