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Topographic cross section around Koseda Coast and location of Onagawa Outcrop modified after Nanayama and Maeno (2019) 16) .
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Previous research indicates that Southern Kyushu Island, southwestern Japan, may have been struck by a large tsunami during the 7.3 ka caldera-forming Kikai eruption, but this has not been confirmed. This paper describes sedimentological and chronostratigraphic evidence of a tsunami during the 7.3 ka Kikai caldera eruption based on examples from Ya...
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Indonesia is exposed to earthquakes, volcanic activities, and associated tsunamis. This is particularly the case for Lombok and Sumbawa Islands in West Nusa Tenggara, where evidence of tsunamis is frequently observed in its coastal sedimentary record. If the 1815 CE Tambora eruption on Sumbawa Island generated a tsunami with well-identified traces...
Citations
... Okamura et al. (2005) interpreted a 20-cm-thick bed of sandy gravel just below the K-Ah tephra in piston cores from Tachibana Bay, about 210 km North of the caldera (TB in Figure 1(b)), as a tsunami deposit from the 7.3 ka Kikai eruption. However, other possible tsunami deposits lie outside the range of the Kikai eruption tsunami simulated by Maeno et al. (2006) and Maeno and Imamura (2007), and for these an interplate earthquake in the northern Ryukyu Trench, the Sea of Hyuga, or the Nankai Trough is also a plausible source (Nanayama, 2019). . This suggests that the sea level is rising slightly on the Southwestern coast of Shikoku. ...
... Naruo and Kobayashi (2002) reported large areas that underwent liquefaction caused by one or more large earthquakes during the K-Ah eruption. Because this liquefaction area extends from Yakushima to the Osumi Peninsula along the Northern Ryukyu Trench, it may instead be evidence of an interplate earthquake in the Ryukyu Trench around 7300 cal BP (Nanayama, 2019). ...
Tsunami deposits in Kyushu Island, southwestern Japan, have been attributed to the 7.3 ka Kikai caldera eruption, but their origin has not been confirmed. We analyzed a 83-cm-thick Holocene event deposit in the SKM core, obtained from incised valley fill in the coastal lowlands near Sukumo Bay, southwestern Shikoku Island. We confirmed that the event deposit contains K-Ah volcanic ash from the 7.3 ka eruption. The base of the event deposit erodes the underlying inner-bay mud, and the deposit contains material from outside the local terrestrial and marine environment, including angular quartz porphyry from a small inland exposure, oyster shell debris, and a coral fragment. Benthic foraminifers and ostracods in the deposit indicate various habitats, some of which are outside Sukumo Bay. The sand matrix contains low-silica volcanic glass from the late stage of the Kikai caldera eruption. We also documented the same glass in an event deposit in the MIK1 core, from the incised Oyodo River valley in the Miyazaki Plain on southeastern Kyushu. These two 7.3 ka tsunami deposits join other documented examples that are widely distributed in southwestern Japan including the Bungo Channel and Beppu Bay in eastern Kyushu, Tachibana Bay in western Kyushu, and Zasa Pond on the Kii Peninsula as well as around the caldera itself. The tsunami deposits near the caldera have been divided into older and younger 7.3 ka tsunami deposits, the younger ones matching the set of widespread deposits. We attribute the younger 7.3 ka tsunami deposits to a large tsunami generated by a great interplate earthquake in the northern part of the Ryukyu Trench and (or) the western Nankai Trough just after the late stage of the Kikai caldera eruption and the older 7.3 ka tsunami deposits to a small tsunami generated by an interplate earthquake or Kikai caldera eruption.
Here, the characteristics of post-LGM (Last Glacial Maximum) in-cised valley fills and their depositional sequence were examined by analyzing the SKM core collected in the Sukumo coastal lowlands, southwestern Shikoku Island, which has been subject to extensive seismic subsidence due to the large Nankai Trough earthquakes. Here we provide results of sedimentological, radiocarbon dating, and paleoenvironmental analysis. Sediments of the SKM core show clear indications that the succession was influenced by post-glacial sea level change. The Matsuda River incised valley formed during the LGM (.-ka), and was infilled by fluvial sand and gravel in the late Pleistocene. Following postglacial transgression, sea level rose by m (a.s.l.) at. ka, and the incised valley became an estuarine environment. As sea level continued to rise, the estuarine environment was replaced by an inner bay mud bottom, and maximum water depth was reached at. ka. The. ka Kikai caldera eruption in southern Kyushu Island caused heavy K-Ah ash fall in southwestern Shikoku Island, and frequent large-scale lahars occurred immediately after the ash fall because of the proximity to the volcanic source. After the ash fall, the K-Ah secondary sediments were deposited rapidly in the inner bay environment and caused rapid sea level regression. After. ka, a delta region began to develop, which might have been due to the large K-Ah ash fall. At ka, sea level reached +. m (a.s.l.), estimated from the Sukumo midden, and is recognized as the Holocene marine limit in this area. This information on relative sea level change during the past , years suggests that the Sukumo Bay area has not subsided as a result of seismic-induced crustal deformation.
