Groundwater level changes on Jeju Island associated with the Kumamoto and Gyeongju earthquakes

Abstract. The largest earthquake since the beginning of instrumental earthquake monitoring (magnitude 5.4) in Korean peninsula occurred in Gyeongju City area, South Korea, at 20:32:54 on September 12, 2016 (local time). Before the Gyeongju earthquake, an earthquake of magnitude 7.0 occurred in Kumamoto prefecture, Kyushu, Japan, at 01:25:06 on April 16, 2016 (local time). This study examined groundwater level changes of the monitoring wells on Jeju Island in relation to the Gyeongju and Kumamoto earthquakes. Groundwater level changes due to the Kumamoto and Gyeongju earthquakes exhibited spikes or oscillations, with the initial water level change occurring 2–3 min after earthquake generation, displaying different behaviors depending on the magnitude of the earthquakes and different sensitivities depending on the aquifer and geological characteristics. On Jeju Island, the groundwater level change caused by the Gyeongju earthquake (M 5.4) was larger than that caused by the Kumamoto earthquake (M 5.4). This was because a smaller energy attenuation occurred during the Gyeongju earthquake along the Yangsan fault on the Korean peninsula extending in the NNE-SSW direction, while a larger energy attenuation occurred during the Kumamoto earthquake along the median tectonic line (MTL) fault on the Japanese island arc extending in the ENE-WSW direction.