abstract
Article history:
Received 5 February 2015
Received in revised form 7 August 2015
Accepted 8 August 2015
Available online 10 August 2015
Keywords:
2011 Tohoku-oki tsunami
Erosion
Redeposition
Coastal lake
Sendai Plain
Erosion of sediment beneath a coastal lake by the 2011 Tohoku-oki tsunami largely removed up to 1100 years of
the geologic record of tsunami and tephra deposition. That lost geologic record included the 1611 Keicho tsunami
deposit and the 915 To-a tephra, amounting to ca. 1.4 m of lake-bottom sediment, as judged from comparison of
stratigraphy at four locations examined before and after the 2011 tsunami. The tsunami replaced the eroded sediment
with a 7–15-cm-thick sand sheet and an overlying 20–60-cm-thick layer of black mud. Both 2011 deposit
components were likely derived from sediment eroded along the tsunami flow path, including granite rock fragments
downstream of the lake basin and mainly reworked lake-floor sediment. Erosion and redeposition of lake
floor sediment by tsunami inundation can introduce uncertainty to sandy tsunami deposit age estimation. The
2011 tsunami sand might be dated a few hundred years older than the actual age if one overlooks the reworked
muddy tsunami deposit. In its facies, the mud closely resembles mud deposited from the usual environment but
is mixed with sediments of various ages. Therefore, it is important to identify the reworked muddy tsunami deposit
based on geochemistry and micropaleontology, to avoid misinterpretation of the depositional age of tsunami
deposits. Erosion of deposits beneath the lake floor also erases evidence, leaving an incomplete paleo-tsunami
history. Reconstructing geological records of tsunamis in coastal lakes requires careful evaluation of the possible
effects of tsunami erosion. Identifying sediment sources, mapping stratigraphic unconformities, and constructing
detailed age-depth models to test for gaps in sedimentation can produce important insights.
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