Detecting earthquakes with optical fibres

An interesting study was published in Science last week, showing how transoceanic optical fibres can be used to detect seismic waves. This is important because the vast majority of seismic receivers are located on land, which limits our knowledge of the earth’s interior. There are attempts to address this imbalance using seismic receivers on the ocean floor, however since they are expensive to deploy their coverage remains limited. 

The present study shows that existing transoceanic optical fibre networks which form the backbone of the internet can help fill this gap in seismic receiver coverage. The authors’ idea is to sense seismic waves by detecting changes in the polarization of light transmitted through oceanic fibers. 

Modern optical fibers use the polarization degree of freedom of light to increase bandwidth by encoding signals into both polarization channels. However, the polarization is not fixed during propagation and is perturbed due to various effects including temperature changes and vibrations. A decoder at the end of the fiber is used to measure and correct for these distortions.

On land there are many sources of noise in the polarization, including vibrations caused by passing traffic. The weak signals caused by seismic waves are typically masked by the noise. For sub-sea fibres the noise is orders of magnitude weaker, enabling their use as seismic sensors.

For their study, the authors used a recently-deployed 10,000km fiber link between Los Angeles, USA and Valparaiso, Chile. By measuring changes in the polarization emerging from the fiber they detected signals from several large earthquakes occurring over a period of several months. 

The fiber could also detect ocean waves caused by storms. Therefore, another useful application of this method may be in the detection of tsunamis. Further work will be required to improve the sensitivity of the technique, such as by combining measurements from several nearby fibres. 

Reference: Optical polarization–based seismic and waterwave sensing on transoceanic cables