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Confirmation of this hunch finally arrived on August 16, when Thiel got in contact with Gideon Rogers, a spokesperson from the plant. The plant disposes of old ammunition each morning around 11 a.m., except on Sundays, in explosions separated by 20-second pauses.
A request for additional details about their activities is still pending. But Carmichael suspects that the ammunition is being split into a small groups that are detonated sequentially, to avoid shocking nearby populations with a single large blast. That would send out pulses of energy that can account for the unusual geologic heartbeat across the state.
Some of the confusing ping-pong effect might because the scientists were too zoomed in when looking for patterns, like examining the individual dots in a pointillist painting. Realizing the source was McAlester—which is located farther to the southeast than the vast majority of the detectors—helped them take a step back and see the whole picture.
“It was just seeing data in a different way than we’re used to,” Thiel says.
By land or by air
But how the energy from presumably small blasts wound up traveling statewide has left researchers debating.
Speed estimates suggest that the signals swept across Oklahoma much slower than energy usually moves through the ground—during an earthquake, for instance, surface waves usually bop along at about 2,200 miles an hour or more. But Oklahoma’s waves weren’t exactly going slow, either, sweeping across the state at sometimes supersonic speeds of nearly 900 miles an hour.
One possibility is that the seismometers are picking up sound waves. Sound can travel stunningly far, says Boise State University’s Jeffrey Johnson, an expert in the use of infrasound to study volcanoes. However, it doesn’t necessarily zip straight to seismometers.
Explosions in particular send out a smattering of acoustic waves at different frequencies, the lowest of which, known as infrasound, travels the farthest. Infrasound can jettison up into the atmosphere, where the air grows steadily cold and then warm again. These layers of heat can bend the waves back toward the ground in concentric zones like a bullseye.
Such sounds could be shaking the ground—similar to a deep bass riff shaking your car window—or even rattling the solar panels above the instruments. These are extremely sensitive detectors, Thiel explains via email: “We even try to install stations away from trees, where the leaves and branches might sway in the wind, producing movement in the tree's roots, which could contribute noise to the sensor.”
According to Johnson, who reviewed OGS’ analysis of the event on National Geographic’s request, this is definitely acoustic waves.”
A wobble below
But Carmichael spotted something that suggests sound might not be the only phenomena at work. The motion of the rollicking pulses logged by many of the seismometers points to a surface wave radiating through the ground. While rare, a slow surface wave is not impossible, he says, and perhaps in just the right atmospheric and geologic conditions, such a wave could sweep far and wide.
“Nature is conspiring to make the signals travel in such a way that we can’t tell whether they’re acoustic or seismic,” he says.
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