Multimodal surface waves during energetic typhoons

Ambient noise tomography is a revolutionary technique in past twenty years, and has many far-reaching applications from exploration, regional and continental to global scales. Recently, surface wave overtones have been efficiently extracted from ambient noise cross-correlations by the frequency-Bessel (F-J) transform method. The excitations of overtones from noise cross-correlations, however, have not be investigated well. Here we collect 16-day continuous seismic data recorded by a seismic array installed in Mongolia during two super typhoons in 2011. Utilizing the F-J transform, we extract daily multimodal dispersion curves with high signal-to-noise ratios from vertical-vertical (Z-Z) component cross-correlations and transverse-transverse (T-T) component cross-correlations. For both the fundamental mode and overtones, our results show that the cross-coefficients between the wind speeds and the Z-Z dispersion amplitudes over 0.1Hz are over 0.6 in typhoon track regions, which suggests that these two typhoons excite Rayleigh waves over 0.1Hz.  For T-T surface waves, however, typhoons only excite the fundament mode over 0.1Hz, and overtones do not relate to typhoons. Our results indicate that energetic typhoons can efficiently excite multimodal Rayleigh waves while Love waves during this period may not come from the microseisms excited by typhoons, which may help us to deepen our understanding of the coupling system among the atmosphere, oceans and the solid earth.