Over the past century or so, seismometers have helped us to detect, study, and even forecast earthquakes. The simplest seismometers are made up of a weight hanging from a spring: A pen attached to the weight records slight movements or vibrations, known as seismic waves, on a rotating spool of paper; the spool, frame, and base of the instrument move with Earth while the suspended weight remains stationary. More modern versions use a computer to record electrical voltages generated by the instrument’s movements.
Scientists’ ability to study the movements of Earth has so far been limited, however, by the logical locations of seismometers. The instruments are sparsely dispersed across the globe and are clustered mainly on land.
In a new publication, Lindsey et al. used a novel seismic recording approach to study seismic waves from earthquakes for the first time. The technique, called distributed acoustic sensing, repurposes underground fiber-optic cables (normally used for telecommunications, such as internet, television, and telephone service) up to tens of kilometers long.
To make the ground motion measurements, the scientists used pulses of laser light traveling inside of a fiber-optic cable. The photons scatter off randomly distributed manufacturing imperfections inside the fiber and travel back to a light detector. Using a reference loop of fiber, the scientists could measure the length of the cable by producing an interference pattern whose time-varying amplitude provides a fingerprint of the individual distances to different scatterers inside the experimental fiber length. By monitoring how this fingerprint changes from one pulse to the next, researchers can measure the time rate of change of the path length in a segment of the experimental fiber and deduce how segments of the fiber undergo compression and tension as the ground moves.
Laser pulses were used to make these measurements continuously at a rate of 1,000 times per second over weeks to months. Comparing information gleaned from the pulses’ interference patterns to data recorded by traditional inertial seismometers allowed the scientists to see how precisely this new distributed fiber-optic method picks up seismic signals during earthquakes.
The researchers tested this technique at three locations: Fairbanks, Alaska; a region of northern California called the Geysers, home to one of the world’s largest geothermal fields; and the Stanford University campus.
In Fairbanks, the researchers found that the ground motion recorded with the fiber-optic cable was comparable to that from a traditional seismometer. At the Geysers, they were able to use an L-shaped array of cables to track the arrival of several phases of an earthquake that occurred in 2016. Beneath Stanford University, they installed fiber-optic cables in a plastic telecommunications conduit and in the process discovered that seismic waves are still detectable when the fiber-optic cable is minimally coupled to the soil.
The spatial resolution of the data collected—one ground motion measurement every meter for kilometers—allowed the researchers to use the array nature of fiber-optic seismology to learn more about the way faults rupture and how seismic waves fan out in the solid Earth. These findings will hopefully lead to more extensive seismic studies using fiber-optic cables for distributed acoustic sensing. By improving the way that scientists study earthquakes, this advancement could help mitigate the damaging effects that earthquakes have on lives, homes, and public infrastructure. (Geophysical Research Letters, https://doi.org/10.1002/2017GL075722, 2017)
—Sarah Witman, Freelance Writer