[S-TT42] Applying optic fiber sensing to earth science

[STT42-01] DAS observation of earthquake swarm in Noto Peninsula using optic fiber cable

★Invited Papers

*Yoshiyuki Tanaka¹, Masatoshi Miyazawa², Eiichiro Araki³ (1.Earth and Planetary Science, The University of Tokyo, 2.Disaster Prevention Research Institute, University of Tokyo, 3.JAMSTEC)

Keywords:DAS, optic fiber, Noto Peninsula, earthquake, earthquake swarm

Earthquake swarms have been active in the Noto Peninsula since the end of 2020, including events with magnitudes exceeding 5 that occurred in 2022. An intensive investigation of earthquake, crustal deformation, electromagnetic and gravitational fields, and chemical observations is in progress, and the existence of crustal fluid is considered to be the cause of the earthquake swarms. Distributed Accoustic Sensing (DAS) observation conducted in this investigation is a technique that uses the optical cable itself as a sensor to measure the expansion and contraction of the fiber at different locations by measuring the scattering of laser pulses emitted from the end point into the fiber. DAS has been used for oil well observation, and its use for seismic measurement in Japan is rapidly increasing. There have been reports of observations using fibers in volcanic areas and those related to transportation infrastructures. This study reports the results of observations using NTT's communication network. The fiber connects Noto Town and Suzu City, with an optical path length of approximately 30 km. The observation period is two months starting in mid-January 2023 and data is acquired at 5 m intervals by DAS manufactured by Neubrex, Co., Ltd. As of mid-February, clear waveforms associated with 8 earthquakes in and around Suzu City were captured. Signals from both aerial and underground cables were captured, and differences in their characteristics were observed. Signals from M1 class earthquakes were also observed. Detailed waveform analysis and epicenter determination are still underway. In the case of using communication cables, the work to identify the fiber path is labor intensive and is being carried out by trial and error. The observation results indicate that NTT optical cables, which have a longer total length than the government-controlled optical fibers for river and road management on land, can be used for earthquake observation to some extent, and that DAS can be used for immediate ultra-high-density temporary earthquake observation in the event of a future sudden earthquake disaster, even under manpower shortage conditions. In the presentation, we will introduce the status of the observations, the path identification work, and the characteristics of the observed waveforms. Other observation techniques using optical fiber, such as optical lattice clocks and absolute gravity observations, will also be introduced, and expectations for observations using light will be discussed.