17:15 〜 18:45
[SCG40-P47] Installation of the new LTBMS system in the Nankai Trough, and evaluation of cement curing process on long-term strain data and SSEs detection.
キーワード：ゆっくり地震、光ファイバ歪センサ、南海トラフ、長期孔内観測システム、DONET、セメンチング
In the Nankai Trough subduction zone, three long-term borehole monitoring systems (LTBMS: C0002, C0010, and C0006) have been installed in the Kumano-nada region. Among the data set, the pore pressure data have revealed that slow slip events (SSEs) occur repeatedly in the shallow part on the plate boundary (Araki et al., 2017). Wide and precise monitoring of the SSEs is necessary for understanding processes of large interplate earthquakes in the Nankai trough. We, therefore, decided to install a newly developed LTBMS equipped with a borehole fiber-optic strainmeter off the Kii-channel, where the occurrence of SSEs is strongly expected.
The LTBMS system is mainly composed of the fiber-optic strainmeter and the borehole pore pressure gauge. The fiber-optic strainmeter can observe changes in the length of a 200 m optical fiber with a resolution of 1 nm by measuring changes in the optical path length between the 200 m fiber of a sensing part and a 200 m reference length fiber. The sensing part consists of a 200m optical fiber helically wound around a stainless steel mandrel. The sensing part is cemented inside the borehole after installing into the borehole, allowing it to measure surrounding crustal defotmations. In addition, the fiber-optic strainmeter can perform broadband and highly accurate seismic observations including microearthquakes, strong ground motions and slow earthquakes (VLFE, shallow low frequency tremors) by sampling at 100Hz.
In November 2023, a fourth LTBMS system was installed into the C9038B borehole off the Kii-channel by the D/V Chikyu. Then the system was connected to the DONET2 system in Jan. 2024, and started a real-time monitoring of the slow slip events in the region. A highly quality of strain data and pressure data was confirmed. Also, when we compared the noise level of the strainmeter with a nearby seafloor fiber-optic strainmeter, we found that the noise level was extremely low, and we were able to confirm clear microseism signals.
Also, as mentioned above, cementing was performed after the LTBMS was installed in the borehole. In general, cement curing process takes in a few days, but it is expected to take several months to several years for the effects of the cementing process to settle to a negligible level for strain measurements in the borehole. Therefore, it is necessary to remove the effects of cementing curing effect in order to correctly evaluate long-term changes in strain variation and slow slips. For this reason, we carried out evaluations about the cement curing process using a demonstration equipment in the 20 m borehole at the Kamioka Mine and in the laboratory using the same formulation as in the field. As a result, it was shown that the cement curing process at the Kamioka Mine continues to cause fluctuations that may be due to the effect of cementing effect even more than half a year after installation, and that it may take several months for the effect to disappear in the laboratory. In this presentation, we will compare these results with long-term strain data obtained in the C9038B borehole, and discuss a method to evaluate strain changes with higher accuracy.
The LTBMS system is mainly composed of the fiber-optic strainmeter and the borehole pore pressure gauge. The fiber-optic strainmeter can observe changes in the length of a 200 m optical fiber with a resolution of 1 nm by measuring changes in the optical path length between the 200 m fiber of a sensing part and a 200 m reference length fiber. The sensing part consists of a 200m optical fiber helically wound around a stainless steel mandrel. The sensing part is cemented inside the borehole after installing into the borehole, allowing it to measure surrounding crustal defotmations. In addition, the fiber-optic strainmeter can perform broadband and highly accurate seismic observations including microearthquakes, strong ground motions and slow earthquakes (VLFE, shallow low frequency tremors) by sampling at 100Hz.
In November 2023, a fourth LTBMS system was installed into the C9038B borehole off the Kii-channel by the D/V Chikyu. Then the system was connected to the DONET2 system in Jan. 2024, and started a real-time monitoring of the slow slip events in the region. A highly quality of strain data and pressure data was confirmed. Also, when we compared the noise level of the strainmeter with a nearby seafloor fiber-optic strainmeter, we found that the noise level was extremely low, and we were able to confirm clear microseism signals.
Also, as mentioned above, cementing was performed after the LTBMS was installed in the borehole. In general, cement curing process takes in a few days, but it is expected to take several months to several years for the effects of the cementing process to settle to a negligible level for strain measurements in the borehole. Therefore, it is necessary to remove the effects of cementing curing effect in order to correctly evaluate long-term changes in strain variation and slow slips. For this reason, we carried out evaluations about the cement curing process using a demonstration equipment in the 20 m borehole at the Kamioka Mine and in the laboratory using the same formulation as in the field. As a result, it was shown that the cement curing process at the Kamioka Mine continues to cause fluctuations that may be due to the effect of cementing effect even more than half a year after installation, and that it may take several months for the effect to disappear in the laboratory. In this presentation, we will compare these results with long-term strain data obtained in the C9038B borehole, and discuss a method to evaluate strain changes with higher accuracy.