RESUMO
Seismic seiche-related oscillations caused by Rayleigh waves from large earthquakes are yet to be explored and elucidated comprehensively, then need to accumulate continuously. Herein, we investigated water level fluctuations in Lake Biwa of Japan from surface seiches following the 2011 Tohoku earthquake. Lake Biwa is the largest freshwater resource in Japan, and a small change in its water level can affect local ecosystems. Field observations were conducted during 2010-2012 using a water level gauge with a 1 mm resolution and 2 min data sampling interval. Fast Fourier transform and maximum entropy methods were used for data spectral analysis to distinguish the effects of inherent oscillations on water levels generated by the earthquake. We considered that water level changes were influenced by long-period Rayleigh waves. We observed a wave with a 3.08-3.10 h duration, which was close to the duration determined for the Rayleigh waves (3.08 h). The 3.08-3.10 h wave was caused by forced oscillation of Rayleigh waves characterised by a frequency close to the natural frequency and excited by the earthquake. Overall, our findings suggest that water level fluctuations can be good indicators of high-magnitude earthquakes.
