Last week we learned that earthquakes occur nearly every day in Illinois -- we just don’t feel most of them because they are so small.
This week, we are looking at why we have so many.
We spoke with Dr. Megan Brown. She's an assistant professor with the Department of Geology and Environmental Geosciences at Northern Illinois University.
First, she described an earthquake.
"It's a rapid release of strain," said Brown. "And that creates movement or slippage along a fault and it causes radiating seismic waves -- energy -- that go out in all directions that causes shaking."
Brown explained that there are "different types of different earthquakes."
"Most earthquakes," she said, "occur along plate boundaries because the earth is broken up into many different tectonic plates that move at all times, kind of like a cracked egg shell." Brown said most earthquakes occur along the edges of the plates.
"But then you also have earthquakes that occur in the middle of those plates," she said. "Those are a lot less common and they're called intraplate earthquakes." Brown said any earthquakes in Illinois are intraplate earthquakes because we are not along a plate boundary.
"We're still doing research on why exactly they happen there," Brown explained. "There could be a variety of reasons -- things like fluids playing a role, or geologic structures, both, you know, ancient structure versus intersecting faults. There’s several different hypotheses of why intraplate earthquakes occur."
Some believe conventional drilling leads to earthquakes. Brown calls that technique the “jelly donut” approach to drilling -- where a drill goes straight down into the “donut” – actually the earth -- and extracts the “jelly” -- oil and natural gas.
"Where that analogy fails is it's not like a lake of oil and gas," said Brown. "The oil and gas is sitting -- just like groundwater does -- in between the grains -- the rock in the pore space -- and so that's one of the reasons you might frack a rock is to try to make higher permeability, which is just the ability for fluid to flow through the rock. So you want to increase that to allow more flow, then the oil and gas is pumped out by pump jacks, which are those kind of those bobbing kind of rotating machinery that are often seen in like fields along interstates occur in the, you know, middle part of the country."
Unconventional drilling (called "hydraulic fracturing" or "fracking") uses sand and water to penetrate the earth. It also travels horizontally thousands of feet below the surface. This process creates wastewater which has to go somewhere. Drilling operations often put it back into the ground. That can have consequences. But, it turns out that, in this state, wastewater isn’t the only culprit.
"So injection-induced earthquakes are earthquakes that occur or are triggered by human activities and specifically injection of fluids, usually into the deep subsurface, but sometimes a little shallower," said Brown. "And in Illinois, the vast majority of induced earthquakes are actually associated with the carbon sequestration."
That’s the removal of carbon dioxide from the atmosphere and placed deep -- that is to say, injected -- into the earth with a goal of slowing or reversing global warming. That took place in Decatur from 2011-2014.
"And so there is some induced seismicity there," said Brown. "And that is from injecting CO2 into the subsurface to try to sequester or keep it down there to reduce carbon emissions. But as far as I know the vast majority are very small magnitude. I'm not sure if they've had any felt earthquakes but I can't say they haven't. I haven't done as much research on that."
Brown explained that conventional drilling can lead to earthquakes.
"If you have fracking and conventional [drilling] those little micro-seismicities from breaking the rock are earthquakes," she said. "But the majority of induced seismicity from conventional drilling are going to be from the extraction. So if you’re extracting a whole bunch of oil and gas and the water that comes with it you have pressure decreasing and that can change the stress along faults and allow earthquakes to occur. It’s a little less of a problem than the wastewater disposal, but it definitely does happen."
What about unconventional drilling -- fracking -- in Illinois?
Brown said it doesn't take place in the state. "There are laws in place that were put in somewhere around 2013-2014 for the high volume, hydraulic fracturing, but it looks like there hasn't been any permits that have actually been used."
Whether you are concerned about the potential side effects of conventional and unconventional drilling or fracking, or are in favor of fracking, or your opinions are somewhere in between, Dr. Brown said the key thing is to stay informed.
"Whenever there’s a permit that goes through, there’s usually a requirement for public notice," she said. And if a group wants to ban fracking or limit it, "Stopping it before it starts is definitely easier."
"It's amazing what kind of things end up having public notices," she said. "Tons of times there's public notices and no one actually comments because, I think, they're just not paying attention, but there's definitely always time to comment on those public notices and a lot of these regulatory agencies have open meeting so that people can stay informed."
To learn about registration, permits and regulatory acts, visit the website for the Illinois Department of Natural Resources. To learn about public notices, visit the website for the Illinois Environmental Protection Agency.