Forward numerical investigation of potential tsunami deposits in the South China sea: A case study of Nan'ao Island

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 (similar to Mw 9.0) covering the northern (16-23.5 degrees N) or southern portion (14-19 degrees 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.