講演情報
[S17-09]2024年11月19日トンガ沖海底火山で発生した非ダブルカップル地震と津波
*末次 大輔1,4、三反畑 修1、井上 公3、中田 光紀2、池永 有弥7、後藤 和久2、有川 太郎6、モアラ アパイ4、ペニソニ マフォア4、ペール アンジェリック4、ヴァイオモウンガ レニー4、クラ タアニエラ5、市原 美恵1 (1. 東京大学地震研究所、2. 東京大学理学部、3. 京都大学防災研究所、4. トンガ地質局、5. トンガ国土資源省、6. 中央大学理工学部、7. 産業総合研究所)
An earthquake occurred at a depth of 5 km close to unnamed Submarine Volcano 1 and 2 (Fig.1, called Volc#1 and #2, hereafter) about 50 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 with vertical T-axis” (CLVD hereafter in this study), which is a characteristic pattern of volcanic earthquakes (Fig.1)（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 since 1999 with an interval of 2-3 years. Although centroid locations determined by TGS andand those determined by GCMT, USGS, GEOFON largely agencies scatter in the Volc#1 and #2 area, making it difficult to have a reliable centroid location. We analyzed P wave particle motion to constrain the location, which indicates that the P-wave was propagated from Volc#1. About 20 minutes after the earthquake, a tsunami arrived at Ohonua on ‘Eua island (Fig. 1, 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 (Fig. 1) shows a 4 cm tsunami at a time little later of the ‘Eua tsunami. The observed tsunami is unusually high for Mw5.5. We conducted a tsunami survey a week after the earthquake with a drone at the west and south coasts of Tongatapu island. Although we found water marks on the beach at height of 1.5m, it is not certain that the water marks are due to the tsunami. We performed tsunami simulation by setting seawater elevation of 1 m at the Volc#1 and #2. The Volc#1 can explain the arrival times and waveform of the observed tsunami better than the Volc#2. From the tsunami simulation and the P-wave particle motion analysis, we conclude the CLVD earthquake occurred beneath Volc#1. CLVD events were reported at Sumisu caldera volcano of the Izu-Bonin volcanic chain in Japan (Sandanbata et al., 2022). Mw5.4-5.7 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 have been still debated, Sandanbata et al. proposed a hypothesisthe 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 tsunami boulders emplaced in the past. We need to estimate whether the Volc#1 eruption could generate a large tsunami enough to explain the tsunami boulders. It is important to monitor the earthquake activity and source mechanism carefully near the Tofua volcanic chain. 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).
Reference Ekström G. (1994), EPSL 128, doi:10.1016/0012-821X(94)90184-8; Flohrich C. (2009), Geology, 37, doi:10.1130/G25277A.1; Sandanbata, O., et al., (2022), JGR 127, doi.org/10.1029/2022JB024213.
Reference Ekström G. (1994), EPSL 128, doi:10.1016/0012-821X(94)90184-8; Flohrich C. (2009), Geology, 37, doi:10.1130/G25277A.1; Sandanbata, O., et al., (2022), JGR 127, doi.org/10.1029/2022JB024213.
