Impact of Repeating Massive Earthquakes on Intertidal Mollusk Community in Japan

Abstract

History repeats itself. In these 3,500 years, massive earthquake like the Pacific coast of Tohoku earthquake in 2011 has occurred at least seven times. Major earthquakes with big tsunami have occurred in 500-year cycles along the Pacific coast of northern part of Japan. The tsunami caused a lot of damage to the Tohoku region including marine life. During the earthquake, wavelike movement observed on the surface of the tidal flats. Liquefaction occurred just after the earthquake before tsunami. Bivalves distributed inside the sediment were dug out to surface with a jet of water. Not only the tsunami but also the liquefaction brought quick damage in the coastal marine life. The earthquake caused sudden land subsidence of about 1 m in maximum where intertidal zone became subtidal zone. It may take at least several 10 years to recover the coseismic land subsidence. We have to clarify not only short-term effect which may explain an initial recovery of intertidal and subtidal animals but also a long-term effect which may explain continuous changes in mollusk population with land subsidence. Construction of tsunami seawalls has begun without environmental assessment. Huge seawalls break the continuity of nature to land from the sea. We have to clarify the impact on the coastal ecosystem of artificial structures.