5:15 PM - 6:45 PM
[SVC26-P20] Seismic observations of Kikai caldera based on DAS using seafloor cables
Keywords:Distributed acoustic sensing, Kikai caldera, volcanic activity
Introduction:
Kikai caldera, located south of Kagoshima prefecture, has experienced devastating eruptions at 130, 95, and 7.3 ka. Recent studies revealed that the lava dome located in the middle of the caldera formed after the latest devastating eruption at 7.3 ka (Tatsumi et al. 2018). Although the Iwo-dake volcano located at the caldera rim is active now, the volcanic activity of Kikai caldera is not well known because most part of the caldera is located beneath the seafloor.
In order to clarify the structure and develop new methods to monitor the caldera activities, JAMSTEC and Kobe university carried out seismic observations based on the distributed acoustic sensing (DAS) using seafloor cables deployed around Kikai caldera (Nakano et al. 2021). They detected several earthquakes that occurred beneath the caldera dome during 1 month observation period. Their signal was clearly observed by DAS records on the cable connecting Iwo-jima and Take-shima islands located at the caldera rim. This result indicate that the caldera is still seismically active, but the activity is still unclear because of the short observation period. Longer observations are necessary to clarify the activity of Kikai caldera.
In this study, we carried out DAS observations around Kikai caldera with longer period.
Observations:
We conducted the DAS observations between August and November, 2023. For DAS observations, we used the seafloor cables for broadband telecommunications of the islands of Mishima village, Kagoshima prefecture. We used an AP Sensing N5226B R120 interrogator for the DAS observations. Since this interrogator can observe fiber strain longer than 100 km, we observed the seismic motion between Makurazaki and Iwo-jima by connecting the cables at Take-shima.
Analysis and results:
We applied the STA/LTA event trigger to the DAS records after 2–20 Hz band-pass filtering. We searched for earthquakes corresponding to the trigger from the JMA earthquake catalog based on the trigger time. About 500 earthquakes with magnitudes between 0 and 5 were recorded during the observation period located within about 200 km from the observation area. We also detected several tens of events that were not listed in the JMA earthquake catalog, which may be small earthquakes occurred close to the seafloor cables.
We determined the hypocenters of the signals without JMA hypocenter information. Assuming that the distinct signals were S waves, we picked their onset times and determined the hypocenter. We note that the onsets were not always clear, and the hypocenter was not well constrained for most of events. Finally, about 5 events were located beneath Kikai caldera, including ones beneath the Iwo-dake.
Although several events were detected beneath the caldera, the number was still small during the four-month observation period, and we consider that the caldera is not seismically active.
Discussion:
Since signals from the caldera events were weak, the first motions were only read at limited segments of the cable which resulted in relatively large location errors. Further studies taking advantage of dense DAS observations may be necessary for improving the location accuracy.
Acknowledgments:
We are grateful to Mishima village, Kagoshima prefecture, for permissions to use the seafloor cables in our DAS experiments. This work was supported by JSPS KAKENHI Grant Numbers JP22H00251 and JP21H05205.
Kikai caldera, located south of Kagoshima prefecture, has experienced devastating eruptions at 130, 95, and 7.3 ka. Recent studies revealed that the lava dome located in the middle of the caldera formed after the latest devastating eruption at 7.3 ka (Tatsumi et al. 2018). Although the Iwo-dake volcano located at the caldera rim is active now, the volcanic activity of Kikai caldera is not well known because most part of the caldera is located beneath the seafloor.
In order to clarify the structure and develop new methods to monitor the caldera activities, JAMSTEC and Kobe university carried out seismic observations based on the distributed acoustic sensing (DAS) using seafloor cables deployed around Kikai caldera (Nakano et al. 2021). They detected several earthquakes that occurred beneath the caldera dome during 1 month observation period. Their signal was clearly observed by DAS records on the cable connecting Iwo-jima and Take-shima islands located at the caldera rim. This result indicate that the caldera is still seismically active, but the activity is still unclear because of the short observation period. Longer observations are necessary to clarify the activity of Kikai caldera.
In this study, we carried out DAS observations around Kikai caldera with longer period.
Observations:
We conducted the DAS observations between August and November, 2023. For DAS observations, we used the seafloor cables for broadband telecommunications of the islands of Mishima village, Kagoshima prefecture. We used an AP Sensing N5226B R120 interrogator for the DAS observations. Since this interrogator can observe fiber strain longer than 100 km, we observed the seismic motion between Makurazaki and Iwo-jima by connecting the cables at Take-shima.
Analysis and results:
We applied the STA/LTA event trigger to the DAS records after 2–20 Hz band-pass filtering. We searched for earthquakes corresponding to the trigger from the JMA earthquake catalog based on the trigger time. About 500 earthquakes with magnitudes between 0 and 5 were recorded during the observation period located within about 200 km from the observation area. We also detected several tens of events that were not listed in the JMA earthquake catalog, which may be small earthquakes occurred close to the seafloor cables.
We determined the hypocenters of the signals without JMA hypocenter information. Assuming that the distinct signals were S waves, we picked their onset times and determined the hypocenter. We note that the onsets were not always clear, and the hypocenter was not well constrained for most of events. Finally, about 5 events were located beneath Kikai caldera, including ones beneath the Iwo-dake.
Although several events were detected beneath the caldera, the number was still small during the four-month observation period, and we consider that the caldera is not seismically active.
Discussion:
Since signals from the caldera events were weak, the first motions were only read at limited segments of the cable which resulted in relatively large location errors. Further studies taking advantage of dense DAS observations may be necessary for improving the location accuracy.
Acknowledgments:
We are grateful to Mishima village, Kagoshima prefecture, for permissions to use the seafloor cables in our DAS experiments. This work was supported by JSPS KAKENHI Grant Numbers JP22H00251 and JP21H05205.