There's been a surge in earthquakes in the U.S. over the last few years. In Texas, there are 10 times the number of earthquakes now than just a few years ago.
Scientists say it's likely linked to the boom in oil and gas activity, meaning that people who never felt the ground shake are starting to.
Here's how Pat Jones of Snyder, Texas, describes the earthquake that struck her town in 2010: "It just sounded like some car hit the back of our house. We got up and checked around and we didn't see anything or hear anything else."
In 2012 in Alvaredo, Craig Bender called 911 about a quake and told the operator, "There was an explosion-type sound somewhere which kind of concerns me, but I haven't seen anything burning anywhere."
In a public forum with state oil and gas regulators, Greg Morrison described the feeling of a quake in Reno as "a semi truck hitting your house with a bomb going off."
Outside Texas, people are hearing those booms as well, often in states where there's been an upsurge in drilling and the use of disposal wells to store drilling waste. Scientists have linked those wells to quakes, and some quakes can get loud.
"They're actually hearing the wave that traveled through the rock all the way to the Earth's surface," says William Ellsworth, a seismologist with the U.S. Geological Survey. "When a fault slips suddenly underground, it radiates two different kinds of seismic waves."
One is a P wave, an acoustic wave that you can actually hear. The other is an S wave.
"It's the wave that carries most of the energy, and it's the one that we typically feel," Ellsworth says.
The P wave travels faster than the S wave, so before the ground even starts to shake, people may hear something.
"It's a little bit like thunder," he says. "You may see the flash in the distance, and then it takes a while for the wave — the air wave, in this case — to propagate. That makes the boom that you associate with lightning."
The U.S. Geological Survey even keeps an online catalog of earthquake noises. They range from a rumbling boom to a sound almost like a bass drum.
Most of the quakes in Texas are weak — but those are the kind that make the loudest noise. Ellsworth says stronger quakes often register at a frequency too low for people to hear.
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