Chapter 5 - Sedimentary Characteristics and Depositional Processes of Onshore Tsunami Deposits: An Example of Sedimentation Associated with the 12 July 1993 Hokkaido—Nansei-oki Earthquake Tsunami

Abstract

Sedimentary descriptions, especially those of sedimentary structures and facies, are limited, with the exception of grain-size distributions. Four years after the 12 July 1993 Hokkaido–Nansei-oki earthquake tsunami, the sedimentary characteristics and facies of the onshore tsunami deposits were examined by the peel and oriented-lunch box sampling methods at Taisei, in an estuary area of the Usubetsu River. In this chapter, several characteristic sedimentary structures and facies were found, including gravel fabric, current ripples and dunes. These structures show that the sediments were supplied primarily in the form of a traction current. The 1993 onshore tsunami deposits can be divided into two sedimentary facies: a gravel facies and a sheet-sand facies. The gravel facies was produced by a powerful turbulent current similar to a high-density turbidity current. Four sedimentary units were identified in the sedimentary facies of the 1993 onshore tsunami deposits and are interpreted as having been deposited by the inflow (run-up) and the outflow (backwash) of two tsunami currents that entered the Usubetsu River.