Abstract
Stratigraphic and paleontological investigations in Mugi Town, on the Pacific coast of Shikoku Island, revealed evidence of as many as five tsunami inundations from events along the Nankai Trough between 5581 and 3640 cal yr BP. Nine event deposits (E1–E9) were identified in cores ranging in length from 2 to 6 m, consisting of sandy and gravelly layers interbedded with organic-rich mud. Sedimentary structures in the event deposits observed by computed tomography included normal grading and sharp lower stratigraphic contacts. Event deposits E3, E6, E7, and E8 contained mainly brackish-marine diatom species, suggesting that they had been deposited during inundation by seawater. In addition, fossil diatom assemblages were markedly different above and below event deposits E3, E4, E6, and E8. For example, assemblages below event deposit E6 were dominated by a freshwater species (Ulnaria acus), whereas assemblages above it were predominantly brackish-marine (Diploneis smithii, Fallacia forcipata, and Fallacia tenera). We attributed these changes to the increase of marine influence due to coastal subsidence associated with subduction-zone earthquakes, as documented in the 1946 Showa-Nankai earthquakes. We conclude that event deposits E3, E6, and E8 and perhaps E4 and E7 were deposited by tsunamis generated by subduction zone earthquakes along the Nankai Trough. The ages of these event deposits, as constrained by ten radiocarbon ages, suggest that some of the tsunamis that impacted Mugi Town were correlated with those reported elsewhere along the Nankai Trough, thereby complementing the existing but still incomplete geological record for these events.
Highlights
The Pacific coast of Japan has been repeatedly affected by subduction zone earthquakes and tsunamis
This study investigated the geological record of prehistoric tsunamis from Holocene coastal sediments in Mugi Town
Tsunami deposits associated with earthquakes along the Nankai Trough were identified from stratigraphic and microfossil evidence, and their depositional ages were estimated using radiocarbon ages from plant macrofossils based on Bayesian statistics
Summary
The Pacific coast of Japan has been repeatedly affected by subduction zone earthquakes and tsunamis. The long-term forecasts for the Nankai Trough region in western Japan relied mainly on historical documents from the last 1300 years (The Headquarters for Earthquake Research Promotion 2001), there. This study was carried out in a coastal lowland in Mugi Town, adjacent to the Nankai Trough, with the aim of identifying prehistoric tsunami deposits and estimating event ages. The post-disaster survey report of the Showa-Nankai earthquake (Central Meteorological Observatory 1947) indicated that the tsunami inundation area included the whole study site. The limiting ages were used to constrain the depositional ages of the events based on Bayesian statistics (Table 3) This calculation was performed in OxCal with the procedure proposed by Lienkaemper and Bronk Ramsey (2009)
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