Geomorphological Study of the Hamada Coast, Southwestern Japan, with Special Reference to the 1872 Hamada Earthquake

Abstract

The 1872 Hamada earthquake (M=7.1±0.2) is one of the major historical earthquakes which accompanied a coseismic uplift and subsidence in the coastal area. This paper intends to examine an implication of the Hamada earthquake for geomorphological development of the Hamada area, based on the investigation of marine terraces, emerged sea level indicators such as sea caves, benches, fossil beds and beach deposits, and fault topography.
Two steps of Pleistocene marine terraces are found in the coastal area. Terrace I, c. 40-60 m high, is underlain by weathered beach gravel covered by dune sand which is interbedded by at least four horizons of paleosol. It is considered, that a major interglacial period resulting in strong weathering of Terrace I deposits occurred after the formation of this terrace. Thus, Terrace I can be correlated to the penultimate interglacial, and Terrace II, c. 15-25 m high, underlain by rather fresh gravel bed, to the last interglacial. Accepting this correlation, the uplift rate of the study area must be small, c. 0.1-0.2 m/ka, and is no significant difference in the uplift rate over the study area. Height of emerged sea level indicators associated with the Hamada earthquake is 0.9-1.7 m above the present mean sea level. At least one sea level indicator higher than the emerged sea level at the time of the 1872 Hamada earthquake was found at several locations through the study area including the coast where coseismic subsidence occurred in 1872. Northeast-southwest trending lineaments predominate in the study area and a fault exposure is observed on one of the lineaments suggesting that they are fault origin. Areas of coseismic uplift and subsidence can be seens by turns along these faults.
Characteristics of the 1872 Hamada earthquake are summarised as follows, based on the above mentioned geomorphological observation : 1) Coseismic deformation as associated with the Hamada earthquake has not accumulated through the late Quaternary, and this earthquake must be a very rare event with coastal deformation during the Holocene. 2) Distribution pattern of coseismic deformation, that is, the occurrence of small blocks with coseismic uplift and subsidence by turns can be interpreted as an effect of strike-slip movement on the faults.