Anomalous frequency characteristics of groundwater level before major earthquakes in Taiwan

Abstract

Abstract. Unusual decreases of water levels were consistently observed in 78 % (= 42/54) of wells in the Choshuichi Alluvial Fan of central Taiwan about 250 days before the Chi-Chi earthquake (M = 7.6 on 20 September 1999) while possible factors of barometric pressure, earth tides, precipitation as well as artificial pumping were removed. Variations in groundwater levels measured on anomalous wells from 1 August 1997 to 19 September 1999, which covers the 250 day unusual decreases, were transferred into the frequency domain to unveil frequency characteristics associated with the Chi-Chi earthquake. Analytical results show that amplitudes at the frequency band between 0.02 day −1 and 0.04 day −1 generally maintained at the low stage and were apparently enhanced a few weeks before the Chi-Chi earthquake. Variations of amplitude at this particular frequency band were further examined along with other Taiwan earthquakes (M > 6) from 1 August 1997 to 31 December 2009. Features of the enhanced amplitudes at the frequency band are consistently observed prior to the other two earthquakes (the Rei-Li and Ming-Jian earthquakes) during the 12.5 yr study period. In addition, surface displacements recorded from GPS, which provides insights into understanding stress status in subsurface during the Chi-Chi earthquake, are also inspected. The result confirms that abnormal rise and fall changes in groundwater level yield an agreement with forward and backward surface displacements around the epicentre prior to the Chi-Chi earthquake.

Unique Pre-Earthquake Deformation Patterns in the Spatial Domains from GPS in Taiwan

“…Disagreements included five M < 5.5 offshore earthquakes (i.e., EQ4, EQ8, EQ21, EQ25, and EQ30) which would have been caused by GPS stations located on land far away from the epicenters. In contrast, the other M = 5.2 land event (i.e., EQ 13) showed different characteristic changes in the GPS azimuths before the earthquake for unknown reasons (see also Reference [40]). This suggests that the analytical methods used in this study are most suitable for large and land earthquakes.…”

“…Meanwhile, the alignments of seismo-crustal displacements anomaly can be utilized as references to examine other promising seismo-anomalies [40,45]. Negative (positive) strain changes can relate to uplift (depression) of seismo-ground water levels.…”

A Mathematical View of Water Table Fluctuations in a Shallow Aquifer in Brazil

“…A new sample interval of 24 h was subsequently adopted to match the interval of available pluviometric data. The 5‐s sampling interval was initially selected with the intent to record high‐frequency phenomena such as those resulting from earthquakes (Todd ; Roeloffs ; Zeumann et al ; Chen et al ). We indeed detected several earthquakes with our monitoring system, including some from as far away as Alaska (results not further shown here).…”

“…Groundwater monitoring constitutes an important tool for aquifer management by providing information about the current state and trends of various hydrological processes, as well as recording responses to both anthropogenic activities and natural phenomena. Detailed monitoring further provides insight into the nature of groundwater table fluctuations in response to such processes as precipitation, evapotranspiration and recharge, regional flow and pumping, barometric pressure changes, earth tides, and even earthquakes (Todd ; Bredehoeft ; Roeloffs ; Healy and Cook ; Zeumann et al ; Cutillo and Bredehoeft ; Chen et al ; and numerous references therein). Understanding the processes that cause variations in the water table and the ability to monitor groundwater levels is critical for estimating recharge, whether on the scale of an aquifer to assess groundwater resources and management in general, or for local situations such as for assessing groundwater contamination.…”

Co-seismic geomagnetic fluctuations and atmospheric disturbances during the 2018 M 6.2 Hualien Earthquake

“…Scientists (Yeh et al 1981;Chen et al 2011Chen et al , 2013Chen et al , 2014Han et al 2014;Liu et al 2017) show that spectrum analyses provide a better understanding on studied signals. Figure 4 illustrates power spectra of the high sampling data of QuakeFinder systems and seismometers, where we can see that the magnetic pulsations are monotonically inversely proportional to the frequency, and those of co-seismic geophone fluctuations and the seismic waves yield peaks around Fig.…”