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[STT36-01] High-frequency tsunamis generated near Torishima island, Japan, observed by distributed acoustic sensing
★Invited Papers
Keywords:DAS, Tsunami, Torishima
Using distributed acoustic sensing (DAS) techniques, various types of wavefield, such as earthquake and ocean waves, have been captured by submarine fiber optic cables. However, tsunami records have been less because whether they can be observed depend on DAS experimental periods and their locations. In southern Japan on October 9, 2023 (JST), sea level changes due to tsunamis were observed by tide gauges, and the tsunami generation is likely to be related to activities of submarine volcanoes. We are now acquiring the continuous records of DAS off Muroto in southern Japan, and attempt to explore signals related to the tsunamis. In this study, if such signals are observed, we confirm whether they correspond to tsunamis by calculating the frequency-dependent phase velocities, and compare the waveforms with those observed at nearby absolute pressure gauges (APGs).
In our DAS records, coherent signals propagating landwards are observed in the northern part of the submarine cable. The dominant frequency is 6–30 mHz. Their arrival times are almost in consistent with those observed the APGs. The frequency-dependent propagation velocities of the observed signals correspond to those of infragravity waves (i.e., deep water waves, ocean surface gravity waves), and the infragravity waves are categorized into high-frequency tsunamis. It seems that lower frequency components than 3 mHz are less in this tsunami event, which differs from frequency components of typical tsunamis excited by large earthquakes.
Moreover, using DAS records, we estimated time-series of the tsunami generation at the source location. Relatively high frequency components within high-frequency tsunamis are excited in early stage of the volcanic activities, and their frequency components are shifted to middle and low ones in later stage. This indicates that the volcanic activities tend to be large at the later stage. The obtained features of the time-series were consistent with those using records of APGs. Our results indicate that DAS records are useful for detecting tsunami propagations and also elucidating excitation mechanisms of tsunamis.
Data citation
DONET: doi:10.17598/NIED.0008
Acknowledgement
This work was supported by JSPS KAKENHI Grant No. JP21H05202, JP21H05204 in Scientific Research on Transformative Research Areas “Science of Slow-to-Fast earthquakes”. We also used DONET data provided by National Research Institute for Earth Science and Disaster Resillience.
In our DAS records, coherent signals propagating landwards are observed in the northern part of the submarine cable. The dominant frequency is 6–30 mHz. Their arrival times are almost in consistent with those observed the APGs. The frequency-dependent propagation velocities of the observed signals correspond to those of infragravity waves (i.e., deep water waves, ocean surface gravity waves), and the infragravity waves are categorized into high-frequency tsunamis. It seems that lower frequency components than 3 mHz are less in this tsunami event, which differs from frequency components of typical tsunamis excited by large earthquakes.
Moreover, using DAS records, we estimated time-series of the tsunami generation at the source location. Relatively high frequency components within high-frequency tsunamis are excited in early stage of the volcanic activities, and their frequency components are shifted to middle and low ones in later stage. This indicates that the volcanic activities tend to be large at the later stage. The obtained features of the time-series were consistent with those using records of APGs. Our results indicate that DAS records are useful for detecting tsunami propagations and also elucidating excitation mechanisms of tsunamis.
Data citation
DONET: doi:10.17598/NIED.0008
Acknowledgement
This work was supported by JSPS KAKENHI Grant No. JP21H05202, JP21H05204 in Scientific Research on Transformative Research Areas “Science of Slow-to-Fast earthquakes”. We also used DONET data provided by National Research Institute for Earth Science and Disaster Resillience.