[S08P] PM-P

[S08P-16] On Cavitation, Shock Waves, and Generation of Seaquakes

*Koji UENISHI¹ (1. Univ. of Tokyo)

Seaquakes are violent vertical shocks experienced on board a floating body like a ship during a seismic event and may cause severe damage to the floating body. Dissimilar to tsunamis, seaquakes are normally felt only near the epicenter, but in the event of the 1995 Hyogo-ken Nanbu, Japan, earthquake, at least four ferry boats were struck by seaquakes. Two of them are subjected to two strong vertical shocks with an interval of one or two seconds while for another boat the interval between the two shocks was as long as a few minutes. Tsunamis, about 5 cm high, were observed in the Osaka Bay but they did not have two clear peaks. Thus, seaquakes are not tsunamis, and unlike the propagation of tsunamis where incompressible movement and gravitational balance of water are considered, seaquakes are often studied in light of the propagation of compressible P acoustic waves in water. In our previous study (Uenishi and Sakurai, SSJ Fall Meeting, 2012; Int. J. Protect. Struct., 2014), based on simplified one-dimensional analyses, we have suggested two possible generation mechanisms of seaquakes: (1) dynamic interaction and quick resonance of linear elastic solid-liquid system that involves a layer of seawater of finite thickness on top of a semi-infinitely extending seabed (first shock); and (2) slow evolution and propagation of a nonlinear wave of velocity discontinuity or a shock wave in seawater that is related to the Burgers' equation in fluid mechanics (second shock) (Uenishi, Proc. Jpn. Acad., Ser. B, 2017). Here, we consider another physical scenario for the generation of seaquakes (and possibly tsunamis) by reviewing the effect of cavitation on wave propagation in fluids and point out air bubbles have indeed a large influence on the generation and propagation of shock waves in compressible water. Such shock waves may play a significant role in generating a seaquake and might even induce tsunamis. We are now trying to investigate whether sufficient air bubbles can be really provided during a seismic event to cause shock waves and how air bubbles may influence the generation, propagation and strength of the shock waves, the initiation of seaquakes and tsunamis as well as the existence of tsunami earthquakes.

Acknowledgements: The research has been financially supported by the Japan Society for the Promotion of Science (JSPS) through the "KAKENHI: Grant-in-Aid for Scientific Research (C)" Program under grant number 23K04021.