The Characteristic Of Microseism Generated By Typhoon And Analysis For The Located Source

The microseism(~0.05-0.5Hz)is mainly divided into single-frequency microseism(~0.05-0.12 Hz,SF)and doubling-frequency(~0.12-0.4Hz,DF),which can be increased significantly by the strong ocean wave triggered.The strong microseism energy was recorded on both of the the land station and the island station during the span of typhoon.In this paper,we will takes the Typhoon Megi and Lupit in the northwest Pacific as examples,and then analyze the characteristics of microseisms generated by typhoon and propagation characteristics of swell using continuous microseism recorded by TWGB from the island Taiwan,Fnet from Japan,the seismic stations in the southeast coastal area of China and array data form north-east of China during the span of typhoon.We have tried various methods to explore the sources of signal energy recorded by the seismic stations in order to monitor and track the typhoon path during the typhoon,such as polarization analysis from single station,the array cross-correlation noise function study,and the array beamforming technology.Polarization analysis is rotating the two horizontal components of east-west and north-south upwards to obtain the direction of the dominant source of energy.The array cross-correlation noise function study is obtaining the cross-correlation noise function through the crosscorrelation in the time domain of the station pairs by array data,and then obtain the spatial and temporal distribution of signal-to-noise ratio at different frequency bands during the typhoon.The above two methods just located the general direction of the microseismic source during the typhoon.We track and monitor the typhoon path using the propagation characteristics of the P waves with array beamforming technology contained in the DF microseism excited by the typhoon Lupit.Furthermore,we introduced a global model of seismic background noise energy radiation to study the microseismic signal source recorded in the island area.Based on the model,we conducted a numerical modeling and compared it with the observation from seismic station.The research shows that:(1)The fluctuating signal of typhoon "Megi" has distinct spatial and temporal distribution characteristics,and the intensity of the DF microseismic signal recorded by the station is closely related to the distance between the stations and the center of the typhoon,and the microseisms recorded by coastal stations are stronger than that recorded by inland stations;(2)There is a obvious characteristic of dispersion from the the record of TWGB.The dispersion curve calculated from several moments extracted from the ECMWF is overlaid on the dominance of the microseism,indicating the applicability of the dispersion relation.(3)It can be used to locate the general direction of the microseismic source region excited by the typhoon with polarization analysis and cross-correlation function study,but the orientation results of the polarization analysis are biased when detecting weak microseismic signals.(4)It can be detected two kinds of DF microseism source by beamforming,pelagic source and coastal coastal source,which is distinguished by frequency.There are several errors through combining the output of the beamforming with the MODIS data,which is resulted from the typhoon movement speed,the moving direction and the array’s shapes and structures in addition to the inherent errors of the array bunching method and the solid-land heterogeneity.(5)Based on the Ardhuin Seismic Spectra Model,microseisms observed at the stations of island mostly are generated by coastal reflection,which is related with the propagation mechanism of p-wave DF microseism generated by typhoons.