Time series decomposition of groundwater level changes in wells due to the Chi-Chi earthquake in Taiwan: a possible hydrological precursor to earthquakes

摘要

The largest and most disastrous earthquake in Taiwan (Mw: 7.3) in the 20th century, the Chi-Chi earthquake, hit central Taiwan at 01:47 local time on September 21, 1999. The groundwater level changes were rapid at that time. Studies have found that the rapid change in groundwater levels was a co-seismic phenomenon. This work analyzes the possibility that the abnormal change in groundwater levels may have occurred before the earthquake. Three well stations with a total of five wells are considered. They are all near the Che-Lung-Pu fault, which caused the Chi-Chi earthquake. The time series decomposition method was applied to decompose the seasonal groundwater level, the trend in groundwater levels, and the period of the change in the groundwater level. Residual groundwater levels were found by subtracting the determined seasonal, trend and period data from corresponding data for the original groundwater level. The computed residual water levels in July, August and September of 1999, were transformed into a frequency spectrum by a Fourier method. Additionally, the effects of barometric pressures on the groundwater level changes were also evaluated. Analytical results show that the spectral density functions of the irregular groundwater level in the confined aquifer at the Chu-Shan well in September behaved differently from those in July and August. We posit that a pre-seismic hydrogeological anomaly may have existed before the Chi-Chi earthquake, and can be considered in future studies of anomalies associated with earthquakes.