Multi-scale free oscillations and resonances over the continental shelf of the East China Sea from the 2011 Tohoku-Oki tsunami

A number of coastal tide gauges and offshore Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys across the Pacific Basin recorded persistent oscillations excited by the 2011 Tohoku-Oki tsunami. The hazardous persistent oscillations prompted authorities to maintain a tsunami warning for dozens of hours. Tsunami waves reached the eastern China coast about five hours after the earthquake. The duration of detected prolonged oscillations was more than 80 h. In this study, we accurately reproduced tsunami generation and its evolution over the shallow and wide continental shelf of the East China Sea (ECS). For resonance analysis, we used an advanced numerical model that can consider simultaneously the effects of kinematic rupture process, horizontal displacement on tsunami generation, and Earth elasticity, seawater compressibility, and wave dispersion on tsunami propagation. The consistency of the simulated results was validated with both coastal and offshore tsunami records. In addition, we performed a combined spectral analysis of the observed tsunami records, background noise, and synthetic tsunami wave fields to identify tsunami oscillation modes and estimate their spatial distribution, including the spectral amplitude and phase angles. Eight main oscillation modes of the 2011 Tohoku tsunami were determined with periods of 24, 28, 37, 47, 57, 93, 136, and 157 min. They were all constrained within the 200-m isobaths for dozens of hours. The 57 min oscillations mode had the highest energy amplification across most of eastern China's coast. The spatial patterns of the resonance oscillations were used to propose hazard-prone areas. Based on the comprehensive spectral analysis results, the local and shelf topography properties played a key role in the dominant resonance modes of 136, 57, and 37 min. Identification of the shelf resonance oscillations coupled with the nearshore basin oscillations can provide valuable information for coastal tsunami hazard mitigation.