Development of a Plate-scale Distributed Strain Sensing System: A Candidate for Earthquake Early Warning

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On the boundary between many continents and their neighboring seafloors, stress builds up from the process in which the seafloor tectonic plate slides beneath the continental tectonic plate. As the seafloor plate (sometimes called the “downgoing slab”) is slowly subducted below the continent (moving at a few inches per year, on average), friction along the boundary causes both plates to elastically deform. The stress built up by this mechanism can ultimately be released in a large, potentially devasting earthquake. This is often accompanied by a tsunami – the combination poses a significant hazard to many coastal areas, including the Pacific northwest portion of the United States. While the built-up stress is readily observed on land using permanent GPS stations, deformation of the seafloor is more difficult to observe because the satellite signals used by GPS cannot penetrate seawater. Consequently, other means must be used to observe the deformation. The project aims to establish the feasibility of measuring deformation of the seafloor with a long optical fiber cable (up to 100 km) connected to a series of shorter, sensing optical fiber cables. Light sent through the optical fibers can detect very slight length changes in the cable. If carefully attached to the seafloor, changes in the length of the optical fiber cable indicate deformation in the underlying material. By observing seafloor deformation, researchers hope to one day be able to detect sudden deformation changes caused by an offshore earthquake and transmit the information to a land-based network faster than the associated shaking reaches land, facilitating an early-warning to the earthquake.The technical approach in this project has been demonstrated on a small scale. An optical fiber, tensioned between two seafloor anchors separated by a few hundred meters is interferometrically probed to track length changes. A solid state laser at one end of the optical fiber cable injects light into an optical fiber beamsplitter. Part of the light travels along the stretched optical fiber cable to a mirror at its far end, which reflects the light back towards the splitter. Another part of the laser light travels to a local mirror at the end of a length of reference optical fiber wound onto a fixed glass mandrel. When the two reflected light beams are recombined at the splitter, they interfere, creating fluctuating light levels that can be processed to reveal changes in length at the nanometer level. In this project, the established method will be expanded to include a series of interferometric sensors attached to a cable whose length could cover the entire continental shelf, where the most hazardous stress buildup occurs. The method of time-division multiplexing in postulated to be capable of probing ten strain sensors distributed along the long cable, thereby expanding coverage adequately to monitor strain changes in a 100 km long profile. The tests to be performed are to be done in the laboratory testing the time-division multiplexing approach in optical fiber lengths appropriate for these geophysical measurements.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.