Excitation of back-arc tsunamis from megathrust ruptures: The underdog hazard in the Sea of Japan

The 2011 Tohoku earthquake created a moderate tsunami in the back-arc Sea of Japan basin. This tsunami went largely unnoticed due to its small size and the significant coverage of the large fore-arc waves. We present a physical dislocation model for the excitation of back-arc tsunamis and identify fault dip as the main geometrical contributor to the propagation of back-arc tsunamis. Using numerical simulations and data from the 2011 event, we show that a combination of near- to intermediate-field horizontal and vertical dislocations as well as transient surface waves is necessary to reconstruct the back-arc propagation. We then simulate potential future earthquakes in the Japan trench and Nankai trough to investigate the back-arc tsunami hazard in the Sea of Japan. Our results show that the coseismic excitation of back-arc tsunamis can result in considerable waves exceeding 1 m from megathrust earthquakes.

Origin of Compositional Diversity of Marine Tephra during the Late Middle Pleistocene B-KY1 Baekdusan Volcanic Eruption

The focus of this study was the Baekdusan-Kita 1 (B-KY1) eruption during the late Middle Pleistocene. We identified B-KY1 tephra between the Toya and Aso-1 tephras in the ODP 794A core from the Japan Basin of the East Sea/Japan Sea. The stratigraphic position of the B-KY1 tephra correlated exactly with the first B-KY1 to be identified, in the 20EEZ-1 core from the Kita–Yamato Trough. However, B-KY1 tephra in the ODP 794A core showed a wide range of geochemical compositions. The textural characteristics of B-KY1 tephra in the ODP 794A core was characterized by higher content of fine-grained bubble-wall shards than that of the B-KY1 tephra in the 20EEZ-1 core. The difference in B-KY1 tephra between the two coring sites may reflect shifts in wind direction during volcanic eruptions. We refined the eruption age of the B-KY1 tephra by examining distinct sedimentary facies related to the start of the penultimate deglaciation of this region at ca. 135 ka. The findings of this study suggest that the compositional diversity of B-KY1 tephra may have been influenced by subsequent mixing of comendite and comenditic trachyte magma with injected pantelleritic magma during the late Middle Pleistocene Baekdusan volcanic eruption.

Seasonal Variability in Middepth Gyral Circulation Patterns in the Central East/Japan Sea as Revealed by Long-Term Argo Data

Trajectories of Argo floats deployed in the East/Japan Sea from 2001 to 2014 reveal that the middepth gyral circulation pattern of the Japan basin, the central part of the East/Japan Sea, undergoes a seasonal variation. The middepth circulation of the Japan basin is found to be characterized usually by the gyres trapped to the east of the Bogorov Rise (E-gyres) and those extending farther westward into the whole basin (BW-gyres). The E-gyre trajectories are generally associated with the turning of the floats toward deeper regions off the isobaths. This occurs in winter either on the northern or eastern side of the eastern Japan basin. It seems that the upstream part of the otherwise BW-gyre is subject to a strong negative wind stress curl in winter, and there the circulating water columns are driven toward the deeper region, thus triggering the formation of the E-gyre. The topographic effect associated with the Bogorov Rise seems to interfere thereafter in the process of determining the passage of the E-gyre. Otherwise, the water columns continue to flow along the isobaths, hence maintaining the BW-gyre. To the knowledge of the authors, this is the first observational evidence of seasonal variability in the middepth gyral circulation pattern in the East/Japan Sea. It suggests that oceanic middepth circulation, usually known to be quasi steady or slowly varying on climatological time scales, might also undergo a significant seasonal variation as it does in the East/Japan Sea.

Spatial Distribution of Radiocarbon in the Southwestern Japan/East Sea Immediately After Bottom Water Renewal

In April 2001, immediately after bottom water renewal in the Japan/East Sea, 7 vertical profiles of radiocarbon were obtained from the area, including the Japan Basin and the Tsushima/Ulleung Basin. The bottom water Δ14C of the Tsushima/Ulleung Basin increased by 24% during 1979–2001 and was 10–15% higher than the values reported for the Japan Basin and Yamato Basin during 1995–2002. Within the depth interval 800–2000 m, Δ14C values at the southernmost station in the Japan Basin were much higher than at other stations, with the maximum difference in Δ14C values being more than 50%. Based on a combination of physical observations with analysis of the Δ14C data, we concluded that the bottom water, which was formed by the sinking of surface water off Vladivostok, Russia, in late January-early February of 2001, had been widely distributed to depths below 2000 m within the southernmost Japan Basin in a timespan shorter than 2 months. However, there was no evidence that the water had been transported into the Tsushima/Ulleung Basin during the same time interval.