Late Holocene sea level changes and tectonic movements inferred from fossil diatom assemblages in Tainohama, Tokushima prefecture, Japan

Abstract:

The average recurrence interval of the interplate earthquakes along the Nankai Trough is estimated from historical literature and archaeological data. However, the details of tectonic movements by past Nankai earthquakes are mostly left unclear from historical literature, therefore, we need to obtain the geological evidence of the tectonic movements. Yuki city, Tokushima prefecture, located in north part of the Nankai Trough, has been subsided and many tsunamis attacked along the coast of the Shikoku islands accompanied by the previous Nankai earthquakes. Therefore, some historical documents and memorial monuments written about the past Nankai earthquakes and tsunamis remain in the city. The study purposed to reveal tectonic movements of the earthquakes from Nankai Trough during the late Holocene at Tainohama in Minami city which is adjacent to the southwest of Yuki city by fossil diatom analysis. We obtained a 700cm long core at a marsh behind a barrier spit probably be not affected directly from sea waves in Tainohama. The core includes more than 13 sand layers in organic-rich muddy or peaty sedimentary succession up to 500cm depth in the core. And the diatom assemblages included in the peat and peaty mud deposits were dominated by fresh and brackish water species, especially Pseudostaurosira brevistriata, P. subsalina and Tabellaria fenestrate. In contrast to the above mentioned sand layers, brackish and marine species, such as Diploneis smithii increased. The diatom assemblages from the organic rich muddy sediments and radiocarbon ages indicate that freshwater marsh or saltmarsh formed in this region during the late Holocene. On the other hand, the sandy layers include the diatoms living in environments where salinities are higher than freshwater or salt marsh, so the assemblages suggest that the sand layers were transported from seaside by past tsunamis. In addition, changes of diatom assemblages in the peaty or peaty mud sediments show increase or decrease of freshwater species, suggesting paleo-sea level changes. The sea level in this area declined gradually to modern sea level from +50cm higher level than modern by eustatic sea level fall during the late Holocene (Sato 2014), thus the fluctuations suggest co-seismic or inter-seismic crustal movements of the past interplate earthquakes along the Nankai Trough.