Fig. 1. System was installed in the South China Sea off the Hainan Island

Fig. 2. (a) Sealed capsule fixed on the bed of a concrete block; (b) Fastening screw and the fixed button on the sealed capsule

Fig. 3. Diagram of the sealed capsule structure

Fig. 4. System block diagram of optical fiber interferometric seafloor seismic tsunami monitoring

Fig. 5. Vibration head structure diagram

Fig. 6. Structure of water level monitor probe

Fig. 7. Location sketch of the Sulawesi earthquake

Fig. 8. Seismic wave propagation paths from the epicenter to the seismometer

Fig. 9. Acceleration waveforms of the ML6.8 earthquake in the adjacent sea area of the Sulawesi, Indonesia

Fig. 10. Water level measurements of three days (April 11–13, 2019)

Fig. 11. Water level measurements of one hour (19:30-20:30 pm during the Sulawesi earthquake)