1:45 PM - 3:15 PM
[STT42-P04] Comparison of fiber-optic strainmeter, seismometer and DAS observation records in a tunnel in Suzu City, Ishikawa Prefecture, Japan
Keywords:Fiber optic strain, Suzu City, Ishikawa prefecture, Earthquake swarms, DAS
Aiming for observation of seismic swarm activity and associated crustal deformation in Suzu City, Ishikawa Prefecture, we have installed a 90-m baseline fiber-optic strainmeter and a 3-component seismograph in a railroad tunnel in Suzu City, and have been continuously observing since November 1, 2022. In addition, DAS observations using NTT optical lines in Suzu City are being conducted for about two months from January 12, 2023. In order to integrate the strain data obtained by optical fiber sensing technology with the ground motion data obtained from the seismic network, it is necessary to understand the relationship between these different physical observed quantities. Therefore, we compared and discussed these three observations in close proximity. In addition, we have observed many felt earthquakes with the strainmeter and DAS. We examined the amount of surface strain change associated with earthquakes.
First, a comparison of earthquakes obtained by fiber-optic strainmeter in the tunnel and ground motions by seismometers showed that for the P-wave portion of the earthquake, contraction strain was observed for the upward rise of the seismometer, which is consistent with plane wave incidence from below. But in general the strainmeter tended to observe the P phase 0.15 seconds earlier than the seismometer. We believe that this suggests that the vibration of the tunnel roadbed where the seismographs are installed is caused by deformation of the tunnel structure. Considering plane incident waves, our strain rate and acceleration records had a proportional relationship when compared in the S-wave portion of the earthquake record, assuming a phase velocity of about 2000 m/s. But for 10 Hz and above, the amplitude response of the strainmeter tended to be smaller. This may be due to the 90 m baseline length of the strain gauges, suggesting that the wavelength of the strain seismic wave propagating through the tunnel is shorter than the baseline length at short periods. In addition, the duration of the observed S-wave tended to be long. This suggests that the response of the tunnel structure to the seismic waves is complex.
The DAS and the fiber-optic strainmeter in the tunnel were compared for the 1/13 6:13 JST earthquake, and the strain changes obtained from the DAS and the fiber-optic strainmeter for both the P-wave and S-wave portions generally agree, but the DAS record shows cycle-skipping in the S-wave portion, which requires careful handling.
First, a comparison of earthquakes obtained by fiber-optic strainmeter in the tunnel and ground motions by seismometers showed that for the P-wave portion of the earthquake, contraction strain was observed for the upward rise of the seismometer, which is consistent with plane wave incidence from below. But in general the strainmeter tended to observe the P phase 0.15 seconds earlier than the seismometer. We believe that this suggests that the vibration of the tunnel roadbed where the seismographs are installed is caused by deformation of the tunnel structure. Considering plane incident waves, our strain rate and acceleration records had a proportional relationship when compared in the S-wave portion of the earthquake record, assuming a phase velocity of about 2000 m/s. But for 10 Hz and above, the amplitude response of the strainmeter tended to be smaller. This may be due to the 90 m baseline length of the strain gauges, suggesting that the wavelength of the strain seismic wave propagating through the tunnel is shorter than the baseline length at short periods. In addition, the duration of the observed S-wave tended to be long. This suggests that the response of the tunnel structure to the seismic waves is complex.
The DAS and the fiber-optic strainmeter in the tunnel were compared for the 1/13 6:13 JST earthquake, and the strain changes obtained from the DAS and the fiber-optic strainmeter for both the P-wave and S-wave portions generally agree, but the DAS record shows cycle-skipping in the S-wave portion, which requires careful handling.