https://www.scientificamerican.com/article/kilometers-of-dark-cable-form-the-newest-seismic-sensors/

Celeste Labedz heard a sound like thunder roll across the ice. She was standing on Alaska’s Taku Glacier, a vast field of snow-smothered ice between towering mountains, when the icequake began: a short-lived seismic tremor caused by the glacier’s sudden movement. Immediately she scrambled for her notebook and jotted down the time. Labedz, a graduate student at the California Institute of Technology, would check that time against data from a fiber-optic cable she and her colleagues had just deployed to study such quakes—a promising new method that is shaking up geology and adjacent fields.

Information travels through a fiber-optic cable via pulses of laser light, most of which moves directly through the hair-thin glass threads. But inevitably a small amount hits microscopic flaws in the cable and scatters back toward the source. This reflection varies when the cable stretches or bends because of ground vibrations, such as those from an earthquake or even a passing truck, and scientists can monitor changes in the backscattered light to quantify those movements. First developed by the petroleum industry a decade ago, this technique—known as distributed acoustic sensing (DAS)—has recently infiltrated the sciences. “The [DAS] community has just exploded in the past couple of years,” says Jonathan Ajo-Franklin, a geophysicist at Rice University. A workshop organized by the American Geophysical Union last December included scientists who had used the technique to image glaciers, monitor thunderstorms and peer into the deep ocean.