Tsunami hazard assessment to South China Sea Islands induced by the earthquake with maximum possible magnitude in the Manila subduction zone

[Objective] The Manila subduction zone is the primary source of potential large tsunamis in the South China Sea, which may result in severe coastal disasters. This study aims to evaluate the tsunami hazards faced by South China Sea Islands caused by earthquakes with maximum magnitudes through assessing the earthquake with maximum possible magnitude in the Manila subduction zone and simulating the process of induced tsunamis.[Methods] The seismic potential was evaluated using the negative dislocation inversion model TDEFNODE based on GPS horizontal velocity field data. The acquired distribution of the locking and slip deficit along the Manila subduction zone was first used to assess the seismic potential. The earthquakes with a magnitude of 8. 9 and a 500-year return period were selected as the maximum possible earthquake to design extreme earthquake tsunami events. This study comprehensively considered the impact of the epicenter, focal depth, and heterogeneity in the fault slip on tsunamis, and about 700 000 tsunami events under the condition of magnitude 8. 9 were simulated for further evaluation. Both uniform and heterogeneous slip models were adopted to describe fault slips in the tsunami events. Considering that a larger fault slip is more likely to occur in areas with a higher degree of fault locking, the distribution of fault locking was also introduced into the heterogeneous slip model as a constraint for the random slip distribution. The tsunami events were simulated by the unit-source superposition method proposed by our group previously, which could efficiently simulate the propagation of tsunami waves based on a precomputed database and provided the offshore tsunami wave heights of major islands with small computational cost.[Results] The findings revealed that even under the same magnitude, the height of tsunami waves exhibited significant randomness. The tsunami wave height in Dongsha Island varied between 1. 8 m and 6. 2 m during 8. 9-magnitude earthquake tsunami events. The heterogeneity of fault slip had a significant impact on tsunami wave height, and conventional models that neglected heterogeneous slip distribution would underestimate the tsunami wave height by approximately 20% - 50%. In terms of spatial distribution, with tsunami wave heights exceeding 4 m, Nanshazhou, Nandao, and Beidao in the Xuande Islands and Dongsha Islands were worst affected, while the tsunami hazard in the Nansha Islands was much smaller.[Conclusions] This work enhances the tsunami hazard assessment model by introducing fault locking into the random slip model as a constraint, enabling the description of the fault slip to be more realistic than the conventional uniform slip assumption. The maximum possible tsunami hazard faced by major islands in the South China Sea has been quantified, which offers effective support for tsunami hazard prevention and reduction in these islands. © 2024 Tsinghua University. All rights reserved.