5:15 PM - 6:45 PM
[U15-P19] Fiber-optic strain observation near the epicenter of the Jan. 1, 2024 Noto Peninsula earthquake.
Keywords:Noto Peninsula earthquake, Fiber-optic strain, near field observation, strong motion
We have been continuously observing the crustal deformation and seismic activity in Suzu City, Ishikawa Prefecture, by installing a 50-m and 76-m fiber-optic strainmeter and a 3-component seismograph in a railway tunnel in Suzu City (Kasuga Tunnel of the Noto Railway). These instruments were in operation when the Mj7.6 earthquake occurred on January 1, 2024, although the observations were stopped 2~3 hours after the earthquake due to a power outage in the area. We obtained ultra-broadband and high-dynamic-range observation records of strong earthquakes and crustal deformation from the optical fiber strainmeters near the epicenter of the Noto Peninsula Offshore Earthquake.
The optical fiber strain recordings were obtained by stretching the 76 m baseline length to the tunnel wall and the 50 m baseline length in the tunnel floor. The optical fiber strain records were in good agreement with each other except for the 16:10 M7.6 earthquake, which was consistent during the initial motion but showed different variation when a large strain variation exceeding 150 μstrain occurred during the S-wave. Both records were elongated at periods longer than 1 s. In the 76-m baseline length strainmeter on the tunnel wall, the light intensity from the optical fiber suddenly decreased during the variation caused by the 16:10 earthquake, and the subsequent recording became unreliable for a certain period of time. After the earthquake, we checked the tunnel where the observation was being conducted, and found that there was a large gap of several centimeters at the joints of the concrete structure at both ends of the tunnel, which caused the optical fiber stretched on the structure to extend and bend significantly. This explains the reason for the decrease in light intensity of the optical fiber and confirms that the tunnel frame was subjected to large inelastic deformation during the earthquake.
The optical fiber strain recordings obtained in this way are important from the viewpoint that they record broadband seismic and crustal deformation near the epicenter without saturation, although it is necessary to note the effect of inelastic deformation caused by strong seismic motion in the tunnel frame. Therefore, we will discuss some of the characteristics found in the record.
1) Crustal deformation during the earthquake was observed not only for the 16:10 M7.6 earthquake, but also for the 16:06 earthquake and many aftershocks after the earthquake. The variations were shortened for the 16:06 earthquake, unlike the extension of the 16:10 earthquake and for many aftershocks
2. A few minutes after the 16:10 earthquake, a large non-seismic strain variation in the direction of shortening was observed. This variation could be explained as the effect of a tsunami hitting the shore in the south side of the tunnel or the effect of pressure waves associated with the large earthquake.
3. Slow, non-seismic strain fluctuations were repeatedly observed for several minutes after the 16:10 earthquake. Most of them were in the direction of elongation. The cause of these fluctuations is unknown and needs to be investigated.
4. We examined whether slow crustal deformation occurred between the 16:06 and 16:10 earthquakes. Although this is difficult to discuss because of the influence of temperature fluctuations in the tunnel, we could not identify any easily identifiable magnitude of deformation between the earthquakes. We will continue to remove the influence of temperature fluctuations and examine the possibility of pre-earthquake crustal deformation.
We thank Jun Nakagawa, Airi Nagaoka (Disaster Prevention Research Institute, Kyoto University), Ryo Matsuo, and Yuki Funabiki (Department of Science, Kyoto University) for their cooperation in conducting the observations. This work was supported by Grant-in-Aid for Scientific Research (23K17482, 21H05204) and "Research Plan for Earthquake and Volcano Observation to Contribute to Disaster Mitigation (2nd)".
The optical fiber strain recordings were obtained by stretching the 76 m baseline length to the tunnel wall and the 50 m baseline length in the tunnel floor. The optical fiber strain records were in good agreement with each other except for the 16:10 M7.6 earthquake, which was consistent during the initial motion but showed different variation when a large strain variation exceeding 150 μstrain occurred during the S-wave. Both records were elongated at periods longer than 1 s. In the 76-m baseline length strainmeter on the tunnel wall, the light intensity from the optical fiber suddenly decreased during the variation caused by the 16:10 earthquake, and the subsequent recording became unreliable for a certain period of time. After the earthquake, we checked the tunnel where the observation was being conducted, and found that there was a large gap of several centimeters at the joints of the concrete structure at both ends of the tunnel, which caused the optical fiber stretched on the structure to extend and bend significantly. This explains the reason for the decrease in light intensity of the optical fiber and confirms that the tunnel frame was subjected to large inelastic deformation during the earthquake.
The optical fiber strain recordings obtained in this way are important from the viewpoint that they record broadband seismic and crustal deformation near the epicenter without saturation, although it is necessary to note the effect of inelastic deformation caused by strong seismic motion in the tunnel frame. Therefore, we will discuss some of the characteristics found in the record.
1) Crustal deformation during the earthquake was observed not only for the 16:10 M7.6 earthquake, but also for the 16:06 earthquake and many aftershocks after the earthquake. The variations were shortened for the 16:06 earthquake, unlike the extension of the 16:10 earthquake and for many aftershocks
2. A few minutes after the 16:10 earthquake, a large non-seismic strain variation in the direction of shortening was observed. This variation could be explained as the effect of a tsunami hitting the shore in the south side of the tunnel or the effect of pressure waves associated with the large earthquake.
3. Slow, non-seismic strain fluctuations were repeatedly observed for several minutes after the 16:10 earthquake. Most of them were in the direction of elongation. The cause of these fluctuations is unknown and needs to be investigated.
4. We examined whether slow crustal deformation occurred between the 16:06 and 16:10 earthquakes. Although this is difficult to discuss because of the influence of temperature fluctuations in the tunnel, we could not identify any easily identifiable magnitude of deformation between the earthquakes. We will continue to remove the influence of temperature fluctuations and examine the possibility of pre-earthquake crustal deformation.
We thank Jun Nakagawa, Airi Nagaoka (Disaster Prevention Research Institute, Kyoto University), Ryo Matsuo, and Yuki Funabiki (Department of Science, Kyoto University) for their cooperation in conducting the observations. This work was supported by Grant-in-Aid for Scientific Research (23K17482, 21H05204) and "Research Plan for Earthquake and Volcano Observation to Contribute to Disaster Mitigation (2nd)".