11:45 AM - 12:00 PM
[SCG40-30] New real-time seafloor borehole observatory at C9038B in the Nankai Trough for shallow Slow Slip Event observation in the Nankai Trough.
Keywords:borehole observation, the Nankai Trough, slow slip event, fiber optic strainmeter
Slow slip events repeatedly occur in the shallow subducting plate boundary in Kumano-nada off the Nankai Trough, which were discovered by an array of deep-seafloor long-term borehole observation system (LTBMS) installed by IODP in the Kumano-nada. There are other regions where shallow slow slip could also repeat, as suggested from offshore GNSS/A observations and slow earthquakes from seismological observations, but the actual state of these shallow slow slip activities is unknown due to the lack of highly sensitive real-time crustal deformation observations in the seafloor.
Therefore, we have developed a new LTBMS to monitor shallow slow slip events in a wide area. The newly developed system is installed in a borehole drilled into the seafloor by the deep drilling vessel Chikyu, and for the real-time observation, connected to the submarine cable observation network such as DONET2 and N-Net (currently under construction). Based on the occurrence of slow earthquakes and other factors, we plan to install the borehole observatories in three locations: off the Kii Channel, off the coast of Shikoku, and in the Hyuganada, in the Nankai Trough.
Installation of the first borehole observatory, in offshore Kii Channel, was conducted by the Chikyu vessel in CK23-03 cruise in Nov. 6-28, 2023. The new LTBMS was installed in Hole C9038B, drilled 500 m below the seafloor at a depth of 2,653 m. The installed borehole sensors are capable of fiber-optic strain measurement, borehole pore-fluid pressure observation, and fiber-optic sensing.
After confirming the performance of these borehole sensors at the time of installation, cement was pumped into the borehole and the fiber-optic strainmeter was cemented into the borehole. At the end of December 2023, when the cement was considered to have consolidated, the borehole sensors were connected to the DONET2 by the Shinsei-Maru cruise KS-23-J11, and continuous real-time observation started on Jan. 2, 2024.
The obtained borehole real-time data are in good condition. The pore-fluid pressure and fiber optic strainmeter records clearly shows deformation of the sedimentary layer at the depth of the sensor under the load of ocean tides, infragravity waves, and seismic motions from earthquakes including teleseismic events. When compared with the records from seafloor fiber-optic strainmeters installed ~10 km away from the C9038B borehole observatory, observed seismic strain variation was smaller than that of the seafloor, probably because the sediments at the installation depth of the strainmeter were quite soft. Ocean tides records from the borehole pore-fluid pressure shows 600-1200 seconds delay, which would suggest the sediment is also permeable. The borehole fiber-optic strainmeter records show some drift, possibly due to cement curing, but the rate of this drift is smoothly decreasing with time.
The newly installed seafloor borehole observatory at C9038B hole, along with nearby seafloor fiber-optic strainmeters at 2F-S2 site, allows array observation of slow slip events, which considered to occur just beneath the array. In the future, we plan to analyze such slow slip events, very-low-frequency earthquakes, and low-frequency tremors in detail. We also develop and prepare two more borehole observatories, in off the coast of Shikoku and in the Hyuganada, in the further western side of the Nankai Trough, reflecting experiences from the first observatory at hole C9038B.
Therefore, we have developed a new LTBMS to monitor shallow slow slip events in a wide area. The newly developed system is installed in a borehole drilled into the seafloor by the deep drilling vessel Chikyu, and for the real-time observation, connected to the submarine cable observation network such as DONET2 and N-Net (currently under construction). Based on the occurrence of slow earthquakes and other factors, we plan to install the borehole observatories in three locations: off the Kii Channel, off the coast of Shikoku, and in the Hyuganada, in the Nankai Trough.
Installation of the first borehole observatory, in offshore Kii Channel, was conducted by the Chikyu vessel in CK23-03 cruise in Nov. 6-28, 2023. The new LTBMS was installed in Hole C9038B, drilled 500 m below the seafloor at a depth of 2,653 m. The installed borehole sensors are capable of fiber-optic strain measurement, borehole pore-fluid pressure observation, and fiber-optic sensing.
After confirming the performance of these borehole sensors at the time of installation, cement was pumped into the borehole and the fiber-optic strainmeter was cemented into the borehole. At the end of December 2023, when the cement was considered to have consolidated, the borehole sensors were connected to the DONET2 by the Shinsei-Maru cruise KS-23-J11, and continuous real-time observation started on Jan. 2, 2024.
The obtained borehole real-time data are in good condition. The pore-fluid pressure and fiber optic strainmeter records clearly shows deformation of the sedimentary layer at the depth of the sensor under the load of ocean tides, infragravity waves, and seismic motions from earthquakes including teleseismic events. When compared with the records from seafloor fiber-optic strainmeters installed ~10 km away from the C9038B borehole observatory, observed seismic strain variation was smaller than that of the seafloor, probably because the sediments at the installation depth of the strainmeter were quite soft. Ocean tides records from the borehole pore-fluid pressure shows 600-1200 seconds delay, which would suggest the sediment is also permeable. The borehole fiber-optic strainmeter records show some drift, possibly due to cement curing, but the rate of this drift is smoothly decreasing with time.
The newly installed seafloor borehole observatory at C9038B hole, along with nearby seafloor fiber-optic strainmeters at 2F-S2 site, allows array observation of slow slip events, which considered to occur just beneath the array. In the future, we plan to analyze such slow slip events, very-low-frequency earthquakes, and low-frequency tremors in detail. We also develop and prepare two more borehole observatories, in off the coast of Shikoku and in the Hyuganada, in the further western side of the Nankai Trough, reflecting experiences from the first observatory at hole C9038B.