  Earthquake monitoring across the globe is currently achieved with networks of
seismic stations. The data from these networks have been instrumental in
advancing our understanding of the Earth's interior structure and dynamic
behaviour. However, almost all seismic stations are located on land and
earthquakes of magnitude smaller than 4 at the bottom of the oceans remain
largely undetected. Here we show that ordinary telecommunication optical fibre
links can detect seismic events when combined with state-of-the-art frequency
metrology techniques. We have detected earthquakes over terrestrial and
submarine optical fibre links with length ranging from 75 to 535 km and a
geographical distance from the earthquake's epicentre ranging from 25 to 18,500
km. In contrast to existing commercial reflectometry-based acoustic sensing
methods used widely in the oil and gas industry, which are limited to only a
few tens of kilometres, the technique presented here can be extended over
thousands of kilometres, paving the way for detection of remote underwater
earthquakes. By using the proposed technique on the existing extensive
submarine optical fibre infrastructure, which already criss-crosses the seas
and oceans, a global seismic network for real-time detection of underwater
earthquakes could be implemented. The ability to detect off-shore earthquakes
closer to the source could also enable a cost-effective solution for early
detection of tsunamis.
