North Korea’s 1-kiloton nuclear test last October showed up in the U.S. on readings from an exquisitely sensitive seismometer array. Seismologists Charles Ammon and Thorne Lay reported the finding last month in Eos: “Nuclear Test Illuminates USArray Data Quality”(PDF here).
Modern readers are quickly jaded: We all know nuclear weapons are powerful and seismographs are sensitive. So let’s add a little scale: First, the bomb North Korea tested was small but still showed up as a magnitude 4.2 jolt on nearby seismometers (pretty scary when you think about Cold War-era payloads in the megatons).
By the time the Pyongyang-induced tremors crossed the Pacific and reached the instruments in the U.S. array, Ammon and Lay write, the size of those vibrations would be about 2 nanometers, or around a thousandth the size of a speck of phytoplankton. It took some sophisticated data processing to sort the signal from the noise, but the signal was there. The rest of the article is a rather well-deserved pat on the back for the array’s sensitivity.
Last December, I marveled at the ability of a seismometer sitting on an Antarctic iceberg to pick up the roll and sway of swells arriving from Alaska. At the time, another seismologist remarked that his instruments, buried in rock at the South Pole, can hear those same waves crashing against the shore. Here is evidence of the Earth itself ringing from half a world away like a locomotive ringing down the railroad tracks.
In a marvelous throwaway remark, the authors noted that some of their seismometers missed the North Korea tremors because they were too close to the “noisy Pacific coast.” I like to think that the noise they’re referring to wasn’t just the creaky San Andreas fault, but the sound of Pacific surf battering the mudstone reefs of Santa Cruz and shivering on eastward through the inland rock.
So what is this USArray? It’s a monumental effort to put seismometers in the ground every 70 km across the entire United States (check out the Earthscope page). The seismometers are installed in broad swaths of about 400 at a time, starting in the West and gradually relocating eastward every 18-24 months. They report their data to the Web, and the latest reports are typically less than an hour old.
But in case you’re wondering, the idea here is not to help us detect bad guys blowing things up half a world away – or even, really, to detect big earthquakes of any kind. This large-scale installation lets people like Ammon and Lay turn the question around, and use earthquakes to learn about Earth. Their continent-sized net of seismometers catches the tremors from earthquakes large and small. Differences in the shape and timing of the same waves hitting different parts of this net let them piece together an image of Earth’s interior. It’s sort of like a great big planetary MRI. Cool.