10:00 AM - 10:15 AM
[SSS03-05] Seismic observation in the Tsugaru Strait using submarine optical fiber cables.
Keywords:Distributed Acoustic Sensing, Fiber optic strain, Tsugaru Strait
Since there are fewer regular seismic stations in the ocean than on land, it is difficult to understand detailed earthquake occurrence in the ocean, especially for those in the shallow crust, without nearby observations. In recent years, distributed acoustic sensing (DAS), a technique for observing earthquakes and other phenomena by observing changes in the strain distributed in optical fibers, has begun to be utilized. In this study, we applied the DAS and the long baseline optical fiber observation technology to the seafloor optical fiber laid near the Tsugaru Strait. We investigated the possibility of observing earthquakes in the sea area using the seafloor optical fiber as a sensor.
For the DAS, AP Sensing's DAS N5200 was connected to the land end of the submarine optical fiber cable, and the observations were conducted continuously for two periods, from September 21 to September 30, 2021 and from December 14 to December 23, 2021. The long baseline optical fiber observation technique uses a different fiber core from the DAS observation of the submarine optical fiber cable. The fiber was connected to a highly stable laser source (SLS-INT-1500-200-1), a laser optical interferometer, and a Faraday mirror at one end of the interferometer, and another Faraday mirror to the other end of the submarine cable. In the period from August 26 to December 23, 2021, we were able to continuously observe the phase change of the laser light as it traveled along the entire length of the submarine optical fiber cable.
More than 50 earthquakes were observed by the DAS (Figure 1), during each observation period. Some of the earthquakes were less than JMA magnitude 1, indicating that the DAS is highly capable of detecting earthquakes. The DAS observation records show that the microseisms noise is dominant in the range of 0.1-2Hz due to the influence of the sea condition in the Tsugaru Strait. The microseisms noise was especially high in winter, although there was no significant difference between seasons above 2Hz. A comparison of DAS seismic records with nearby land seismic records shows that DAS records for several earthquakes showed later phases continuous to those on land, in addition to the P and S phases. This indicates that it is possible to perform detailed seismic wave structure analysis by integrating DAS seafloor records with land seismic records, which has not been possible with sparse land observation records alone.
Using the long-baseline fiber-optic observation technique, we were able to obtain records of long-period seismic waves for some teleseismic earthquakes (Fig. 2), as well as local seismic events. This indicates the possibility of broadband seismic observation in the seafloor and observation of longer-period seafloor crustal deformation, but it also clarifies technical issues such as removal of noise on the land part of the optical fiber cable and the necessity of an observation system with a higher speed that can respond to large seismic motions.
For the DAS, AP Sensing's DAS N5200 was connected to the land end of the submarine optical fiber cable, and the observations were conducted continuously for two periods, from September 21 to September 30, 2021 and from December 14 to December 23, 2021. The long baseline optical fiber observation technique uses a different fiber core from the DAS observation of the submarine optical fiber cable. The fiber was connected to a highly stable laser source (SLS-INT-1500-200-1), a laser optical interferometer, and a Faraday mirror at one end of the interferometer, and another Faraday mirror to the other end of the submarine cable. In the period from August 26 to December 23, 2021, we were able to continuously observe the phase change of the laser light as it traveled along the entire length of the submarine optical fiber cable.
More than 50 earthquakes were observed by the DAS (Figure 1), during each observation period. Some of the earthquakes were less than JMA magnitude 1, indicating that the DAS is highly capable of detecting earthquakes. The DAS observation records show that the microseisms noise is dominant in the range of 0.1-2Hz due to the influence of the sea condition in the Tsugaru Strait. The microseisms noise was especially high in winter, although there was no significant difference between seasons above 2Hz. A comparison of DAS seismic records with nearby land seismic records shows that DAS records for several earthquakes showed later phases continuous to those on land, in addition to the P and S phases. This indicates that it is possible to perform detailed seismic wave structure analysis by integrating DAS seafloor records with land seismic records, which has not been possible with sparse land observation records alone.
Using the long-baseline fiber-optic observation technique, we were able to obtain records of long-period seismic waves for some teleseismic earthquakes (Fig. 2), as well as local seismic events. This indicates the possibility of broadband seismic observation in the seafloor and observation of longer-period seafloor crustal deformation, but it also clarifies technical issues such as removal of noise on the land part of the optical fiber cable and the necessity of an observation system with a higher speed that can respond to large seismic motions.