Significant coherence for groundwater and Rayleigh waves: Evidence in spectral response of groundwater level in Taiwan using 2011 Tohoku earthquake, Japan

Summary

It was found that groundwater can be fluctuated by a variety of disturbances. For instance, seismic activity inducing the dilatation of earth can disturb groundwater level fluctuation as observed in groundwater wells. In this research, spectral analysis in the frequency domain was used to quantitatively evaluate coherence between groundwater head fluctuations and the ground motions recorded in Taiwan from a distant earthquake, the 2011 Tohoku earthquake in northern Honshu, Japan with magnitude of 9. The relationship between groundwater fluctuations and the decomposed ground motions of Rayleigh waves are clearly identified. By analyzing autospectral density, cross-spectral density, and resultant coherence for the seismograms and groundwater head, it was found that the Rayleigh waves dominated the groundwater fluctuations at period of about 21–32 s for six pair of groundwater and seismograms of broadband distributed around Taiwan; fluctuations of groundwater are highly coherent with the radial and vertical components of ground motions. Our analysis also shows the time from event to station for Rayleigh waves ranged from 780 to 900 s approximately. Wave parameter for seismic event to station of groundwater and seismograms were also identified as 3.0–3.5 km/s and 64–110 km for wave velocity and wave length, respectively. The relationship of groundwater fluctuations and ground motions induced by seismic activity become feasible to assess using spectral analysis.

Introduction

The study demonstrates that the polarities of the observed coseismic water-level and river discharge changes are in good agreement with those of the static volumetric strain calculated by a dislocation model, using the well-constrained rupture model of the seismogenic Chelungpu fault (Lee et al., 2002). Analysis of strong-motion instrument recordings in Seattle, Washington, resulting from the 2002 M_w 7.9 Denali, Alaska, earthquake reveals that amplification in the 0.2–1.0 Hz frequency band is largely governed by the shallow sediments both inside and outside the sedimentary basins beneath the Puget Lowland (Barberopoulou et al., 2006). For the great Sumatra earthquake of M_w 9.3 on 2004, Chadha et al. (2008) indicated that large water level changes are attributed to the dynamic strain induced by the passage of seismic waves, most probably long-period surface waves. The Sumatra–Andaman Islands Earthquake with magnitude 9 on 26 December 2004 at 00:58:53.45 UTC was used as the seismic disturbance to estimate groundwater storage in well-aquifer system located in the eastern Taiwan (Shih, 2009).

The studies have observed that water level fluctuations in an aquifer-well system demonstrated strong correlations to nearby boundary disturbances (Shih et al., 2008a, Rotzoll et al., 2008. Cooper et al. (1965) studied the water level response to pressure-head fluctuations due to dilatation of the aquifer and to vertical motion of the well-aquifer system induced by seismic waves. The response of a confined well-aquifer system to tidal dilatations, consisting of the earth tidal dilatation, the harmonic tidal dilatation, and the ocean tidal dilatation, has also been demonstrated (Robinson and Bell, 1971). Shih (2009) derived the spectral relationship between groundwater level and the vertical components of seismic Rayleigh waves. By using the derived spectral relationship, the groundwater storage properties, storage coefficient and specific storage were evaluated. In the study by Shih (2009), the Sumatra–Andaman Islands Earthquake with magnitude 9 on 26 December 2004 was used as the seismic activation to the well-aquifer system located in the eastern Taiwan while a nearby groundwater monitoring well and a broadband seismic station were served as the receivers. The resultant spectral estimates were used to determine the storage coefficient and specific storage of the case site to be on the order of 10⁻³ and 10⁻⁴ (m⁻¹), respectively.

It is known that short-term hydraulic head in an aquifer is strongly affected by incident earthquake seismic waves since the pioneering study by Cooper et al. (1965). In this study, six pairs of groundwater level and seismogram in the Taiwan have been collected utilizing the seismic source located in the eastern Honshu, Japan. Generally, the fluctuations of groundwater level in the study aquifer can be traced by the three-component in seismograms. However, the relationships behind those phenomena are still not fully identified in the present time. Therefore, this research aims to provide demonstrative evidence by analyzing the correlation between groundwater level and the seismic Rayleigh waves in vertical and radial components. We interpreted the structure and examined the particle motions of Rayleigh waves. Spectral analysis was also used to identify the dominant period for groundwater fluctuations and the segments associated with Rayleigh wave observations. Coherence between groundwater fluctuation and components of seismograms is well discussed and indicative evidence is then demonstrated.