Abstract

Changes in groundwater levels on Jeju Island, which is ∼1500  km from the epicenter of the 2011 Mw 9.0 Off the Pacific Coast of Tohoku earthquake in Japan, were analyzed. The results show a series of water level fluctuations related to the foreshock (Mw 7.3 and Mw 6.1) and aftershock (Mw 7.9) in addition to the mainshock (Mw 9.0). The groundwater‐level changes in response to the earthquake were oscillatory, and the groundwater levels at some wells had an irregular pattern after the Mw  9.0 earthquake, although they recovered in five to seven days. This phenomenon may reflect the unstable elastic aquifer properties that are present for a period of time after a large earthquake. In addition, the successive groundwater‐level change was different for each magnitude and well location. The magnitude increases the groundwater‐level change, but the response amplitude is also dependent on the hydrogeological characteristics at the well. On Jeju Island, the groundwater‐level changes due to the earthquake generally increased where there was more volcanic hard rock and more permeable layers, but these changes were inversely correlated with the presence of sedimentary deposits with less permeability and less restrictive characteristics.

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