[H-QR03] Quaternary, Diachronic dynamics of human-environment interactions

[HQR03-P09] Chronology and sedimentary architectures of submerged beach ridges in Kiritappu Marsh, eastern Hokkaido

Yuki Negoro^1,2, *Toru Tamura^1,2, Koichiro Tanigawa¹, Yumi Shimada¹, Rina Okada^1,3, Yuki Sawai¹ (1.Geological Survey of Japan, AIST, 2.Graduate School of Frontier Sciences, The University of Tokyo, 3.Graduate School of Science and Technology, Hirosaki University)

Keywords:beach ridge, coastal uplift/subsidence, eastern Hokkaido, Kuril Trench, luminescence dating

Eastern Hokkaido is characterized by tectonic vertical displacements that are variable with respect to the time scale. The Pleistocene marine terraces indicate long-term uplifts while tide gauges have recorded rapid subsidence up to 1 cm/year over the last decades. These trends are supposed to be results of a combination of inter-seismic subsidence and uplift after great earthquakes that happened along the Kuril Trench every hundreds of years but are yet to be understood quantitatively. This study investigated chronology and sedimentary architectures of submerged beach ridges in Kiritappu Marsh for examining their formative processes related to tectonic uplift and subsidence. A shore-normal transect from the shoreline to 900 m inland in the northern part of the marsh was surveyed with handy geoslicer excavation at 10 sites and ground-penetrating radar (GPR). Sediment samples were analyzed with feldspar post-IR IRSL dating of beach-ridge sand, radiocarbon dating of peat, and correlation of tephra in peat. The surface of the marsh is characterized by peat accumulation, below which beach ridges are submerged. In many locations the beach-ridge sand is divided into the upper and lower layers by a thin peat layer, indicating initial regression was followed by transgression. At the most landward site, the lower and upper layers of the beach-ridge sand were dated as 2700 years and 1600 years, respectively. Ages of beach-ridge sand are younger seaward and consistent with the radiocarbon ages and tephrochronology. The lower part of the beach-ridge sand is characterized by GPR reflection that are inclined similarly to the present beach face, revealing the shoreline progradation. According to these features, the formation of the beach ridge is accounted for by the following steps 1 to 3. 1) The relative sea level falls owing to post-seismic uplift. 2) The shoreline is then prograded to abandon a beach ridge inland, above which peat is accumulated. 3) Following inter-seismic subsidence causes a relative sea-level rise that allows for another beach-sand deposition on peat accumulation. The submergence of the oldest beach ridges below the present sea level means the net subsidence over the last 2700 years while the beach ridge abandoned after the 13th century earthquake is as high as the present foredune and thus suggests a balance between subsidence and uplift associated with two great earthquakes in the last 800 years. The temporal variations in uplift–subsidence trends revealed here by the beach ridges are not only critical for the quantitative understanding of the regional tectonic displacement but also a clue for clarifying earthquake variability along the Kuril Trench.