CHARACTERISTICS OF UNCONFINED AND CONFINED GROUNDWATER LEVEL FLUCTUATION IN TOKYO BY THE 2011 OFF THE PACIFIC COAST OF TOHOKU EARTHQUAKE

Abstract

Groundwater observation wells have been bored in 42 sites in Tokyo Metropolis by the Civil Engineering Support and Training Center of Tokyo Metropolitan Government. Large fluctuations of unconfined and confined groundwater levels in Tokyo were observed following the 2011 off the Pacific coast of Tohoku earthquake. In this study, the characteristics of unconfined and confined groundwater level fluctuation caused by the earthquake were investigated. As for the results, three and seven fluctuation patterns were identified for the unconfined and confined groundwater levels, respectively. The increase after short drawdown was a major pattern for the confined groundwater levels, whereas no change in pattern was observed for about 70% of the unconfined groundwater levels. The causes of those patterns were additionally outlined in this study.