Characterization of groundwater response and hydrogeological properties surrounding the Pohang earthquake, South Korea
摘要
The hydrogeological properties in and around two geothermal wells at depths of approximately 4300 m in the Pohang Enhanced Geothermal System (EGS), South Korea, an area of public concern due to the 15 November 2017 Pohang earthquake, were examined. The subsurface at the study site is geologically composed of Permian granodiorite; Cretaceous andesite, rhyolite, sandstone, and tuff; and Tertiary tuff and mudstone. Hydraulic conductivities of the subsurface to a depth of 250 m ranged from 1.46 × 10−6 to 4.65 × 10−1 cm/s and exhibited an exponentially decreasing trend with depth. The hydraulic conductivities of the two EGS wells, screened several hundred meters from the well bottoms, ranged from 4.54 × 10−12 to 1.50 × 10−10 cm/s, which are lower than the predicted values based on exponential regression. This suggests that the five hydraulic stimulations conducted for the EGS were not effective. Some surrounding wells exhibited water level declines in response to the earthquake, particularly those screened to relatively deep bedrock aquifers. Over the long-term (2016–2018), most surrounding wells showed generally declining groundwater levels. This overall decline was mainly attributed to decreasing rainfall, while the influence of groundwater use was relatively minor during this period. The findings contribute to a better understanding of the origin and mechanism of the Pohang earthquake.