The Effects of Liquefaction, Tsunami, and Land Subsidence on Intertidal Mollusks Following the Great East Japan Earthquake

The Great East Japan Earthquake that occurred on 11 March 2011 caused a great deal of damage to the organisms that inhabit the coast of the Tohoku district of Japan. In this paper, I mainly focus on the earthquake’s impact on bivalves that inhabit the sandy intertidal zone, with specific emphasis on the short-term effects from liquefaction and the tsunamis and the long-term effects of land subsidence. As previous studies have not discussed the details of the separate physical events of this type of natural disaster, this study divided it into three main events: the initial earthquake including liquefaction, subsequent tsunamis, and land subsidence. During the liquefaction period, bivalves were brought to the surface with jets of water and were then swept away by the tsunamis. As such, both the magnitude of the tsunamis and length of the liquefaction period were factors critical to the survival of clams. The tsunamis have also contributed to dispersion of benthic organisms. For example, the redistribution of oysters across the mud bottom increased their range. In addition to these events, land subsidence resulted in marked changes to the vertical position of the intertidal zone. These large changes in the physical environment cannot recover in the short term, and the readjustment of tidal communities is expected to take several decades. However, land subsidence has also formed new intertidal zones, which will ultimately promote the colonization of various organisms. At some sites where the silt content of the bottom sediments increased due to land subsidence, the dominant species have changed from those that prefer a sandy bottom to those that prefer a muddy bottom. These disturbances to the ecosystem have destabilized these bivalve species resulting in fluctuations in abundance and community structure for at least 3 years following the earthquake. Finally, continual monitoring studies are needed to understand the ecological impacts resulting from the earthquake, and the artificial seawall and tidal flat that have been constructed since the quake, on benthic organisms inhabiting the coastline of the Tohoku district.