EARTHQUAKES make seismic waves, which seismologists use to investigate the Earth’s inner structure. Seismic waves are typically recorded on seismometers, which are instruments placed in the ground that are sensitive to the shaking of the Earth after an earthquake happens.

If there are a lot of seismometers in a certain region, seismologists are able to calculate more accurate and precise models of the Earth’s inner structure beneath. 

For global models, the “resolution” of the model is a way to understand how accurate and precise it is; for regions with a lot of seismometers, the resolution is better.

This variation is most starkly seen beneath the oceans; it is not possible to deploy the same seismic instruments on the seafloor as on land, leading to poor resolution under oceanic regions in global models. 

Since the Earth is 70 per cent ocean, this is problematic for seismologists seeking to produce an accurate understanding of the layers that make up the Earth — the crust, mantle and core.

One solution that has been developed over the past few decades is the ocean-bottom seismometer, a specialist seismometer that can be placed on the seafloor.

These ocean-bottom seismometers have been specially designed to withstand the pressures at seafloor depths (as much as 5,500m), with the electronics kept inside a glass sphere.

The seismometers are dropped from the sea surface by a ship and then free-fall to the bottom of the ocean, from there they begin to record seismic waves.

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At the end of the experiment, the seismometers receive a signal from a surface ship, which triggers their release from the seafloor so the data can be analysed. 

The deployment of these ocean-bottom seismometers therefore requires a considerable amount of work and expense, with ships and personnel needing to be fitted for long sea voyages.

An alternative measurement technique that has become favoured by seismologists since 2010 is known as distributed acoustic sensing (or DAS). 

DAS exploits something that all of us use — the fibre optic telecommunications cables that run underneath the ocean. Globally there are approximately 1.5 million kilometres of these submarine cables, which form the basic infrastructure of global information and data exchange.

Some 99 per cent of all intercontinental internet traffic travels through submarine cables. In other words, the cables are used for almost all of our internet use. 

Data is transferred down optical fibres in laser light pulses. The fibres consist of a thin glass rod (known as the core) surrounded by a cladding layer. Both materials are specially selected to allow light to travel along the core by being totally reflected off the core-cladding boundary.

The reflection is important, because it means the fibre can be bent and the light will still travel down it. Submarine cables often consist of multiple optical fibres grouped together.

Optical fibres have multiple advantages for telecommunications. The fibres have low loss, meaning the light travels for a long time before the signal requires amplification. They also have high bandwidth, meaning that multiple signals can be sent through each fibre. 

Submarine fibre optic cables can be exploited to detect seismic waves by using the cable as a sensor for strain: a measure of the amount of deformation experienced by the cable. When seismic waves from an earthquake reach the cable, their motion stretches and compresses the fibre, causing a change in strain. 

To detect the change in strain, seismologists attach a piece of equipment known as an interrogator to the optical fibre. The interrogator fires pulses of laser light down the fibre.

The glass core of the fibre contains microscopic imperfections, which cause some of the laser light to “backscatter” towards the interrogator.

This is the same mechanism that causes the sky to appear blue: as light from the Sun travels through the atmosphere, it scatters off molecules. Blue light is more likely to be scattered, because its wavelength is shorter, so the sky looks blue when we observe it on the Earth’s surface. 

In submarine fibre optic cables, the backscattered light is affected by changes in strain due to seismic waves. This difference can be analysed by the interrogator to detect seismic waves arriving and leaving, similar to seismometer readings.

The length of the cable means that the DAS measurements are "distributed” (where the name comes from) across many kilometres. A DAS interrogator can sense an earthquake at many points along the cable.

Instead of acting like a single seismometer, it mimics the measurements you would get if you had an array of seismometers across a large distance. 

Distributed acoustic sensing has multiple applications, from detecting earthquakes on the seafloor to investigating so-called icequakes on glaciers, a motion similar to an earthquake but caused by the movement of ice.

It has also been used to monitor whale presence and movement through the oceans, as described in a recent study in Frontiers in Marine Science, where a team of scientists led by Robin Rorstadbotnen demonstrated that DAS can be used to record low-frequency whale vocalisations. 

The physical architecture of the internet is invisible to most users of it. We have become so used to near-instantaneous global connection that we often don’t stop to think about the considerable amount of infrastructure that physically facilitates our phone and video calls, streaming services, data transmission and access to the web.

However, these facilities are very much material, and can be strategic or inadvertent targets of military attacks. In early 2024, an apparently inadvertent example of damage of this kind happened after a British cargo ship that had been attacked by a Yemeni Houthi missile seems to have ripped up three of the 15 optical cables in the Red Sea, which altogether carry 90 per cent of internet traffic between Europe and Asia. 

Internet cables could easily be a more targeted arena of combat in future. Scientific innovation could also mean that this technology can be used to advance our knowledge and understanding of the oceans and what lies beneath them; regions of the globe that have been historically inaccessible to exploration.

However, such innovation is currently limited by private corporations, who own 99 per cent of submarine cables, and whose permission is required to experiment on the cables.

Free, publicly owned telecommunications could enable new, exciting methods of underwater exploration. 

Science and Society is a fortnightly column.