When it comes to earthquakes, the earlier the warning, the better. While current seismometers can pick up minor tremors with precision, they're relatively short-ranged and expensive, but now, a Stanford study has demonstrated that an extensive earthquake-detection network could already exist right under our feet: the optical fiber cables piping high-speed internet throughout cities.

Since optical fibers work by bouncing light signals down a glassy cable, minor disturbances to that signal can be measured as they come back. This technology, called distributed acoustic sensing (DAS), is already at work in the oil and gas industries.

"How DAS works is that as the light travels along the fiber, it encounters various impurities in the glass and bounces back," says Eileen Martin, co-author of the study. "If the fiber were totally stationary, that 'backscatter' signal would always look the same. But if the fiber starts to stretch in some areas — due to vibrations or strain — the signal changes."

To test whether these kinds of cables could be used to monitor and measure earthquakes, three miles (4.8 km) of optical fiber was installed under Stanford in a figure-8 shape, fitted with laser interrogators designed to record any movements.

This fiber optic seismic observatory, as they called it, was fired up in September 2016, and it was able to record more than 800 events in its first year of operation. That included manmade rumbles like blasts from a nearby quarry, small local quakes and even the huge magnitude 8.2 event that devastated central Mexico on September 8 this year, some 2,000 miles (3,220 km) away from the Stanford setup. In one case, the sensor picked up two earthquakes from the same source, with magnitudes of 1.6 and 1.8.

"As expected, both earthquakes had the same waveform, or pattern, because they originated from the same place, but the amplitude of the bigger quake was larger," says Biondo Biondi, lead researcher on the study. "This demonstrates that fiber optic seismic observatory can correctly distinguish between different magnitude quakes."

In the same vein, the seismic observatory was also able to tell the difference between S and P waves, shockwaves that ripple through the ground at different speeds. P waves usually arrive much earlier than S waves but are far weaker, so detecting them is a key part of an effective early warning system.

While their fiber optic seismic observatory has shown some promising early results, the researchers point out that traditional seismometers are still more sensitive for monitoring earthquakes, but their system has other advantages. Namely, it's already in the ground in many places, just waiting to be tapped into. That makes it much cheaper and more widespread, allowing for a more affordable way of researching earthquakes and improving early warning systems.

"We can continuously listen to – and hear well – the Earth using preexisting optical fibers that have been deployed for telecom purposes," says Biondi. "Every meter of optical fiber in our network acts like a sensor and costs less than a dollar to install. You will never be able to create a network using conventional seismometers with that kind of coverage, density and price."

The research was published in the SEG Technical Program Expanded Abstracts 2017.

Source: Stanford University

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