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[SGD03-11] Analysis of pre- and post-seismic deformations at Kamioka observed with a 1500-m laser strainmeter during the Noto earthquake swarm
Introduction
Seismic activity has been increasing since about December 2020 near the Noto Peninsula in Ishikawa Prefecture, preceding the M7.6 on the Japan Meteorological Agency (JMA) scale earthquake on January 1, 2024. Researches on the vibration phenomena across a wide range of frequencies have been conducted and some studies suggest that fluids are involved in the earthquake processes (Yoshida et al., 2023).
In this study, we analyzed deformations before and after relatively large earthquakes during the swarm using data from a laser strainmeter with a baseline length of 1500 m installed in an underground tunnel in Kamioka, Hida City, Gifu Prefecture, aiming to understand the process of earthquake generation.
The 1500 m baseline laser strainmeter
The laser strainmeter is installed in a tunnel constructed for the gravitational wave telescope KAGRA in Kamioka. The azimuth of the instrument is oriented in the N60°E direction and it is located about 0.5 km north of the Atotsugawa fault, running almost parallel to it.
The laser strainmeter primarily consists of a light source, two vacuum chambers containing retro-reflectors and a beam splitter, and a 1,500m vacuum pipe connecting the chambers forming an asymmetric Michelson interferometer. The instrument measures the change in distance between the chambers by detecting the phase change of the interference fringes (Araya et al., 2017).
Compared to seismometers, which measure ground velocity or acceleration, laser strainmeters are more suitable for observing low-frequency ground deformations because they directly measure the displacements of two distant points.
Although Kamioka is about 100 km away from the Noto Peninsula, the laser strainmeter may be capable of detecting subtle deformations associated with the earthquakes due to the superior strain resolution (10-13-10-12) compared to GNSS.
Data analysis and results
We analyzed ground strains recorded before and after earthquakes that occurred in the Noto region with a magnitude 5 or higher. These include the earthquakes on June 19, 2022 (M5.4), May 5, 2023 (M6.5), and January 1, 2024 (M7.6).
We performed spectral analysis on the time series data from the laser strainmeter. After averaging the strain data every second to reduce high-frequency noise, we subtracted the theoretical tidal strain—computed using GOTIC2 (National Astronomical Observatory of Japan, NAOJ) — from the observational data. We then applied Fourier transform to the residual signals to obtain power spectra and spectrograms and used them to search for any anomalous signal patterns. We also incorporated data from broadband seismometers and barometers installed in the same tunnel to identify the cause of the signals.
Strain deformations with a period of about 10 minutes were found before and after the earthquakes on June 19, 2022, and January 1, 2024. The origin of the latter was identified as being caused by atmospheric pressure fluctuations, while the analysis of the former is currently underway. In our presentation, we report the results of our analyses of additional earthquakes as well.
Acknowledgements
This research was partly supported by the Joint Research Program of the Institute for Cosmic Ray Research, The University of Tokyo (FY2023 G24) and the Joint Research Program of the Earthquake Research Institute, The University of Tokyo (2023-B-03).
References
Araya et al. (2017): Design and operation of a 1500-m laser strainmeter installed at an underground site in Kamioka, Japan, Earth, Planets and Space., 77, 69-77. https://doi.org/10.1186/s40623-017-0660-0
Yoshida et al. (2023): Upward Earthquake Swarm Migration in the Northeastern Noto Peninsula, Japan, Initiated From a Deep Ring-Shaped Cluster: Possibility of Fluid Leakage From a Hidden Magma System, Journal of Geophysical Research: Solid Earth, 128. https://doi.org/10.1029/2022JB026047
Seismic activity has been increasing since about December 2020 near the Noto Peninsula in Ishikawa Prefecture, preceding the M7.6 on the Japan Meteorological Agency (JMA) scale earthquake on January 1, 2024. Researches on the vibration phenomena across a wide range of frequencies have been conducted and some studies suggest that fluids are involved in the earthquake processes (Yoshida et al., 2023).
In this study, we analyzed deformations before and after relatively large earthquakes during the swarm using data from a laser strainmeter with a baseline length of 1500 m installed in an underground tunnel in Kamioka, Hida City, Gifu Prefecture, aiming to understand the process of earthquake generation.
The 1500 m baseline laser strainmeter
The laser strainmeter is installed in a tunnel constructed for the gravitational wave telescope KAGRA in Kamioka. The azimuth of the instrument is oriented in the N60°E direction and it is located about 0.5 km north of the Atotsugawa fault, running almost parallel to it.
The laser strainmeter primarily consists of a light source, two vacuum chambers containing retro-reflectors and a beam splitter, and a 1,500m vacuum pipe connecting the chambers forming an asymmetric Michelson interferometer. The instrument measures the change in distance between the chambers by detecting the phase change of the interference fringes (Araya et al., 2017).
Compared to seismometers, which measure ground velocity or acceleration, laser strainmeters are more suitable for observing low-frequency ground deformations because they directly measure the displacements of two distant points.
Although Kamioka is about 100 km away from the Noto Peninsula, the laser strainmeter may be capable of detecting subtle deformations associated with the earthquakes due to the superior strain resolution (10-13-10-12) compared to GNSS.
Data analysis and results
We analyzed ground strains recorded before and after earthquakes that occurred in the Noto region with a magnitude 5 or higher. These include the earthquakes on June 19, 2022 (M5.4), May 5, 2023 (M6.5), and January 1, 2024 (M7.6).
We performed spectral analysis on the time series data from the laser strainmeter. After averaging the strain data every second to reduce high-frequency noise, we subtracted the theoretical tidal strain—computed using GOTIC2 (National Astronomical Observatory of Japan, NAOJ) — from the observational data. We then applied Fourier transform to the residual signals to obtain power spectra and spectrograms and used them to search for any anomalous signal patterns. We also incorporated data from broadband seismometers and barometers installed in the same tunnel to identify the cause of the signals.
Strain deformations with a period of about 10 minutes were found before and after the earthquakes on June 19, 2022, and January 1, 2024. The origin of the latter was identified as being caused by atmospheric pressure fluctuations, while the analysis of the former is currently underway. In our presentation, we report the results of our analyses of additional earthquakes as well.
Acknowledgements
This research was partly supported by the Joint Research Program of the Institute for Cosmic Ray Research, The University of Tokyo (FY2023 G24) and the Joint Research Program of the Earthquake Research Institute, The University of Tokyo (2023-B-03).
References
Araya et al. (2017): Design and operation of a 1500-m laser strainmeter installed at an underground site in Kamioka, Japan, Earth, Planets and Space., 77, 69-77. https://doi.org/10.1186/s40623-017-0660-0
Yoshida et al. (2023): Upward Earthquake Swarm Migration in the Northeastern Noto Peninsula, Japan, Initiated From a Deep Ring-Shaped Cluster: Possibility of Fluid Leakage From a Hidden Magma System, Journal of Geophysical Research: Solid Earth, 128. https://doi.org/10.1029/2022JB026047