Past and potential tsunami sources in the South China Sea: A brief synthesis

Extending across some 3.5 million square km and with coastlines populated by in excess of 80 million people, the South China Sea (SCS) is bordered by seven independent states. Understanding the risk of tsunamis in the SCS is therefore an imperative, especially given the new priorities for the science community following the endorsement by the UN General Assembly of the Sendai Framework for Disaster Risk Reduction 2015–2030. Our review presents a synthesis of existing literature on the current state of knowledge concerning known tsunamigenic threats, but also highlights a number of other potential sources that have so far received less attention or gone largely un- or under-recognised. An overview of the geological evolution of the SCS is presented in order to appreciate how the existing structural configuration of the ocean basin is responsible for the presence of a variety of potential tsunamigenic mechanisms. Traditional thinking places the generation of a large-magnitude earthquake at the Manila trench subduction zone at the top of the spectrum for tsunami potential, with several simulations already available in the literature illustrating wave characteristics resulting from the worst-case rupture. In addition to the Manila trench and other possible earthquake sources, however, volcanic activity and submarine landsliding of sediment burden on continental shelves on the SCS margins represent hitherto fundamentally unquantified tsunami threats, although revealed nonetheless by, for example, the existence of the giant Brunei and Baiyun submarine slides. Furthermore, in the central SCS, previously considered essentially devoid of tsunami sources, numerous near-surface carbonate platforms and coral atolls show evidence of arcuate bight-like structures in their planform geometry, likewise suggesting their propensity for possible tsunamigenic lateral flank collapse. In the current absence of well-constrained tsunami probability estimates and scenarios of wave size and propagation features for non-earthquake tsunami sources, future research priorities lie in two main areas. These are, 1. improvement of modelling for tsunamigenic submarine failures as well as for earthquakes; and 2. a greater focus on geomorphic investigation to identify palaeotsunami deposits on coastlines considered likely to have been impacted in the past and where good preservation potential exists. This is needed to establish the magnitude–timing–frequency of events on Holocene timescales (the last 10,000 years) well beyond historical records, thereby improving our capabilities for understanding and forecasting the likelihood and severity of future tsunami risk in the South China Sea.