Toshiya Fujiwara

An increase in seismic activity was observed near the Izu-Torishima and Sofugan islands in the Izu-Ogasawara arc of Japan in early October 2023. Several M6-class earthquakes occurred from October 3 to 6, and tsunamis were recorded on October 5 and 6, associated with M6-class normal faulting earthquakes. On October 8, tsunamis were observed without any significant earthquakes but accompanied by distinct T-phase signals. Analyses of T-phases, tsunami waveforms, and bathymetry surveys suggested that the October 8 tsunamis were linked to the volcanic activity at the Sofu Seamount, located west of Sofugan Island. However, the precise location of the entire activity and its connection to the tsunamis on October 5 and 6 remain unclear. We conducted earthquake observations near the Sofu Seamount from November to December 2023 using ocean bottom seismographs (OBSs). These observations revealed that seismic activity extended approximately 50 km in the NNW direction, from the Sofu Seamount to the Torishima Rift, a back-arc rift between the Izu-Torishima and Sofugan Islands. Although it is difficult to constrain the hypocenter depths owing to the limited number of OBSs and their large separations, earthquakes in the Torishima Rift were likely located at depths of 15–20 km or even shallower, at approximately 5 km. The tsunami observed at Hachijo-jima Island on October 5 can be attributed to the rupture of a normal fault in the Torishima Rift, as inferred from OBS observations. Shallow normal faults and intrusions reaching the seafloor were previously imaged in the back-arc rift zone near the Torishima Rift. Moreover, the earthquake activity in early October showed a northward migration from the Sofu Seamount to the Torishima Rift, followed by a rapid decrease after the volcanic activity of the Sofu Seamount, which was accompanied by tsunamis on October 8. The results suggest that the seismic activity in early October 2023, accompanied by tsunamis, is likely related not only to the volcanic activity at the Sofu Seamount but also to the tectono-magmatic activity in the Torishima Rift. Graphical

On 8 October 2023 (UTC), unique earthquakes occurred in the Izu‐Ogasawara Arc, Japan, in which the P‐ and S‐phases were barely visible and only the T‐phases were evident, followed by tsunamis that reached islands in the Izu‐Ogasawara Arc and a wide area of the Pacific coast of southwest Japan. Our estimated T‐phase source area coincides with the Sofu Seamount, which was previously unrecognized as an active submarine volcano. A bathymetric survey of the seamount conducted 1 month after the event revealed characteristics of the seamount with a caldera and a central cone. Compared to the bathymetry in 1987, the topography in the caldera had changed significantly such as a crater forming in the central cone. This seamount is likely to be an active volcano. The topographic changes on the caldera‐sized scale that occurred at the caldera can be explained as a source of the October tsunami.

On October 8, 2023 (UTC), unique earthquakes occurred in the Izu-Ogasawara Arc, Japan, in which the P- and S-phases were barely visible and only the T-phase was evident, followed by tsunamis that reached islands in the Izu-Ogasawara Arc and a wide area of the Pacific coast of southwest Japan. In the location where the T-phase source was estimated, there is the Sofu Seamount which was previously unrecognized as an active submarine volcano. A bathymetric survey of the seamount conducted one month after the event revealed characteristics of the seamount with a caldera and a central cone. Compared to the bathymetry in 1987, the topography in the caldera had changed significantly such as a crater forming in the central cone. This seamount is likely to be an active volcano. The topographic changes on the caldera-sized scale that occurred at the caldera can be explained as a source of the October tsunami.

症例は73歳男性．PET-CTで左肺上葉にFDGの異常集積を伴う結節を2ヵ所指摘された．全身麻酔下，右側臥位で固定し，胸腔鏡補助下左上葉切除術，ND2a-1リンパ節郭清を施行した．2個の腫瘍はともに低分化腺癌で，pT1c（2）N0M0 stage IA3であった．手術翌日より左三角筋の脱力による左上肢の挙上困難を訴え，腋窩神経麻痺と診断，原因として四辺形間隙症候群が疑われたため，理学療法を開始した．術後1ヵ月目より徐々に左上肢の挙上が可能となり，術後2ヵ月目には日常生活に支障ない程度まで回復，間もなく筋力の回復もみられた．術後5年現在，無再発で左上肢の不自由もなく外来通院にて経過観察中である．胸部手術における合併症としての四辺形間隙症候群は稀であるが，症例を蓄積するとともに腋窩神経とその周辺の解剖構造を理解し，同部への圧迫や過伸展のかからない肢位作りを定型化していくことが重要である．

Monitoring slow earthquake activity in subduction zones can give important insight into the stress build-up and subsequent rupture extent of megathrust earthquakes. Extensive slow earthquake activity occurs up-dip of the seismogenic zone of the Nankai Trough subduction zone, an area that might be awaiting a large (M _w {greater than or equal to}8) earthquake in the near future. Mechanisms used to explain the occurrence of slow earthquakes are often linked to temporal changes in fluid transport along faults. This study utilises this theory in evaluating the usage of 4D gravity measurements on the seafloor for monitoring changes in fluid flow, hence monitoring the slow earthquake activity and the mechanisms behind them. We model the gravity response from fluid-related density changes in an area of the Nankai Trough accretionary prism that experiences several slow earthquake episodes in the interseismic period. The forward modelled 4D gravity response is used to estimate volumes of fluid at specific locations of the accretionary prism and plate interface corresponding to a minimum gravity signal of 5 µGal. This accuracy in the gravity signal is obtainable through technology monitoring micro-gravity effects at the seafloor. Based on the results we have formulated a hypothesis on how small fluid volume changes can be detected through a gravimetry survey at the seafloor of the Nankai Trough. The results can also be used to design a survey layout for obtaining valuable 4D gravity data at the Nankai Trough.

Repeated multibeam bathymetric surveys played an important role for understanding the distribution of coseismic seafloor displacement caused by the March 11, 2011 Mw 9.0 Tohoku-oki earthquake. After the earthquake, we collected bathymetric data along the same tracks obtained before the earthquake. The selected tracks were crossing the trench and extending from the landward to seaward trench slopes. We examined the seafloor displacement on the landward relative to the seaward by means of the difference in bathymetry before and after the earthquake. The multibeam bathymetric survey has the advantage of areal coverage. The repeated surveys clarified the areal distribution of the coseismic seafloor displacement. In the main rupture area, very large seafloor displacement was observed. Sharp bathymetric change at the trench axis provided solid evidence that the fault slip on the shallowest part of the megathrust reached the trench axis and peaked at the trench axis. The very large displacement is limited to the particular area. Smaller seafloor displacements were observed in the area tens of kilometers away from the main rupture area. We present methods and results of the repeated multibeam bathymetric surveys and an application to the seafloor displacement caused by the 2011 Tohoku-oki earthquake in the northern Japan Trench. Less than several meters in seafloor vertical displacements and less than 20 m in seafloor horizontal displacement were estimated in the northern Japan Trench. The estimated smaller displacements are comparable in magnitude to error of the seafloor displacement observation from our bathymetric survey. Nevertheless, three adjacent survey tracks showed coherent relative differences in seafloor elevation, which suggests the relative difference enables us to discuss the along-track variation in seafloor displacement in the area. However, our survey was affected by uncertainties of roll and pitch biases and sound velocity errors. Well-prepared repeated multibeam bathymetric survey for the purpose of seafloor geodesy could lead to a higher resolution and more accurate result. Repeated acquisition of high resolution and accuracy bathymetric data using state-of-the-art technology will be important to quantitative discussion of the seafloor displacements caused by even smaller magnitude earthquakes and tsunamis.

We conducted multibeam bathymetric surveys crossing the northern Japan Trench offshore Sanriku and examined seafloor displacement by means of the difference in bathymetry before and after the 11 March 2011 Tohoku‐oki earthquake. The seafloor displacements were less than several meters in vertical displacement and less than 20 m in trench‐normal horizontal displacement; consequently, lower amplitude seafloor displacements were evaluated. Comparatively, the landward lower slope near the trench was uplifted, the middle slope was subsided, and the midslope terrace was depressed after the earthquake. Therefore, localized very large fault slip or very large submarine landslide is unlikely. Instead, smaller uplift of the lower slope near the trench extending broader along the trench seems to be plausible explanation for the devastating tsunami source in the northern Japan Trench area.

We detected and measured coseismic displacement caused by the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (M_W) 9.0] by using multibeam bathymetric surveys. The difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the Japan Trench, suggesting that the fault rupture reached the trench axis. The sea floor on the outermost landward area moved about 50 meters horizontally east-southeast and ~10 meters upward. The large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement.