5:15 PM - 6:45 PM
[SCG54-P06] Detection of volcanic earthquakes by DAS observation using Tonga seafloor telecommunications cable
Keywords:Distributed Acoustic Sensing, volcanic earthquake, Hunga Tonga-Hunga Haʻapai volcano, volcano monitoring
(Background and Purpose)
The massive eruption of the Hunga Tonga-Hunga Haʻapai volcano (HTHH) in January 2022 caused extensive damage from ashfall and tsunami. Disaster prevention requires enhanced monitoring of submarine volcanoes and volcanic islands. Compared to land volcanoes, sea-area volcanoes have fewer observation stations, and the understanding and monitoring of their activities are insufficient. Ocean bottom seismometers are one way of observing sea-area volcanic activity. However, these are very expensive and not widely installed. In particular, few regions operate ocean-bottom cable seismometers that provide real-time data. Therefore, Distributed Acoustic Sensing (DAS) using existing submarine fiber optic cables attracts attention. This technology can measure the propagation of seismic waves at much higher densities with less cost than ocean-bottom seismometers. To utilize DAS for monitoring submarine volcanoes and volcanic islands, this study aimed to detect volcanic earthquakes from the data of DAS observations conducted in Tonga from February 6 to 13, 2023 [1].
(Methods)
The DAS observations analyzed in this study were conducted using the Tonga seafloor telecommunications cable that had been cut on the seafloor 31 km from the land-based station at Nukuʻalofa (about 40 km south-southeast of HTHH) to be repaired from the damage caused by the HTHH eruption. Nakano et al. [1] extracted about 160 earthquakes focused on 1-20 Hz for the entire DAS observation period (about one week). The hypocenters of 17 earthquakes with distinct P- and S-waves and whose hypocenters were expected to be in the vicinity of Tonga were determined. One of them occurred directly under HTHH [1].
In this study, we focused on the low-frequency band (2-4 Hz) that characterizes volcanic tremor and detected events, by taking the median power in the section with the low noise level. Referring to the apparent velocity of seismic waves whose sources had been determined by Nakano et al. [1], we calculated the relationship between the apparent velocity and the propagation angle to the cable. Then, we estimated the source directions of the detected small events. We also calculated the amplitude ratio between the low-frequency and high-frequency bands and investigated the features of the signals.
(Results and Discussion)
Approximately 100 small new events which were not detected by Nakano et al. were newly detected per day. Many of these events had apparent velocities (2.5-3 km/s) similar to that of the S waves of the M2.8 earthquake located just beneath the HTHH [1], suggesting that they came from the direction of Tonga's volcanic chain. They also had nearly identical maximum amplitudes in the low-frequency (2-4 Hz) and high-frequency (5-20 Hz) bands. We infer that these small earthquakes had common mechanisms and were distinct from the M2.8 earthquake dominated by the high-frequency band. These results suggest that we successfully detected many earthquakes or tremors associated with volcanic activity at or near HTHH.
(References)
[1] Nakano, M., Ichihara, M., Suetsugu, D. et al., Monitoring volcanic activity with distributed acoustic sensing using the Tongan seafloor telecommunications cable, 2024, Earth, Planets and Space, doi:10.1186/s40623-024-01972-2.
Acknowledgements:
This study used the data obtained by the collaboration with Tonga Cable Ltd, JICA, Dr. Takao Ohminato and Mr. Takeo Yagi (ERI, U-Tokyo), Dr. Shigeaki Ono (JAMSTEC), and Tonga Geological Services, including Dr. Daisuke Suetsugu and Mr. V. Tovi. This study is supported by JST-JICA SATREPS (ID 23727132).
The massive eruption of the Hunga Tonga-Hunga Haʻapai volcano (HTHH) in January 2022 caused extensive damage from ashfall and tsunami. Disaster prevention requires enhanced monitoring of submarine volcanoes and volcanic islands. Compared to land volcanoes, sea-area volcanoes have fewer observation stations, and the understanding and monitoring of their activities are insufficient. Ocean bottom seismometers are one way of observing sea-area volcanic activity. However, these are very expensive and not widely installed. In particular, few regions operate ocean-bottom cable seismometers that provide real-time data. Therefore, Distributed Acoustic Sensing (DAS) using existing submarine fiber optic cables attracts attention. This technology can measure the propagation of seismic waves at much higher densities with less cost than ocean-bottom seismometers. To utilize DAS for monitoring submarine volcanoes and volcanic islands, this study aimed to detect volcanic earthquakes from the data of DAS observations conducted in Tonga from February 6 to 13, 2023 [1].
(Methods)
The DAS observations analyzed in this study were conducted using the Tonga seafloor telecommunications cable that had been cut on the seafloor 31 km from the land-based station at Nukuʻalofa (about 40 km south-southeast of HTHH) to be repaired from the damage caused by the HTHH eruption. Nakano et al. [1] extracted about 160 earthquakes focused on 1-20 Hz for the entire DAS observation period (about one week). The hypocenters of 17 earthquakes with distinct P- and S-waves and whose hypocenters were expected to be in the vicinity of Tonga were determined. One of them occurred directly under HTHH [1].
In this study, we focused on the low-frequency band (2-4 Hz) that characterizes volcanic tremor and detected events, by taking the median power in the section with the low noise level. Referring to the apparent velocity of seismic waves whose sources had been determined by Nakano et al. [1], we calculated the relationship between the apparent velocity and the propagation angle to the cable. Then, we estimated the source directions of the detected small events. We also calculated the amplitude ratio between the low-frequency and high-frequency bands and investigated the features of the signals.
(Results and Discussion)
Approximately 100 small new events which were not detected by Nakano et al. were newly detected per day. Many of these events had apparent velocities (2.5-3 km/s) similar to that of the S waves of the M2.8 earthquake located just beneath the HTHH [1], suggesting that they came from the direction of Tonga's volcanic chain. They also had nearly identical maximum amplitudes in the low-frequency (2-4 Hz) and high-frequency (5-20 Hz) bands. We infer that these small earthquakes had common mechanisms and were distinct from the M2.8 earthquake dominated by the high-frequency band. These results suggest that we successfully detected many earthquakes or tremors associated with volcanic activity at or near HTHH.
(References)
[1] Nakano, M., Ichihara, M., Suetsugu, D. et al., Monitoring volcanic activity with distributed acoustic sensing using the Tongan seafloor telecommunications cable, 2024, Earth, Planets and Space, doi:10.1186/s40623-024-01972-2.
Acknowledgements:
This study used the data obtained by the collaboration with Tonga Cable Ltd, JICA, Dr. Takao Ohminato and Mr. Takeo Yagi (ERI, U-Tokyo), Dr. Shigeaki Ono (JAMSTEC), and Tonga Geological Services, including Dr. Daisuke Suetsugu and Mr. V. Tovi. This study is supported by JST-JICA SATREPS (ID 23727132).