A cluster of earthquakes hit Oklahoma over the past few days, unsettling thousands of the state's residents. As of 11 a.m. ET Monday the U.S. Geological Survey says that 2,274 people reported feeling a 4.3 magnitude quake Sunday night. There have been at least 16 noticeable earthquakes (above 2.5 in magnitude) observed by the Geologic Survey since Friday, April 6. While nerve-rattling, the quakes are normal for the area — at least since 2009. That's when the problematic quakes began, Jeremy Boak, Director of the Oklahoma Geological Survey, said in an interview.  SEE ALSO: Hey, how about we helicopter grizzly bears into this remote National Park? "It's not out of the ordinary," said Boak. "In the frame of what’s been going on, it’s normal." Oklahoma's dramatic rise in quakes has been stoked by oil and gas extraction activity in the region.  There have been 8 earthquakes with magnitudes ranging from 2.5 to 4.6 between Perry and Covington in northern Oklahoma in the past 24 hours. The latest, having a preliminary magnitude of 4.6, occurred at 7:16 CDT this morning. #okquake t.co/JwfpIrHgSb pic.twitter.com/UbqUwya6jX — USGS in Oklahoma (@USGS_Oklahoma) April 7, 2018 This quake activity — associated with the "fracking revolution" that has also propelled historically high U.S. oil exports — comes in two forms. The first is fracking itself, an oil extraction process more formally known as "hydraulic fracking." Broadly, this means injecting millions of gallons of water, sand, and a small percentage of chemicals into a deeply-drilled hole. This breaks apart rocks to release oil deposits, sometimes creating earthquakes. But most Oklahoma quakes aren't caused by fracking itself, but by a secondary process called "wastewater injection." After water is used to fracture apart rocks thousands of feet below, it comes back up as "wastewater," and is usually injected back into the ground nearby (the mixture has to go somewhere). Water is extremely heavy, so, this can put pressure on deep-lying faults. And if enough pressure is applied to these cracks in the Earth's crust, they'll rupture and move, causing sizeable quakes.  While a U.S. Geologic Survey spokesperson said it's too early to officially confirm the cause of the northwestern Oklahoma earthquake burst, Boak said it's almost certainly due to wastewater injection. That's the common cause of quakes in this part of northwestern Oklahoma, and generally, has been the prevailing story for years. Earthquakes of 3.0 magnitude or higher measured in Oklahoma as of July 2017.Image: U.S. Geologic survey But, overall, earthquakes have been on the decline in Oklahoma since the especially rattling years of 2014, 2015, and 2016.  The year 2015 saw nearly 900 quakes of 3.0 or higher in Oklahoma (around 2.5 or above is noticeable to most people). For perspective, before 2009, Oklahoma usually recorded one or two quakes of 3.0 magnitude or higher each year. By 2015, earthquake activity peaked for a time at around 4 and a half quakes each day, Boak previously said.  But this year, Boak expects around 200 noticeable quakes to occur in Oklahoma. This recent cluster of quakes, then, is "part of the continuing pattern which in general is declining," he said. There are two major reasons for the decline, said Boak. One is the falling price of oil. This means that oil and gas extraction isn't quite as lucrative as it once was a few years ago (it's a famously boom and bust industry). Accordingly, there's a bit less fracking activity. Oklahoma resident Lisa Griggs believes cracks in her home have been caused by Oklahoma's manmade earthquakes.Image: The Washington Post/Getty ImagesThe second reason is mandatory state requirements that oil and gas companies find ways to reduce quaking. The rattled citizens of Oklahoma made quite clear to the Oklahoma Corporation Commission, which regulates the gas industry, that the quakes must stop — or at least be limited. "We needed to shut this down and it actually appears to have worked," said Boak. Oil and gas companies accomplish this reduction in a variety of ways, which includes stopping wastewater injections when seismic activity begins. As for Boak, he has still yet to feel one of Oklahoma's big quakes — even though he studies them. He's too far south of most the activity, in the quieter confines of Norman, Oklahoma.  "I’ve never had the privilege of feeling one of the Oklahoma earthquakes," he said.  WATCH: Scientists found a weird galaxy without dark matter

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