Over the last few months, I’ve been reveling in a stunning chorus of sounds obtained from digital recordings of seismograms that I’ve converted to multichannel audio, as part of the Deep Earth Dome project. Some of these sounds have never before been heard by human ears. Now and then I uncover one that particularly catches my ears and triggers my imagination.
This is one of them.
It’s a recording of the motion of the ground in Alibek, Turkmenistan, speeded up by a factor of 1100. In this 1½ minute recording, you’ll hear just over 24 hours of real Earth time unfold. Listen for a series of chattering “chirps” emerging from out of the natural microseismic background noise. It sounds a bit like a flock of frenzied sparrows. Over the course of several tens of seconds (the better part of a day, in real Earth time), it swells and fades, eventually slipping back into the noise. After a pause, it returns.
What are these sounds? Are they evidence of a rare tectonic phenomenon peculiar to this part of the Eurasian plate? Are they the creaks and groans of the Earth’s crust responding to the stresses induced by the sudden load of snow dropped by an early winter storm? Is there previously undiscovered bird life in the crust? Perhaps I’d made a groundbreaking sonic discovery, worthy of a sound installation all its own.
But as I kept listening, something bothered me. First, the frequency (pitch) of these sounds was unusually high: about 3-4Hz in the time frame of the real Earth. Vibrations of that frequency don’t travel very far in the Earth’s loose and fractured crustal rock. So whatever generated these sounds must have been quite close to the recording site. Furthermore, these sounds often come and go very abruptly — almost as if someone were flipping a switch. It just didn’t sound natural.
So, putting on my forensic seismologist hat, I did a little digging. I assembled a continuous recording of 10 days of seismic data from this site, and applied a sharp bandpass filter to strip away the sounds not associated with these chirps. The mystery soon unraveled.
The chirps tend to occur in long bursts, once a day, starting around about 7am local Alibek time. At around noon, there’s a slight reduction in amplitude, which lasts for a couple of hours. The noise then picks up again until around 5pm. Most days it’s quiet all night, until the next morning, when the cycle starts all over again. Hmm… Does this remind you of anything? Heck, there’s even one day when you can clearly see the night shift going at it for a few hours after dinner, before calling it quits at around 9pm.
The pieces begin to fall into place. Imagine a construction site just over the hill from the recording station with a bunch of bulldozers and trucks lumbering about. A big Soviet-era front-end loader bashes into a dirt pile, dumps its load into a huge dump truck, which drives away, bouncing heavily at about 3Hz on its rubber balloon tires. The ground rumbles. The nearby seismometer in its underground vault dutifully captures it all.
With one mystery solved, others rush in to fill the gap: What are these workers in Alibek up to? Turkmenistan has a notoriously secretive government, so all we can do is speculate. But given the depressed state of the national economy, it’s a safe bet that they’re not building condos. Maybe they’re building gas pipelines. Or maybe they’re building a Silk Road offramp to their own version of the US’s Area 51. We can only wonder.
Ours is a world layered in mystery. Although these particular sounds turned out to be of human origin, they’re still the voice of the planet speaking to us, in the language of heavy machinery rather than of falling snow and creaking crust. It’s the sound of environmental intervention on a planetary scale. But more about that in another post…