Analysis of recent atypical tsunami source events worldwide and their implications for tsunami research in the South China Sea
Since 2018,several"atypical"tsunami events have occurred globally.These events include the 28 September 2018 Palu Bay tsunami in Sulawesi,triggered by a Mw-7.5 strike-slip earthquake,which led to more than 4300 deaths;the 22 December 2018 tsunami event,caused by the flank collapse of Anak Krakatau volcano in the Sunda Strait,Indonesia,that killed 437 people;the 30 October 2020 Samos tsunami event in the Eastern Mediterranean Basin,the largest since 1956,originating from the Mw 6.9 normal fault earthquake located in the northern Samos Island to the east of the Aegean Sea;the 2022 Tonga tsunami event generated by the VEI5 explosive eruption of Hunga Tonga-Hunga Ha'apai submarine volcano in the South Pacific;and the 2023 Turkey-Syria tsunami event triggered by the onshore strike-slip earthquake which induced a 40-cm wave amplitude.Currently,scientists have limited understanding of the generating mechanisms and physical processes of these"atypical"tsunami sources,and existing disaster response systems are unable to react promptly to these"unexpected"extreme events,resulting in casualties and economic loss.Hence,this paper aims to provide an in-depth analysis of the triggering mechanisms of these"atypical"tsunami events.Factors contributing to the formation of these"atypical"tsunamis and their complexities include the formation of landslide tsunamis from the cascading effect of earthquakes-induced liquefaction of coastal/submarine sand,distinct coastal morphology and complex coupling effect of atmosphere-ocean-land interactions accompanied by intense volcanic activities.These findings challenge our conventional understanding that tsunami generation requires large-scale vertical deformation,which is commonly associated with submarine earthquakes.With this newfound knowledge,strike-slip earthquakes,onshore earthquakes near the coast,and intense volcanic eruptions can all be classified as potential tsunami sources due to the chain effect.A careful review of historical documents detailing the major earthquakes in the northern South China Sea(hereinafter,SCS)and volcanic tsunami events in the surrounding region suggests that SCS and its surrounding regions are not lacking in such"atypical"tsunami events.Historical documents reveal that a majority of major historical earthquakes in the northern SCS,that were accompanied by tsunami phenomena,are documented in local records and genealogies.These events include the 1605 M 7.5 Qiongzhou event,the 1604 M8.0 Quanzhou event,the 1918 M7.5 Nanao event,the 1992 Hainan event and the 1994 Taiwan Strait earthquake event.Historical and modern instrumental records have also identified 22 volcanoes in this region that are responsible for 41 past tsunami events,accounting for 24％of the global historical volcanic tsunami events.These tsunami-triggering volcanoes are mainly located along the Philippine Island Arc and the Sunda Island Arc.Notable examples include the lliwerung volcano in the southern part of the Banda Sea along the eastern Sunda Island Arc,Teon volcano and Ijen volcano in central Java Island,and the Agung volcano and Peuet Sague volcano in western Sumatra Island.Lessons learnt from recent and past"atypical"tsunami events highlight the need for more effective tsunami warning operations,which in turn require a better scientific understanding of the tsunami genesis of those potential"atypical"sources.In the SCS and its surrounding region,future research should prioritize several key aspects:(1)Conducting detailed marine geophysical surveys to better understand the geometry and seismogenic behaviours of active faults;(2)understanding the triggering mechanism of cascading hazard effects through in situ monitoring and seafloor surveys;(3)developing theoretical and numerical approaches to reproduce the complex generation processes of volcanic tsunamis.
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