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The South China Sea (SCS) coastlines are particularly vulnerable to tsunamis due to rapid urbanization and dense infrastructures. However, whether the SCS has experienced ocean-wide tsunamis is still under debate. Some geological records from the offshore islands inside the SCS imply a regional tsunami event may have occurred approximately 1000-yr-ago. We address these questions for the first time using forward numerical simulations in the SCS: what source could be responsible for such deposits, how the deposits were formed and at what conditions do the deposits could be preserved? Here, we assume the validity of the tsunami deposit hypothesis and use the forward model COMCOT-SED to investigate if these tsunami deposits can be linked to potential seismogenic tsunami sources inside the SCS, including megathrust earthquakes (Mw8.8–9.2) from the Manila subduction zone (MSZ) and the 1918 Mw7.5 Nan'ao earthquake from the Littoral Fault Zone (LFZ) in the northern continental shelf of the SCS. We also utilize a reported tsunami deposit from Qing'ao Embayment, Nan'ao Island, China, to exemplify the deposit formation process in a typical barrier-low coastal plain system. Quantitative analysis suggests large earthquakes from the MSZ may account for the reported geological deposits. The full-rupture earthquake scenario could best explain the depositions if such extreme case is geophysically and geologically plausible. If not, earthquakes (∼ Mw 9.0) covering the northern (16–23.5°N) or southern portion (14–19°N) of the MSZ could also be suitable candidates. Such great earthquakes can generate large tsunami waves (>7 m) that inundate the Qing'ao Embayment, transporting sediments and leaving tsunami deposits in low-lying areas. Conversely, tsunamis triggered by nearshore earthquakes in the LFZ cannot cause deposition in Qing'ao Embayment. The barrier-low coastal plain system is well-suited for preserving tsunami deposits, which remain unaltered by subsequent tsunami waves. Our findings offer a quantitative reference for paleo-tsunami research in the SCS and enhance global understanding of the tsunami deposition process in barrier-low coastal plain systems. • The full-rupture earthquake (Mw 9.0) from the MSZ best explains the reported plausible tsunami deposits in the South China Sea. • Tsunami waves >7 m generated by MSZ earthquakes could inundate the Qing'ao Embayment anddeposit sediments in low-lying areas. • The barrier-low coastal plain system in Qing'ao Embayment forms favorable conditions to preserve overwash tsunami deposits. [ABSTRACT FROM AUTHOR]

We propose an optimal deployment scheme of the Seafloor Observation Network (SON) in the South China Sea (SCS), aiming at early warning of potential tsunami hazards based on the data assimilation approach. The SON is composed of offshore bottom pressure gauges (OBPGs). We design the initial location of OBPGs along the isobaths of 500, 1000, and 2000 m. The energy distribution of the tsunami wavefield based on Empirical Orthogonal Function (EOF) analysis is used as a reference. Then, synthetic tsunamis generated by stochastic slip sources along the Manila Trench are adopted to evaluate the performance of the SON. The OBPGs at a depth of 1000 m can make an accurate early warning based on tsunami data assimilation at a reasonable engineering cost. Finally, based on the quantitative evaluation, the optimal deployment scheme of the SON is suggested. Our results indicate that at least three stations are required to cover the coast along southern China to forecast the tsunami in the SCS successfully. We note that our method could also be applied to other regions to design the SON against potential tsunami hazards. • A tsunami early warning system is designed for the South China Sea. • Data assimilation approach is adopted for tsunami forecasting. • Offshore stations along the 1000-m isobath are most cost-effective. • At least three stations are required for constructing tsunami warning system. [ABSTRACT FROM AUTHOR]