Tsunami free oscillations excited by potential local earthquake sources in Bohai Sea, China

The Bohai Sea is the most seismic active region offshore mainland China. Its coastal region is highly populated. The present work proposed a comprehensive scheme for identifying and examining free oscillations with limited number of tide gauges in an irregular semi-enclosed basin. We combined modal analysis, spectral analysis, and multiple-scenario simulation to investigate tsunami free oscillations in the Bohai Sea Basin and three local bays including Liaodong Bay, Bohai Bay, and Laizhou Bay. The Bohai Sea had broadband periods of free oscillations ranging from 228 to 30 min. There existed both along-shore and cross-shore oscillations in most eigenmodes. The ultra-long dominant periods above 50 min were attributed to the shallow water depth (<20 m). Some free oscillation modes appeared in both the Bohai Sea and local bay domains. In addition, the spatial patterns of dominant eigenmodes were characterized and validated by performing spectral analysis for an Mw 7.8 tsunami scenario. The energy was mainly concentrated in the first three modes (171, 146, and 93 min). This is the first study to characterize the areas prone to tsunami hazards in the Bohai Sea, and the methodology can be further extended to other coastal regions.