Deposition of sediments of diverse sizes by the 2011 Tohoku-oki tsunami at Miyako City, Japan

• Tsunami deposits of diverse grain size ranging from mud to boulder were observed.
• Gravelly layered tsunami deposits generally thinned and fined inland.
• Many boulders were found on the lowland, mainly in three clusters.
• Major sedimentation occurred due to the decrease of the flow velocity.
• The boulders were deposited on top of the layered deposit, not buried within it.

This paper reports the transport and deposition of sediments of various sizes, ranging from mud to boulders, by the 2011 Tohoku-oki tsunami at Miyako City, Japan, where the tsunami reached 28.1 m above sea level at the coast and inundated up to 1750 m inland. Mud- to cobble-sized sediments formed layered deposits that covered the lowland where paddy fields had been. These layered deposits were remarkably thick (> 0.5–1.0 m) near the beach, where they were composed mainly of gravels, and they generally thinned and fined inland to sand and mud deposits a few centimeters thick. Separate from the layered deposits, many boulders, consisting of fragments of the tsunami barrier and other structures, wave-dissipating blocks, and volcanic rocks, were found on the lowland, mainly in three clusters. Boulder size decreased abruptly about 750 m inland from the shoreline, where the thickness and grain size of the layered deposits also showed a sharp decrease. These findings suggest that a sudden deceleration of the wave flow velocity, constrained by the local topography, caused rapid deposition of both the layered deposits and boulders there. Interestingly, the boulders were deposited on top of the layered deposit, not buried within it. This finding suggests that sand and gravel were transported in the bore front of the first wave and deposited before the arrival of the boulders. Seaward-extending scours around large boulders showed that the boulders were not moved by the backwash flow, although the backwash eroded the layered deposits to some extent. The stratigraphic relationship between the layered deposits and boulders therefore suggests that both were probably deposited by the first wave.