Tsunami deposits formed and preserved in accordance with the local environment

Abstract:

To identify and utilize paleo-tsunami deposits, it’s important to understand the diversity in characteristics of the deposits due to the deposit materials and environmental settings. We excavated a L-shape trench (10m land-sea direction and 5m along the shore) in a wetland located ca. 500m inland from the beach. Along the eastern coast of Hokkaido, Japan, including our trench site Urahoro, it has been revealed that unusually large earthquakes occurred about every 400 years on average over several thousands years (eg. Nanayama et al., 2003). Along the trench walls (total length of 28.7m), we could observe distribution and sedimentary features of sandy or muddy tsunami (or tsunami like) layers and five widespread tephras in ca. 2m-thick peat developed in 4000-5000 years. Characteristics of these layers are, although they are deposited at the same site, not resemble each other. The youngest two layers lying between the Ta-b tephra (AD 1667) and B-Tm tephra (10^th Century) are relatively thick and coarse, and their thickness varied 34-7 cm for the first layer and 16-0 cm for the second. For both layers, upper boundaries are relatively flat but the lower ones are rising periodically with intervals of 50cm to 1m. Since these intervals are similar to the present-day spacing of tussocks on the wetland, the both tsunami layers might be formed on a tussock-dominated wetland. The deposit thickness of the first sand layer was probably thicker than the tussock heights at that time. Grain size patterns are also affected by the original surface undulation and change between seaside and landside of a buried tussock. On the other hand, the older tsunami deposits seem to be deposited on flat plains. Thickness of the layers ranges 2 to 10cm and they show sheet-like distribution and stable grain size characteristics along the trench. One layer accompanies a thick (10cm) mud cap, suggesting existence of a pond or lagoon at seaside of the trench at that time.