09:15 〜 09:30
[SCG45-12] Long-term continuous seafloor fiber optic observation aimed for capturing slow and fast processes in the Nankai Trough megathrust.
キーワード：光ファイバーセンシング、DAS、南海トラフ、ゆっくり滑り、超低周波地震
Optical fiber sensing is a technology that can observe strain distribution in optical fibers. The technology has large potential for capturing slow and fast processes at the subducting plate interface in that strain distribution of the seafloor may be observed in high time and space resolution across the whole span of the wedge.
So we began applying optical fiber sensing technologies on the JAMSTEC off-Muroto submarine fiber optic cable laid over 120 km across Philippine Sea subducting plate interface in the Nankai Trough, where we already have a dense ocean floor network for Earthquake and Tsunamis (DONET) nearby and very low frequency earthquake (VLFEs) activity are monitored by the DONET.
Very broadband and high dynamic range observations are needed to target slow processes such as VLFEs and slow slip events (SSEs), but that was beyond fiber optic sensing technologies can achieve when we considered application in the Muroto cable. Therefore, we tried to improve the long-period sensitivity of DAS (distributed acoustic sensing) and developed another (TW-COTDR) which would be better in geodetic periods (days and longer).
As the wavelength stability of the laser source used in the DAS directly relate to observed noise in long period, we experimented to operate our DAS (AP Sensing N5225b) with a very stable cavity-locked laser (Stable Laser Systems SLS-INT-1550-200-1) to measure off-Muroto cable. This simple approach resulted in significant improvement in long-period noise such as that we can expect observe VLFEs (very low frequency earthquakes) in the vicinity with very high density. So we started continuous observation with the very stable DAS in Muroto cable since June 2022, after the last major occurrence of local VLFEs in Jan-Mar. 2022. We also expanded reachable distance with the DAS observation. Now our observation of the very stable DAS is up to 120 km from shore in Muroto cable, which is almost full length of the cable.
DAS is tracking optical phase in time. It is difficult to keep optical phase tracking in the events of large earthquakes. We are aiming to evaluate crustal deformation caused by large earthquakes and slow slip after the events. DAS apparently is not suitable in such situations. So, we have developed a fiber optic sensing instrument (TW-COTDR) based on the principle of measuring the optical fiber Rayleigh scatter intensity pattern in wavelengths, which shifts by strain changes in optical fibers. We have started observations with the TW-COTDR unit and the off-Muroto cable since April, 2022. Although seafloor water temperature variation causes significant apparent strain change, we deployed seafloor thermometers to correct the effects of temperature variation from our fiber optic strain data and obtain long-term crustal deformation along the plate subduction.
So we began applying optical fiber sensing technologies on the JAMSTEC off-Muroto submarine fiber optic cable laid over 120 km across Philippine Sea subducting plate interface in the Nankai Trough, where we already have a dense ocean floor network for Earthquake and Tsunamis (DONET) nearby and very low frequency earthquake (VLFEs) activity are monitored by the DONET.
Very broadband and high dynamic range observations are needed to target slow processes such as VLFEs and slow slip events (SSEs), but that was beyond fiber optic sensing technologies can achieve when we considered application in the Muroto cable. Therefore, we tried to improve the long-period sensitivity of DAS (distributed acoustic sensing) and developed another (TW-COTDR) which would be better in geodetic periods (days and longer).
As the wavelength stability of the laser source used in the DAS directly relate to observed noise in long period, we experimented to operate our DAS (AP Sensing N5225b) with a very stable cavity-locked laser (Stable Laser Systems SLS-INT-1550-200-1) to measure off-Muroto cable. This simple approach resulted in significant improvement in long-period noise such as that we can expect observe VLFEs (very low frequency earthquakes) in the vicinity with very high density. So we started continuous observation with the very stable DAS in Muroto cable since June 2022, after the last major occurrence of local VLFEs in Jan-Mar. 2022. We also expanded reachable distance with the DAS observation. Now our observation of the very stable DAS is up to 120 km from shore in Muroto cable, which is almost full length of the cable.
DAS is tracking optical phase in time. It is difficult to keep optical phase tracking in the events of large earthquakes. We are aiming to evaluate crustal deformation caused by large earthquakes and slow slip after the events. DAS apparently is not suitable in such situations. So, we have developed a fiber optic sensing instrument (TW-COTDR) based on the principle of measuring the optical fiber Rayleigh scatter intensity pattern in wavelengths, which shifts by strain changes in optical fibers. We have started observations with the TW-COTDR unit and the off-Muroto cable since April, 2022. Although seafloor water temperature variation causes significant apparent strain change, we deployed seafloor thermometers to correct the effects of temperature variation from our fiber optic strain data and obtain long-term crustal deformation along the plate subduction.