Microseisms associated with the southwest summer monsoon recorded on the seafloor of South China Sea

• Strong monsoon-associated secondary microseisms are recorded in the South China Sea. • The microseisms occurred prior to meteorologically determined summer monsoon onset. • Ocean-bottom seismic data can be used for monsoon early detection and monitoring. As the strongest component in the worldwide ambient noise field, secondary microseisms are dominantly excited by cyclonic storms. Whether unidirectionally propagating monsoon winds can induce seismically observable microseisms remains unclear. In this study, we use broadband seismic data recorded by seven ocean-bottom seismometers (OBSs) to systematically examine the possible excitation mechanism associated with the summer monsoon 2020 in the South China Sea. Spectrograms of the OBS data reveal remarkable power characterized by two notable peaks during May 18–31 in the short-period secondary microseism band of 0.25−0.5 Hz. In this time period, a monsoon onset occurred (May 23) and no tropical cyclones were recorded either in the South China Sea or Western Pacific. Around the summer monsoon onset, wind fields are transformed from northeasterly to southwesterly, driving ocean waves to propagate northeastward and exciting secondary microseisms by interactions with opposing wave trains driven by the pre-existing northeasterlies, which corresponds to the first power peak observed during May 18–20. After the dissipation of the northeasterlies-driven swell, a cyclone associated with the monsoon developed around Taiwan produces northeasterlies and excites secondary microseisms, resulting in the second peak emerged during May 22–26. Polarization analyses indicate that the microseisms are probably amplified by wave shoaling when ocean waves passing from deep to shallow water. Overall, this study provides evidence of a causal relation between monsoon onset and secondary microseisms, highlighting the potential of collecting ocean-bottom seismic data for the early detection and monitoring of monsoons.