Researchers Lay Out Case for DAS in Earthquake Early Warning

13 April 2022–The rise of fiber-optic distributed acoustic sensing (DAS) in seismology could prove a useful addition to earthquake early warning systems, researchers write in the Bulletin of the Seismological Society of America.

DAS uses the tiny internal flaws in a long optical fiber as thousands of seismic sensors. An instrument called an interrogator at one end of the fiber sends laser pulses down the cable that are reflected off the fiber flaws and bounced back to the instrument. When the fiber is disturbed by seismic activity, researchers can examine changes in the reflected pulses to learn more about the resulting seismic waves.

One feature that DAS can detect is dynamic strain, or the rock expansion or compression that occurs as a seismic wave propagates through the rock. Although dynamic strain calculations aren’t a part of current earthquake early warning applications, Noha Farghal, formerly at the U.S. Geological Survey and now at Risk Management Solutions, Inc. and her colleagues think they could be helpful to include in warning systems.

“Before discussing using strain data from DAS arrays, we started by introducing the idea of incorporating strain from borehole strainmeters, which are well-established instruments. We think this strengthens our argument for strain to the earthquake early warning (EEW) community,” said Farghal and colleagues.

But drilling boreholes is costly, and the small network of borehole strainmeters on the U.S. West Coast, for example, can’t provide uniform and large coverage around the region’s hazardous faults, the scientists say.

Compared to the single sensing point offered by a borehole instrument, one DAS array could potentially provide strain observations at every meter along a fiber optic cable, providing measurement redundancy. Data from dense sensing could also help seismologists better understand wave propagation in a way that allows them to pinpoint the source of an earthquake with more accuracy, Farghal and colleagues noted.

DAS might also be a way to get instruments as close as possible to a large earthquake source, they added. Large earthquakes pose problems for traditional seismometers, which can “clip” or fail to record when the shaking amplitude of an earthquake is too high. Accelerometers are more sensitive to high-frequency shaking, compared to the low-frequency shaking needed to estimate an earthquake’s magnitude from data collected at close range.

Strainmeters like those provided by DAS, on the other hand, “are truly broadband with high dynamic ranges,” said Farghal and colleagues, “meaning that when strain amplitudes are very large and over a wide range of frequencies, which will be the case in the near-field of a large rupture, strainmeters will continue to represent the amplitudes and frequencies of the seismic signal without clipping or saturating.”

The researchers say one of the keys to including DAS into an earthquake early warning system will be to develop new strain-based warning algorithms for the system or find a way to derive ground motion metrics, such as peak ground velocity, that can be used with established earthquake early warning algorithms.

Urban and highly populated areas with few traditional seismometer stations are good candidates for fiber optic DAS deployments, Farghal and colleagues said. “A DAS array can be installed around buildings and in existing infrastructure, where the installation of traditional seismic instruments can be difficult,” they said. “Additionally, fiber optic cables could be deployed during planned expansions of telecommunications and fiber optic internet infrastructure in rural areas.”