Detecting seismic precursors in groundwater fluctuations using masking empirical mode decomposition and modified zero-crossing techniques

Earthquakes can cause significant hazards, leading to loss of human life and property damage.In this study, we applied the masking empirical mode decomposition (EMD) to the Gyeongju earthquake (M L 5.8) in South Korea and the Abe Pura earthquake (M L 7.0) in Indonesia and then revealed that the 7th intrinsic mode function (IMF7), composed of waveforms with a period band of approximately 150 to 750 minutes, is decomposed without mode mixing in raw groundwater-level time series at a well on Jeju Island, South Korea.The dominant period of the waveform consisting of the IMF7 is about 375 minutes, which is the first even harmonic of the semidiurnal tide.The remainder mainly consists of abnormal waveforms interpreted as seismic precursors, unknown waveforms, and the higher harmonics of the semidiurnal and diurnal tides.The seismic precursors caused by the earthquakes appeared at least 17 hours before the Gyeongju earthquake.For the Abe Pura earthquake, which occurred over 4,000 km away from the study area, the seismic precursors were detected more than 24 hours prior to the main event.