Satoru Hamanaka, graduated this spring and Assoc. Prof. Kentaro Emoto investigated subsurface structure beneath the National Route 3 in Kumamoto prefecture using fiber optic cable. This research result was published in the journal, Earth, Planets and Space.
Title：Estimation of shallow structure along the Hinagu fault by applying seismic interferometry to DAS observations conducted along national route 3 in Kumamoto prefecture, Japan
Author：Satoru Hamanaka and Kentaro Emoto
Journal：Earth, Planets and Space
Publish date：19 November 2024
URL：https://doi.org/10.1186/s40623-024-02088-3
This paper summarized the Master’s thesis by Satoru Hamanaka.
Fiber optic cable as distributed sensors
Some amount of the laser pulse emitted into a fiber optic cable returned by reflecting at subtle impurities. When the fiber optic cable expands or contracts due to the vibration, the arrival time of the reflected laser pulse changes. We can extract small vibrations at arbitrary points by analyzing changes in reflected waves. This cutting edge technology, Distributed Acoustic Sensing (DAS), rapidly spread in seismic observation.
We measured the vibration records at every 4 m along a 40 km long fiber optic cable installed beneath National Route 3 in Kumamoto Prefecture (Figure 1). The total data amount is 19 TB, a huge size in general seismology. We were especially interested in the detailed structure around the Hinagu Fault parallel to National Route 3.
National Route 3 is one of the main roads, so traffic is heavy even at night. Signals from small earthquakes were masked by such traffic noise. (see the article on the installation).
Estimation of subsurface structure using vibration by vehicles
The ground is always shaking with small amplitudes that humans do not feel. Using the technology called seismic interferometry, we can extract the waves propagating in the earth from the continuous shaking. We applied the seismic interferometry to the shaking generated by vehicles along National Route 3 and tried to extract waves propagating just beneath the cable.
However, we couldn’t extract good-quality waves due to differences in amplification and traffic. Therefore, we repeatedly applied the seismic interferometry to the extracted waves and succeeded in obtaining good-quality waves that can be used for further analyses.
We estimated the subsurface velocity structure using the information on the difference of the wave speed for different wavelengths. We found the thick low-velocity layer across the Hikawa River. This reflects the result of the sedimentation by Hikawa River (Figure 2).
We can estimate the detailed subsurface structure by using super-dense seismic records by DAS with the fiber optic cable.
The DAS observations were supported by Kumamoto River and National Highway Office, Kyushu Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism (MLIT).
