Shore platform lowering on a Pacific coast of Japan: Present, past, and future — Applications of a weathering-controlled erosion model and a long-term platform surface evolution model

Abstract

In order to investigate the lowering process of a shore platform in a tectonically active region along the Pacific microtidal coast of Japan, MEM measurements were carried out seven times during 5.8 years (2004–2010). Four measuring sites were installed on a nearly horizontal, smooth platform in the upper intertidal zone, two sites on the platform surface composed of lapilli tuff and the remaining two on that of sandstone. A time- and space-averaged lowering rate of the lapilli-tuff surface is 0.46 mm/yr and 0.35 mm/yr on the sandstone surface. Evaluations for the assailing force of waves on the platform surface, FA, and the resisting force of unweathered bedrock, FR, indicate that FA ≪ FR, which leads to a conclusion that weathering-induced rock-strength reduction is necessary for the occurrence of platform erosion. A weathering-controlled erosion model is constructed with the aid of results from laboratory experiments on rock disintegration by salt weathering. The field application of the model enables us to determine unknown coefficients included in the model. By use of the coefficient values weathering properties of lapilli tuff and sandstone are explored: lapilli tuff is more susceptible to weathering than sandstone, and weathering rates of both rock types are found to be highly time-dependent. A second model is built to examine the long-term evolution of the platform surface. The model calculations are based on the assumption that downward erosion will cease at the Mean Low Water Spring level (1.0 m below the present MSL) and that both lapilli tuff and sandstone surfaces started to erode immediately after a coseismic uplift event of 1.6 m, estimated to have occurred at 880 AD on the basis of 14C dating of fossil assemblages. It is modeled that the platform will continue to lower toward a steady state in approximate 13,000 years after the present time. Because long-term platform downwearing shows a strong nonlinearity, it is suggested that a linear extrapolation of decadal time scale erosion data to millennial and further to Holocene time scales will lead to inaccurate results. The relationship between platform lowering rates and elevations, derived from the model, indicates that the maximum rates occur in the upper to middle intertidal zone; the result is almost consistent with the previous elevational pattern reported from Canada and Australia.