2:45 PM - 3:00 PM
[SCG51-05] Real-time SSE monitoring by optical borehole tiltmeter in the Nankai Trough, Japan
Keywords:Nankai Trough, Slow slop event, Optical tiltmeter
In the Nankai Trough subduction zone, a submarine cable network observation system named DONET has been in operation. The DONET system was originally equipped with broadband seismometers and pressure gauges to observe broadband real-time seismic and tsunami activities and vertical displacement of the seafloor above the source region of the mega-thrust earthquake which occurs every 100 to 150 years. The DONET system has an ability to expand observation by connecting new sensors to the observation node. To monitor small and slow crustal deformation, three borehole observatories, each having a tiltmeter, a strainmeter and a pore-fluid pressure gauge installed in the bottom of boreholes with depths of 500 to 900 mbsf, have been connected to the DONET nodes since 2013. Long-term data observed by the borehole sensors revealed that a slow slip event (SSE) of several weeks period has occurred repeatedly and accommodated about half of the plate motion (Araki et al., 2017). SSE monitoring in a wide area along the plate subduction zone enables us to estimate spatio-temporal change of interplate coupling, which would be a key parameter for the assessment of the rupture region of the future mega-thrust earthquakes.
In recent efforts, we have developed and installed new sensors, including a fiber optic strainmeter on seafloor and a bubble type tiltmeter in the shallow borehole with diameter of 104 mm and depth of 8.6 mbsf drilled by BMS (Boring Machine System) in the DONET 2F and 1B areas since 2019. These pilot observatories have been installed and operated to observe SSE distributed along the DONET region. Initial data observed by these sensors suggested that they have the ability to capture weak signals related to SSE. However, the observed data suggested that the coupling condition of sensors on the seafloor and in the borehole is very important to measure weak signals. Also, new sensors which have higher stability and lower self-noise than conventional sensors are still needed.
Accordingly, we prepared a new sensor and installation scheme for high-quality real-time SSE monitoring. For the new sensor, we fabricated an optical tiltmeter, which was originally developed by Scripps Institution of Oceanography. The tiltmeter has two long pendulums that swing with length of ~40 cm around orthogonal horizontal axes while their weighted ends are tracked by Michelson interferometers (Zumberge et al., 2018). The power unit and data transmission scheme were modified for DONET protocol. The optical tiltmeter was tested in a shallow borehole drilled in the Kamioka mine for 6 weeks from Oct. to Dec. 2020, and was compared with a broadband seismometer, CMG3T, same as DONET seismometers. We performed power spectral density analysis and phase analysis using ambient noise records and teleseismic records. Results confirmed that the optical tiltmeter can observe broadband signals from approx. 0.001 Hz to several ten Hz with the same flat amplitude and phase responses as those of the broadband seismometer. In long-term data, the stability of the optical tiltmeter was almost the same level as the bubble type tiltmeter. These results suggest that the optical tiltmeter can observe broadband seismic signals and geodetic signals. We developed a new installation scheme for the tiltmeter in a high stability environment created by drilling a borehole and cementing in the sensor. BMS was modified for drilling a large diameter of Φ230 mm with 20 m depth for the optical tiltmeter. Cement-based mortar with special formulation for sensor installation was developed. A mortar deployment system by ROV (Remotely operated vehicle) was developed including the pumping system. A special winch unit was also developed for the tiltmeter installation by ROV.
The tiltmeter was successfully installed into a 19 m borehole on 26 Jan. 2021 by KM-ROV. Before the installation, the borehole was drilled by the BMS, and a greater than 2 m interval at the bottom of the borehole was filled with cement. The optical tiltmeter was then installed by the winch system. On 11 Feb. 2021, the tiltmeter was connected to the DONET network by ROV Hyper Dolphin. The optical tiltmeter is now on-line transmitting continuous tilt data. Preliminary results of timeseries and PSD analysis suggest that the optical borehole tiltmeter has lower noise level than that of horizontal component of the DONET broadband seismometer in long-period seismic band. We expect that the tiltmeter can measure weak horizontal signal related to teleseismic and very low frequency earthquakes, which cannot be captured by DONET seismometer. In this presentation we will show initial data observed by the tiltmeter including seismic signals and tidal response.
In recent efforts, we have developed and installed new sensors, including a fiber optic strainmeter on seafloor and a bubble type tiltmeter in the shallow borehole with diameter of 104 mm and depth of 8.6 mbsf drilled by BMS (Boring Machine System) in the DONET 2F and 1B areas since 2019. These pilot observatories have been installed and operated to observe SSE distributed along the DONET region. Initial data observed by these sensors suggested that they have the ability to capture weak signals related to SSE. However, the observed data suggested that the coupling condition of sensors on the seafloor and in the borehole is very important to measure weak signals. Also, new sensors which have higher stability and lower self-noise than conventional sensors are still needed.
Accordingly, we prepared a new sensor and installation scheme for high-quality real-time SSE monitoring. For the new sensor, we fabricated an optical tiltmeter, which was originally developed by Scripps Institution of Oceanography. The tiltmeter has two long pendulums that swing with length of ~40 cm around orthogonal horizontal axes while their weighted ends are tracked by Michelson interferometers (Zumberge et al., 2018). The power unit and data transmission scheme were modified for DONET protocol. The optical tiltmeter was tested in a shallow borehole drilled in the Kamioka mine for 6 weeks from Oct. to Dec. 2020, and was compared with a broadband seismometer, CMG3T, same as DONET seismometers. We performed power spectral density analysis and phase analysis using ambient noise records and teleseismic records. Results confirmed that the optical tiltmeter can observe broadband signals from approx. 0.001 Hz to several ten Hz with the same flat amplitude and phase responses as those of the broadband seismometer. In long-term data, the stability of the optical tiltmeter was almost the same level as the bubble type tiltmeter. These results suggest that the optical tiltmeter can observe broadband seismic signals and geodetic signals. We developed a new installation scheme for the tiltmeter in a high stability environment created by drilling a borehole and cementing in the sensor. BMS was modified for drilling a large diameter of Φ230 mm with 20 m depth for the optical tiltmeter. Cement-based mortar with special formulation for sensor installation was developed. A mortar deployment system by ROV (Remotely operated vehicle) was developed including the pumping system. A special winch unit was also developed for the tiltmeter installation by ROV.
The tiltmeter was successfully installed into a 19 m borehole on 26 Jan. 2021 by KM-ROV. Before the installation, the borehole was drilled by the BMS, and a greater than 2 m interval at the bottom of the borehole was filled with cement. The optical tiltmeter was then installed by the winch system. On 11 Feb. 2021, the tiltmeter was connected to the DONET network by ROV Hyper Dolphin. The optical tiltmeter is now on-line transmitting continuous tilt data. Preliminary results of timeseries and PSD analysis suggest that the optical borehole tiltmeter has lower noise level than that of horizontal component of the DONET broadband seismometer in long-period seismic band. We expect that the tiltmeter can measure weak horizontal signal related to teleseismic and very low frequency earthquakes, which cannot be captured by DONET seismometer. In this presentation we will show initial data observed by the tiltmeter including seismic signals and tidal response.