Hydrologic responses to local and distant earthquakes in Korea

Water level, water temperature and electric conductivity (EC) has been monitored by 6-hr interval at the National Groundwater Monitoring System (NGMS) of Korea that consists of a total of 320 stations around the country. This study covers the difference in responses of groundwater to local and distant earthquake which occurred seven days apart in January, 2007. The local earthquake occurred at Jan 20, 2007 in the mid-eastern part of Korea (37.68°N, 128.59°E) with the magnitude of 4.8. Eight out of 320 monitoring wells tapped at deeper aquifer responded. Water-level changes ranged from -76 cm to 16 cm (average in modulus: 29.88 cm). One well showed decline in EC by 112 μS/cm, too. In the wells of lower transmissivity values, changed water-levels were recovered its former level in up to 9 days. An earthquake of M8.1 ‘us2007xmae (USGS, 2007)’ occurred on Jan. 13, 2007 at the east of the Kuril Islands (46.272°N, 154.455°E), located approximately 2,500 km northeast from the Korean Peninsula. A total of 10 out of 320 wells appeared to show the responses corresponding to the earthquake. Nine wells showed water-level changes ranging from -42 cm to 4 cm with average of 12 cm, but only one well, the Hanam-Hasangok station, recovered its former level after abrupt change. Three out of the nine wells also showed changes in EC up to 400 μS/cm (average: 240 μS/cm). One well located in coastal area showed abrupt oscillatory-change of 1°C in temperature for two days without any changes in water level and EC. Although the wells are located in nearby areas no well that showed hydrologic responses to both of two earthquakes. Yeoju-Geumsa monitoring station which has the lowest transmissivity value recovered its normal value, while no recovery in the others. Transmissivity of the aquifer that the monitoring wells tapped into shows not clear relationship to the amount of water-level changes. However, wells with low transmissivity (<3m2/day) only showed recovery, contradicting to our assumption that groundwater in aquifers of higher T value would have relatively easier and faster recovery. Successive observation on groundwater changes corresponding to earthquake events and more analyses are warranted.