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

Abstract

Abstract:The largest and most disastrous earthquake in Taiwan (Mw: 7Ð3) 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.

Hydrologic Precursors

“…Another reported possible precursor to the Chi-Chi earthquake is a change in the spectral characteristics of water level fluctuations in some wells in the month preceding the earthquake compared with those 2 and 3 months before the earthquake (Gau et al 2007). This is not a compelling comparison as the amount and character of precipitation also changed (Fig.…”

Assessing aquifer responses to earthquakes using temporal variations in groundwater monitoring data in alluvial and sedimentary bedrock aquifers

“…Groundwater flowing under the forces of gravity and fluid pressure along pores and fractures in geologic media reacts differently depending upon the aquifer type/characteristic and external pressures, such as earthquakes and changes in atmospheric pressure (Freeze and Cherry 1979). Thus, analyses of fluctuations in groundwater levels have been used to indicate abnormal changes (oscillations) caused by seismic activity (Kitagawa et al 2006;Gau et al 2007;Lee et al 2018). We analyzed the changes in groundwater levels caused by the Gyeongju earthquakes by discriminating between the responses of confined and unconfined aquifers, considering their distances from the fault.…”

“…Observational data collected over long periods of time are essential for understanding and analyzing natural phenomena, and many countries such as European Union (Mendizabal and Stuyfzand 2009), United States (Rosen and Lapham 2008; US Geological Survey, https://cida.usgs.gov/ ngwmn; Texas, https://www.waterdatafortexas.org/groundwater), The Netherlands (Van Duijvenboodem et al 1993), and Taiwan (Hsu 1998) have been operating groundwater monitoring systems. A spatially distributed network of hydrologic monitoring wells established for groundwater management can be used for identifying seismic mechanisms and for selecting points for further earthquake monitoring (King et al 1999;Gau et al 2007;Chen et al 2013;He and Singh 2019). Groundwater monitoring networks on a regional or national level were used to identify the co-seismic changes in groundwater level due to local earthquakes and offshore earthquakes around the world (Lee and Woo 2012;Lee et al 2013;Yun et al 2019).…”