Recurrence of intraplate earthquakes inferred from tsunami deposits during the past 7300 years in Beppu Bay, southwest Japan
Recurrence of intraplate earthquakes inferred from tsunami deposits during the past 7300 years in Beppu Bay, southwest Japan
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DOI：
10.1016/j.quascirev.2021.106901
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发表时间：
2021-04-10
影响因子：
4
通讯作者：
Takeda，Daisuke
中科院分区：
文献类型：
--
作者：
Yamada，Masaki;Fujino，Shigehiro;Takeda，Daisuke
我们研究了从日本九州岛北部别浦湾南岸海岸沼泽中获取的陆上沉积物岩心，以确定该海湾史前板内地震是否引发了海啸，并建立了这些地震的年表和复发间隔。后者是从对海上浅层断层的调查中推断出来的，但断层运动的时间具有很大的不确定性。在长达8.8米的沉积层序中识别出5个砂层，主要由陆相富有机质泥浆和海相贫有机泥浆组成。大多数砂层厚度为1-20厘米，与周围泥浆表现出尖锐的上下接触，这意味着它们是由突发事件沉积的。砂层地球化学特征明显，以硅、钾、钙、钛、锰、铁、锶等元素峰为主，硅藻组合以微咸水-海相和海洋生物为主。这表明沙子来自海底和/或海岸，而不是来自淡水沼泽。根据放射性碳年龄模拟，这些砂层的沉积年龄分别为：3270-3450年龄段、4250-4510年龄段、4970-5280年龄段、5750-6750年龄段和6430-7070年龄段。据推测，最古老的沉积物记录了与日本南部Kikai火山7.3ka火山口形成的喷发有关的海啸。因此，我们将其排除在海湾海啸板内地震复发间隔的计算之外。重现期约为460~1850年，包括1596年公元1596年的Keicho Bungo地震和1700~2200年前的断层破裂，在浅海走滑断裂和正断层破裂的研究中已有报道。在非海相富含有机质的泥质上，上部泥质砂层(2750-2870年代)硅藻化石组合以微咸水和海洋生物为主，表明泥质砂体沉积于内湾或潮坪环境。2750-2870年代以来从陆相沉积到海相沉积的变化不能用区域海平面的变化来解释，因为后者在过去的6000年里已经下降了。因此，这种环境变化最可能的解释是沼泽局部的同震下沉，可能归因于位于沼泽南部的活动断层的破裂。这项研究首次根据对陆上海啸沉积的研究报告了反复发生的板内海啸地震，并表明调查海啸沉积有助于评估其他地方板内地震的海啸风险。
We studied onshore sediment cores obtained from a coastal marsh on the south coast of Beppu Bay, northern Kyushu Island, Japan, to ascertain whether prehistoric intraplate earthquakes in the bay had generated tsunamis, and to establish a chronology and recurrence intervals for these earthquakes. The latter had been inferred from investigations on shallow offshore faults, but the timing of the fault movements has large uncertainties. We identified five sand layers in the up to 8.8 m long sedimentary sequence, which is mainly composed of non-marine organic-rich mud and marine organic-poor mud. Most sand layers, 1–20 cm thick, exhibit sharp upper and lower contacts with the surrounding muds, implying that they were deposited by sudden events. The sand layers are geochemically distinct, with peaks in Si, K, Ca, Ti, Mn, Fe, and Sr, and diatom assemblages dominated by brackish–marine and marine species. This suggests that the sand came from the sea bottom and/or shore rather than from the freshwater marsh. Based on radiocarbon age modeling, the depositional ages of these sand layers are: 3270–3450 cal yr BP, 4250–4510 cal yr BP, 4970–5280 cal yr BP, 5750–6750 cal yr BP, and 6430–7070 cal yr BP. The oldest deposit is inferred to record a tsunami associated with the 7.3 ka caldera-forming eruption of the Kikai volcano, southern Japan. Thus, we exclude it from the calculation of the recurrence interval of tsunamigenic intraplate earthquakes in the bay. Recurrence is approximately 460–1850 years, including the 1596 CE Keicho Bungo earthquake and a fault rupture 1700–2200 years ago, which have been reported in studies on ruptures of shallow offshore strike-slip and normal faults. Brackish–marine and marine species dominate the fossil diatom assemblages in the upper muddy sand layer (younger than 2750–2870 cal yr BP) above the non-marine organic-rich mud, implying that the muddy sand was deposited in an inner bay or tidal flat environment. The facies change from non-marine to marine sediments after 2750–2870 cal yr BP cannot be explained by a change in the regional sea level because the latter has fallen in the last 6000 years. Therefore, the most likely interpretation for this environmental change is a local coseismic subsidence of the marsh, probably attributed to a rupture of the active fault located south of the marsh. This study is the first to report on recurrent tsunamigenic intraplate earthquakes based on research of onshore tsunami deposits, and suggests that investigating tsunami deposits can contribute to an assessment of tsunami risks for intraplate earthquakes elsewhere.