In Istanbul, these fibres have revealed potentially life-saving information about how to protect people and infrastructure against future earthquakes.

 

Elsewhere, they are allowing researchers to measure the subsurface hum of London's bustle, track the rumbling of Iceland's volcanoes and map the upper reaches of our planet's mantle.

 

This new view of the underground has the potential to transform our understanding of the world's constant vibration.

"The overall goal is an almost transparent Earth," says Jonathan Ajo-Franklin at Rice University in Texas.

 

These fibers have revealed
potentially life-saving information

 

 

The road to that point begins with firing rapid pulses of laser light down a fibre-optic cable.

 

For normal telecommunications, the aim is to send information encoded in these pulses that will be decoded on the other end - the cable is just the conduit.

 

But the light can also be used to detect changes within the cable itself: if you were having a phone call or video chat over a fibre-optic cable and an earthquake occurred in the area of that cable, the frequencies in the other person's voice would come through distorted.

 

These distortions happen when underground vibrations stretch and bend the fibre through which a call is being carried.

 

The warping in the fibre causes subtle delays in the light that is reflected back to the beginning of the cable by tiny imperfections in the glass it is made of, known as the backscatter.

 

A device called an interrogator interprets shifts in this backscatter to determine exactly where the vibrations strained the fibre and by how much, which researchers use to reconstruct the tremors themselves.

 

A little more than a decade ago, seismologists realized they could repurpose fibre-optic cables to monitor underground trembling.

 

This would be a cheap way to extend the coverage of existing networks of seismometers, which are expensive to install and operate.

 

The high spatial resolution achievable with fibre optics could also create a far more detailed picture of the subsurface - at least at shallow depths - than what we can achieve with seismometers measuring shaking at a single point.

 

This gave rise to the field of distributed acoustic sensing (DAS), referring to how the cables make it possible to measure acoustic waves at many points along their length.

 

The oil and gas industry pioneered the use of DAS; as early as 2009, the industry was testing ways to monitor shaking in wells by snaking a fibre-optic cable down with the drill.

 

The approach was soon adopted for a broad range of applications, from tracking the movement of animals by their footsteps to observing changes in soil moisture by measuring the speed of waves moving through the ground.

"You have a new hammer, you have to go for every nail you can possibly find," says Andreas Fichtner at ETH Zurich in Switzerland.

Much of the research up to this point has used dedicated cables installed specifically for monitoring purposes.

 

For instance, fibre optics are commonly installed to monitor infrastructure like pipelines and train tracks. But more ambitious projects have sought to exploit fibres in existing telecommunication networks.

 

This approach has major practical benefits: it is cheaper because there is no need to build anything new, and existing networks cover far more of the planet than could ever be monitored with traditional seismometers.

 

Cables that are actively carrying signals generally can't be used to take reliable measurements, because the DAS lasers tend to interfere with the other signals being sent, but there is another option.

Most telecommunication networks are replete with unused cables laid down in anticipation of future traffic.

 

Making use of this global network of "dark fibres" could help fill the gaps in existing arrays of seismometers, says Andreas Wüstefeld at the NORSAR seismology foundation in Norway.

 

While that includes hard-to-reach places like the bottom of the ocean, the locations that stand to gain the most are seismically active cities that lack robust systems to monitor for shaking.

 

Istanbul is one of the largest such cities, among the world's most seismically vulnerable places.

 

That potential danger became tragically clear in February 2023, when a series of powerful earthquakes shook southern Turkey and Syria, killing more than 55,000 people and destroying hundreds of thousands of buildings. Istanbul, more than 800 kilometers from the epicentre, wasn't damaged, but low-level shaking did reach the city.

 

By chance, a group of researchers led by Fichtner was recording on a dark fibre when the shock wave arrived.

"As soon as we heard about it, we immediately rushed to the laptop and opened the data," says Daniel Bowden, also at ETH Zurich.

 

This map showed that during an earthquake, some areas along the fibre could see 10 times more shaking than adjacent streets. The arrival of an actual earthquake enabled the researchers to validate their model results, supporting the idea that the dark fibres can reliably predict seismic risk.

 

The city government of Istanbul now has that data to help inform building codes and construction projects above the fibre, says Bowden.

 

 

 

 

However, a single 8-kilometre fibre is just the start.

 

The researchers are now expanding this approach across an entire city. In May, they began using four 50-kilometre dark fibres to map the ground beneath about 900 square kilometers of Athens, Greece.

"We've never done something of this magnitude," says Fichtner.

Together, the cables encircle Athens and cross it in a big "X", which should enable Fichtner's team to create a detailed, three-dimensional map of the ancient city's underlying geology and seismic risk.

"For a seismologist, it's a dream geometry," he says.

They will also test ways to monitor "microquakes" that could help forecast more dangerous shaking.

 

Rafael Mestre at the University of Southampton in the UK and his colleagues are preparing an even bigger project using a dark-fibre network linking Southampton, London and Cambridge.

"Nobody has done large-scale continuous data acquisition in really large cities like this," says Mestre.

These areas aren't prone to earthquakes, but the data could have a slew of applications, from identifying leaky underground pipes to assessing the stability of building foundations.

 

Urban DAS could even detect vibrations above ground that penetrate Earth, like noise from a nearby airport. This extraordinary sensitivity does bring up concerns about privacy and surveillance.

 

The fibres are so sensitive that they could, in theory, be used to detect footsteps or even voices, says Mestre.

"It's going to be used and people don't even know about it."

DAS is also not without its technical issues. Much of the early interest in the technique focused on monitoring earthquakes in real time.

 

Unfortunately, the flood of extremely noisy data produced by the cables has made that idea look more challenging than researchers initially thought.

"The data are very difficult. They're just a mess," says Fichtner.

 

This extraordinary sensitivity
does bring up concerns
about privacy and surveillance

 

 

The cables are also concentrated where people live, so aren't useful in more remote places.

 

And getting access from telecommunication companies and governments can be a headache, he says.

"[But now] we are discovering what is doable, and what is not."

 

 

There have already been plenty of useful results.

 

For instance, Ajo-Franklin's group used a dark fibre in California to map ambient seismic waves, which revealed a previously overlooked geothermal reservoir that could be tapped to generate electricity.

 

Researchers at the California Institute of Technology used dark fibres around a volcanic system in Iceland to detect early signs of eruption.

 

Another team, based at the University of California, Berkeley, has made progress using offshore fibres to show how monitoring earthquakes in ocean basins could shave a few seconds off warning times. Tsunami warnings could, in turn, improve by life-saving minutes.

 

Meanwhile, other researchers are using these networks to peer into the planet's depths.

 

James Atterholt at the US Geological Survey (USGS) and his colleagues recently used a 100-kilometre dark fibre in California to map the boundary between Earth's crust and mantle in unprecedented detail.

 

Known as the Moho line, this boundary occurs at varying depths across the planet and marks a distinct transition in the composition and properties of the rock as the crust gives way to the upper mantle.

"Major structures that we see at the surface - volcanoes, faults - interact with the Moho," says Atterholt.

Being able to monitor such deep interactions could shed light on how large an earthquake a fault might generate, or on the plumbing of a particular volcanic system.

 

The USGS is now exploring ways to use a dark fibre in northern California to study one of the most seismically active areas of the continental US:

a meeting point of three tectonic plates called the Mendocino Triple Junction.

Monitoring shaking there could enable researchers to map this complex region in more detail, shedding some light on how it generates such powerful quakes.

"It's really exciting to think about imaging the Earth at a much, much higher resolution," says Atterholt.

 

"We're trying to use this fibre to do some interesting science."

The potential may even extend beyond the network of dark fibres.

 

Researchers have recently shown that it may actually be possible to reliably collect DAS measurements on "lit cables", those that are simultaneously transmitting other signals.

 

This "multiplexing" approach works by carefully selecting the wavelength of laser light fired through a cable so that it doesn't interfere with other network traffic.

 

This could make it possible to measure the vibrations on any part of the global fibre-optic network.

 

Measuring these quakes amid the signals of the internet could, in turn, expand our view into the planet's interior wherever cables are in place:

under cities, beneath the oceans, all around the world...

If it is possible, it could be a huge leap towards a truly transparent Earth.

 

Return Home

Return to Tsunamis and Earthquakes