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 an 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.

中文翻译：

---

从日本西南部宿毛湾地区的海啸沉积推断出 7.3 ka 的大地震

日本西南部九州岛的海啸沉积物归因于 7.3 ka Kikai 火山口喷发，但其来源尚未得到证实。我们分析了 SKM 核心中一个 83 厘米厚的全新世事件沉积物，该沉积物是从四国岛西南部 Sukumo 湾附近沿海低地的下切山谷填充物获得的。我们确认事件矿床包含 7.3 ka 喷发的 K-Ah 火山灰。事件矿床的底部侵蚀了下伏的内湾泥浆，矿床包含来自当地陆地和海洋环境之外的物质，包括来自小内陆暴露的角状石英斑岩、牡蛎壳碎片和珊瑚碎片。沉积物中的底栖有孔虫和介形动物表明了各种栖息地，其中一些栖息地位于 Sukumo 湾外。砂基质包含来自 Kikai 火山口喷发后期的低硅火山玻璃。我们还在 MIK1 核心的事件矿床中记录了相同的玻璃，来自九州东南部宫崎平原的大淀河谷。这两个 7.3 ka 海啸沉积物与日本西南部广泛分布的其他有记载的例子一起，包括东九州的丰后海峡和别府湾、西九州的橘湾、纪伊半岛的 Zasa Pond 以及火山口周围。火山口附近的海啸沉积物被分为较老和较年轻的 7.3 ka 海啸沉积物，较年轻的与广泛分布的沉积物相匹配。我们归于年轻的 7。