Ground-water level changes to multiple distant earthquakes at earthquake-groundwater monitoring stations in Jeju Island, Korea

Hydrological responses caused by earthquakes have been documented for thousands of years and recently many studies have been conducted for investigating its underlying mechanisms. Such responses include level changes, temperature changes, chemical composition variation, and liquefaction. Among these phenomena, this study focuses on ground-water level changes. There are two kinds of ground-water level changes: one is co-seismic change that occurs simultaneously with earthquake, and the other is gradual change that occurs gradually after earthquake.　A temporary seismograph network which is consisting of 20 broadband seismographs has been operated in Jeju Island since October 2013, and all these seismographs are colocated with some of ground-water monitoring stations distributed in the island. Continuous simultaneous observations of such two different physical quantities provide a good chance for direct comparison between them. Sampling rate of seismographs was set to 200Hz and ground-water monitoring time interval was set to 1 minute. Raw data from the ground-water monitoring stations were corrected for the effects of atmospheric pressure and earth tide to get only ground-water level changes.　On May 30, 2015, an earthquake of M 7.8 occurred at Chichi-shima, Japan. In response to the earthquake, ground-water level changes were observed at both the earthquake and groundwater monitoring stations in Jeju Island. Ground-water level began to change after three minutes of the earthquake origin time. The largest change with the maximum amplitude of 8.1 cm was observed after six minutes after the origin time. Seismograms at the same stations were compared with the ground-water level data. The results will be presented and the physical relationship between earthquake ground motions and the corresponding hydrogeological response will be discussed.