The fibre-optic cables that carry internet traffic across the ocean floor turn out to be sensitive instruments in their own right. The same strands of glass that move data can register the faint strain of an earthquake hundreds of kilometres away.
The technique borrows from telecoms engineering. Pulses of light sent down a fibre scatter very slightly as they travel, and changes in that scattering reveal whether the cable is being stretched or squeezed along its length.
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