Editing Seismometer (section)

=== Fiber optic cables as seismometers ===
A new technique for detecting earthquakes has been found, using [[fiber optic]] cables.<ref><!-- {{Harvnb|Marra|Clivati|Luckett|Tampellini|2018 -->
{{Citation |first1=Giuseppe |last1=Marra |first2=Cecilia |last2=Clivati |first3=Richard |last3=Luckett |first4=Anna |last4=Tampellini |first5=Jochen |last5=Kronjäger |first6=Louise |last6=Wright |first7=Alberto |last7=Mura |first8=Filippo |last8=Levi |first9=Stephen |last9=Robinson |first10=André |last10=Xuereb |first11=Brian |last11=Baptie |first12=Davide |last12=Calonico |date=3 August 2016 |title=Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables |journal=Science |volume=361 |issue=6401 |pages=486–490 |doi=10.1126/science.aat4458 |pmid=29903881 |doi-access=free |hdl=11696/59747 |hdl-access=free}}.</ref>
In 2016 a team of metrologists running frequency [[metrology]] experiments in England observed noise with a wave-form resembling the seismic waves generated by earthquakes. This was found to match seismological observations of an {{m|w|6.0|link=y}} earthquake in Italy, ~1400&nbsp;km away. Further experiments in England, Italy, and with a submarine fiber optic cable to [[Malta]] detected additional earthquakes, including one 4,100&nbsp;km away, and an {{m|l|3.4|link=y}} earthquake 89&nbsp;km away from the cable.

Seismic waves are detectable because they cause [[micrometre|micrometer]]-scale changes in the length of the cable. As the length changes so does the time it takes a packet of light to traverse to the far end of the cable and back (using a second fiber). Using ultra-stable metrology-grade lasers, these extremely minute shifts of timing (on the order of [[femtosecond]]s) appear as phase-changes.

The point of the cable first disturbed by an earthquake's [[p wave]] (essentially a sound wave in rock) can be determined by sending packets in both directions in the looped pair of optical fibers; the difference in the arrival times of the first pair of perturbed packets indicates the distance along the cable. This point is also the point closest to the earthquake's epicenter, which should be on a plane perpendicular to the cable. The difference between the P wave/S wave arrival times provides a distance (under ideal conditions), constraining the epicenter to a circle. A second detection on a non-parallel cable is needed to resolve the ambiguity of the resulting solution. Additional observations constrain the location of the earthquake's epicenter, and may resolve the depth.

This technique is expected to be a boon in observing earthquakes, especially the smaller ones, in vast portions of the global ocean where there are no seismometers, and at much lower cost than ocean-bottom seismometers.