Somewhere beneath the Adriatic Sea, a rogue block of the African tectonic plate is burrowing under southern Europe, stretching Italy eastward by a few millimetres each year. On October 26, 2016, the stress triggered an earthquake in the Apennine Mountains, one in a series of quakes which toppled buildings in Italian towns.
On the day of the tremor, Giuseppe Marra, a principal research scientist at the National Physical Laboratory in Teddington, England, was running an experiment that beamed an ultra-stable laser through underground fibre-optic cables. It was part of a larger effort to build one of the world’s most accurate clocks, capable of measuring time to the nearest quintillionth of a second. Almost a thousand miles away from his native Italy, Marra did not feel the quake, but he heard about it on the news. The next morning, he walked to work to review the results of his experiment.
The light of a laser can be studied as a wave, and, as Marra looked at the data on his computer screen, he noticed what he called “a small wiggle” in the oscillations. In the language of physics, the phase had changed. As Marra tried to understand why, he recalled the earthquake, checked the timing, and found that his wiggle occurred at the same time as the squiggles on a British Geological Survey seismogram. In other words, the earthquake had caused a miniscule delay in the arrival of the laser’s light. He calculated that it must have nudged the underground cable by less than a millimetre. Marra had stumbled onto a new way to detect earthquakes. [Continue reading…]