The east coast of Japan is prone to tsunamigenic megathrust earthquakes, as tragically demonstrated in 2011 by the Tōhoku earthquake (Mw 9.0) and tsunami. The Nankai Trough subduction zone, to the southwest of the area affected by the Tōhoku disaster and facing the densely populated and heavily industrialized southern coastline of central and west Japan, is expected to generate another megathrust earthquake and tsunami in the near future. This subduction zone is, however, segmented and appears to be characterized by a variable rupture mode, involving single- as well as multi-segment ruptures, which has immediate implications for their tsunamigenic potential, and also renders the collection of sufficiently long time records of past earthquakes and tsunami in this region fundamental for an adequate hazard and risk assessment. Over the past three decades, Japanese researchers have acquired a large amount of geological evidence of past earthquakes and tsunami, in many cases extending back in time for several thousands of years. This evidence includes uplifted marine terraces, turbidites, liquefaction features, subsided marshes and tsunami deposits in coastal lakes and lowlands. Despite these efforts, current understanding of the behaviour of the subduction zone still remains limited, due to site-specific evidence creation and preservation thresholds and issues over alternative hypotheses for proposed palaeoseismic evidence and insufficiently precise chronological control. Within the QuakeRecNankai project we are generating a long and coherent time series of megathrust earthquake and tsunami recurrences along the Nankai Trough subduction zone by integrating all existing evidence with new geological records of paleo-tsunami in the Lake Hamana region and of paleo-earthquakes from selected lakes in the Mount Fuji area. We combine extensive fieldwork in coastal plain areas and lakes, with advanced sedimentological and geochemical analyses and innovative dating techniques.
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Among obligate groundwater organisms (stygobiontes), the Stenasellidae is a diverse family of isopods (Crustacea) that is mainly found in tropical Africa. However, our knowledge of the diversity and endemism of stygobiotic crustaceans found in Africa remains limited due to a lack of studies and a deficit in taxonomic expertise. Within the Stenasellidae, the genus Metastenasellus Magniez, 1966 is currently composed of nine species with a trans-Saharan distribution, but the recent descriptions of two new species in Cameroon suggest that its diversity is largely underestimated. Here, we further explore the diversity of Metastenasellus by sequencing the DNA barcode fragment of 57 Beninese and 27 Cameroonian specimens from 24 and 29 localities respectively. A data set of 84 new and 8 GenBank sequences was analysed with four species delimitation methods (ABGD, PTP, GMYC and KoT). All methods agreed on a minimum of 23 putative species, 11 from Benin and 12 from Cameroon. Ongoing morphological scrutiny of the male pleiopods of these putative species confirm that they correspond to distinct species. The coexistence of two putative species in some localities is further evidence that they represent separate evolutionary lineages. The geographical distribution of the 23 putative species suggests a restricted distribution and a low dispersal capacity of Metastenasellus species, as already observed for macrostygobionts in Europe. Given the number of potential species currently observed per sampled station, it is to be expected that many new species could be discovered by increasing the sampling effort. There also appears to be a decoupling between watersheds and species distribution, suggesting that watersheds are a rough approximation of water table boundaries or that the current distribution of Metastenasellus is more the result of historical factors than of dispersal constraints related to the current configuration of the catchment areas.
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RBINS Staff Publications 2022
