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

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.