# Precise Positioning of BDS, BDS/GPS: Implications for Tsunami Early Warning in South China Sea
## 2 Real-Time Kinematic Precise Positioning Performance of BDS in South-East Asia
At the southwest coast of Taiwan and southeastern coast of China, tsunami wave heights may reach 1-2 m.  
In order to issue a reliable tsunami early warning as soon as possible, seismic parameters (e.g., epicenter, magnitude, slip distribution) should be inverted from co-seismic signal immediately after the earthquake breaks. Since the Luzon Island is located close to the northern Manila trench, our scenario rupture causes significant horizontal co-seismic displacements throughout the island. As shown in Figure 4a, stations distributed along the northwest part of the Luzon Island will experience northwest-directed surface displacements in excess of 10 cm. Taking into account the horizontal accuracy of real-time BDS and GPS processing of about 1 cm (Table 1), we conclude that co-seismic displacements due to an earthquake with a size of Mw = 8.0 should be well-detected and accurately measured with the present-day BDS/GPS observation and processing precision, which, in turn, should enable reliable source inversion and tsunami forecasting. To illustrate this, we retrieved static offsets from our simulated kinematic BDS and BDS/GPS displacements (Figure 3) and invert them back into the source parameters (Figure 4b,c). Finally, we compute tsunami propagations from the \"inverted sources\" and compare them to the forward scenario (vertical bars on Figure 5). In order to get static offsets at virtual BDS/GPS stations from the contaminated displacement waveforms (Figure 3), we employ a method introduced by [PERSON] _et al._[21]. Resulting static displacements were subsequently inverted into the slip distribution along the predefined geometry of the Manila trench using the SDM software [47]. Here, in order to make the inversion test closer to the real world scenario, we did not constrain the possible rupture  
Figure 5: Simulated tsunami scenarios. The color map shows the maximum wave height in the forward model (Figure 4a) after 6 h of tsunami propagation. Vertical bars are maximum wave heights as recorded at selected coastal locations for the three propagation models: forward model (black); with source as inverted from BDS-displacements (dark grey in Figure 4b); with source as inverted from joint BDS/GPS-displacements (light grey, Figure 4c). geometry to the forward model area by length and width. Additionally, we employed alternative fault discretization with 23 \(\times\) 9 subfaults. Since we were expecting predominantly thrust events at the Manila trench, the rake angle was allowed to vary between 70 and 100 degrees. The two horizontal components were weighted twice as much as the vertical component, and a smoothing constraint was imposed to avoid unrealistic slip patterns. Figure 4b,c present slip distributions as inverted from the simulated real-time co-seismic BDS (Figure 4b) and joint BDS/GPS (Figure 4c) displacements. Inverted slip distributions from single BDS and BDS/GPS show a high degree of consistency, which is obvious since the processing accuracy is very similar in both cases (Table 1). In contrast, comparison with the forward model (Figure 4a) shows some bias. To be exact, inverted slip distribution covers a somewhat larger area than the original model; the maximum slip of the forward model is 8.4 m while the inverted maximum slips are 7.31 and 7.32 m, respectively, and the derived moment magnitudes are both Mw = 8.06, slightly greater than the input value of Mw = 8.0. This insignificant discrepancy is explained, first of all, by the fact that our virtual observational network is located, due to natural reasons, only on one side of the Manila trench. Nevertheless, tsunami wave heights as predicted from the inverted source models are very similar to the forward computations (compare vertical bars on Figure 5 and refer to the supplementary Table S1 for more detailed information). Note that the difference in estimated wave heights is larger along the Luzon coast compared to the distant coasts of China and Taiwan.