15:30 〜 15:45
[MGI25-01] トンガにおける２０２４年海底火山地震と津波：巨大津波石の起源に関連して
★招待講演
キーワード：海底火山、CLVD 地震、津波、津波石
An earthquake occurred at a depth of 5 km close to “Unnamed Submarine Volcano 3 and 4" (Global Volcanism Program) about 60 km west of Tongatapu and Eua islands at 3:41pm on Nov. 19, 2024 (Mw5.5). There is no felt report of the event. The source mechanism determined by TGS is “compensated linear vertical dipole (CLVD) with vertical T-axis” (CLVD-VT hereafter), which is a characteristic pattern of volcanic earthquakes (e.g., Ekström, 1994).
The source mechanisms reported internationally are almost the same as that determined by TGS. According to the Global Centroid Moment Tensor (GCMT) catalog, one of the most reliable earthquake catalogs that has been operated for several decades, the CLVD-VT events have been taking place in the area repeatedly at least since 1999 with an interval of 3-5 years.
About 20 minutes after the earthquake, a tsunami arrived at Ohonua on ‘Eua island (epicentral distance of 52 km). The peak-to-trough amplitude is 40 cm on a tide gauge operated by Tonga Meteorological Services (TMS). A tide gauge at Nuku’alofa of Tongatapu shows a few-cm tsunami a little after the ‘Eua tsunami. The tsunami is likely to be generated by the earthquake, since theoretical tsunami travel times from the Unnamed volcanoes to ‘Eua and Nukualofa is 20~25 minutes (JICA, 2023). The observed tsunami is unusually high for Mw5.5. We conducted a tsunami survey with a drone at the west and south coasts of Tongatapu island a week after the earthquake and found water marks on the beach, though it is not certain that the water marks are due to the tsunami, since a previous drone survey shows that a part of the water marks existed before Nov. 19. Assuming the tsunami water mark represents a tsunami run-up location, the tsunami run-up height are 1.2~1.5 m.
CLVD-VT events were reported at Sumisu caldera volcano of the Izu-Bonin volcanic chain in Japan (Sandanbata et al., 2022). Mw5.5-6 events have occurred under the Sumisu caldera repeatedly with an interval of about 10 years and generated tsunami waves of 1 m with no precursory ground shaking. Although details of the mechanism of earthquake and tsunami occurrence has been still debated (e.g., Ekström, 1994; Sandanbata et al., 2022), the events are considered to be caused by magma intrusion beneath the caldera. The latest study proposed that the events were generated by magma intrusion and associated reverse faulting along the caldera rim. The Tongan event in the present report may have been caused by a magma activity.According to reports from local residents, the west coast of Tongatapu island had been hit by an ocean wave of unusual height with no precursory ground shaking since1990s. Some of the high waves may have been tsunamis generated by an earthquake beneath the Unnamed volcanoes. Several tsunami boulders are located along the west coast, which are considered to be brought by tsunami (e.g., Frohlich et al., 2009), although a tsunami source is unidentified. The volcanic earthquake and associated tsunami may be relevant to the emplacement of tsunami boulders in the past. It is important to monitor the earthquake activity and source mechanism carefully near the Tofua volcanic chain.
References
Ekström G. (1994), EPSL, 128, doi:10.1016/0012-821X(94)90184-8.
Flohrich C. (2009), Geology, 37, doi:10.1130/G25277A.1.
Global Volcanism Program, Smithsonian Institute National Museum of Natural History, https://volcano.si.edu.
Japan International Corporation Agency, Oriental Consultant Global Co. Ltd., Pacific Consultant, Co. Ltd., (2023), Survey report for planning a project of a recovery from volcanic and tsunami disasters.
Sandanbata, O., et al., (2022), JGR Solid Earth, 127, e2022JB024213. https://doi.org/10.1029/2022JB024213.
Acknowledgement
We thank Tonga Meteorological Services for providing tide gauge data. This research was supported by Science and Technology Research Partnership for Sustainable Development (SATREPS) in collaboration between Japan Science and Technology Agency (JST, JPMJSA2309) and Japan International Cooperation Agency (JICA).
The source mechanisms reported internationally are almost the same as that determined by TGS. According to the Global Centroid Moment Tensor (GCMT) catalog, one of the most reliable earthquake catalogs that has been operated for several decades, the CLVD-VT events have been taking place in the area repeatedly at least since 1999 with an interval of 3-5 years.
About 20 minutes after the earthquake, a tsunami arrived at Ohonua on ‘Eua island (epicentral distance of 52 km). The peak-to-trough amplitude is 40 cm on a tide gauge operated by Tonga Meteorological Services (TMS). A tide gauge at Nuku’alofa of Tongatapu shows a few-cm tsunami a little after the ‘Eua tsunami. The tsunami is likely to be generated by the earthquake, since theoretical tsunami travel times from the Unnamed volcanoes to ‘Eua and Nukualofa is 20~25 minutes (JICA, 2023). The observed tsunami is unusually high for Mw5.5. We conducted a tsunami survey with a drone at the west and south coasts of Tongatapu island a week after the earthquake and found water marks on the beach, though it is not certain that the water marks are due to the tsunami, since a previous drone survey shows that a part of the water marks existed before Nov. 19. Assuming the tsunami water mark represents a tsunami run-up location, the tsunami run-up height are 1.2~1.5 m.
CLVD-VT events were reported at Sumisu caldera volcano of the Izu-Bonin volcanic chain in Japan (Sandanbata et al., 2022). Mw5.5-6 events have occurred under the Sumisu caldera repeatedly with an interval of about 10 years and generated tsunami waves of 1 m with no precursory ground shaking. Although details of the mechanism of earthquake and tsunami occurrence has been still debated (e.g., Ekström, 1994; Sandanbata et al., 2022), the events are considered to be caused by magma intrusion beneath the caldera. The latest study proposed that the events were generated by magma intrusion and associated reverse faulting along the caldera rim. The Tongan event in the present report may have been caused by a magma activity.According to reports from local residents, the west coast of Tongatapu island had been hit by an ocean wave of unusual height with no precursory ground shaking since1990s. Some of the high waves may have been tsunamis generated by an earthquake beneath the Unnamed volcanoes. Several tsunami boulders are located along the west coast, which are considered to be brought by tsunami (e.g., Frohlich et al., 2009), although a tsunami source is unidentified. The volcanic earthquake and associated tsunami may be relevant to the emplacement of tsunami boulders in the past. It is important to monitor the earthquake activity and source mechanism carefully near the Tofua volcanic chain.
References
Ekström G. (1994), EPSL, 128, doi:10.1016/0012-821X(94)90184-8.
Flohrich C. (2009), Geology, 37, doi:10.1130/G25277A.1.
Global Volcanism Program, Smithsonian Institute National Museum of Natural History, https://volcano.si.edu.
Japan International Corporation Agency, Oriental Consultant Global Co. Ltd., Pacific Consultant, Co. Ltd., (2023), Survey report for planning a project of a recovery from volcanic and tsunami disasters.
Sandanbata, O., et al., (2022), JGR Solid Earth, 127, e2022JB024213. https://doi.org/10.1029/2022JB024213.
Acknowledgement
We thank Tonga Meteorological Services for providing tide gauge data. This research was supported by Science and Technology Research Partnership for Sustainable Development (SATREPS) in collaboration between Japan Science and Technology Agency (JST, JPMJSA2309) and Japan International Cooperation Agency (JICA).