An effective method, involving time and frequency domains was developed to interpret seismic precursors by comparing groundwater-level fluctuations recorded immediately and long before the occurrence of a known earthquake. The proposed method, consisting of the pre-processing (3-point filtering, band-pass filtering, and spectrum analysis) and post-processing (weighted moving average method and histogram and spectrum analyses) stages, was applied to the groundwater-level time series measured at three monitoring wells on Jeju Island, South Korea, from 00:00 on 8 September 2016 to 00:00 on 22 September 2016. The Gyeongju earthquake (Mw 5.4) occurred at 20:32 on 12 September2016. The histogram analysis exhibited an accentuating bellshape as the total number of waveforms, including those caused by the earthquake, of the groundwater-level fluctuations increased. The weighted moving average analysis indicated that various abnormal waveforms with different periods occurred in the fluctuations approaching the occurrence of the earthquake. The periods of seismic precursors in the groundwater-level fluctuations were determined by spectrum analysis and varied among the monitoring wells. Seismic precursor responses attributable to the Gyeongju earthquake were identified at least 8 hours before the earthquake, and the method used in this study indicates its good potential to predict an impending earthquake.

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In order to find out the correlation between depletion of groundwater and tunnel excavation, a series of investigation has been executed including geological and geophysical investigation. Instead of direct measurements of inflow rates of groundwater into the tunnel, water discharge rates were analysed. Electrical resistivity tests were adopted in geophysical survey. Fracture zones like shear bands and faults that may serve as conduits of groundwater flow were detected through geological and geophysical surveys. Comparing resistivity profile with water discharge rates, it was known that a great extent of groundwater must have flowed into the tunnel through these fracture zones.

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Transient/time-domain electromagnetic (TEM) soundings were carried out on reclaimed land, since the TEM sounding has good resolution of a conductive and thin overburden. The reclamation material is marine clay dredged from the seafloor, which is currently undergoing consolidation in the upper part of the dredged layer. The conductivity of the marine-origin clay is generally more than 0.3 S/m. The aim of the TEM sounding was to determine the depths of weathered and soft rocks overlain by thick and conductive overburden. The TEM responses were measured at delay times of 0.050-20.575 ms with a 30 × 30 m coincident loop array. Data from the TEM inversion and core logging indicate that the resistivities of the conductive overburden are less than 2 Ω-m, while those of the weathered and soft rocks are 10-20 and 70 Ω-m, respectively. The depth to weathered rock is 26-58 m and the depth to soft rock is 46-75 m.