Abstract:Based on the records of four-component borehole strain gauge of Sheshan station in Shanghai during the six western Pacific typhoons,the methods of wavelet decomposition and predominant polarization direction were applied to extract the significant low-frequency disturbance signals excited by the typhoon,and the characteristics of disturbance signal in time-frequency domain and spatial domain were then analyzed in detail.The results showed that in the low frequency range,the dominant period of disturbance signal excited by the typhoon is 2-4 min;the signal energy varies with the progress of typhoon,and is mainly related to the magnitude of typhoon,the location of typhoon center (sea or land),and the distance between the station and the typhoon center.As the typhoon center approached the coast,the disturbance signals recorded by the four components of Sheshan station began to increase,but the signal intensity recorded in different directions were not consistent.The predominant polarization direction of the signal is 160° and closely related to the strike of the coastline where the station is located.Finally,based on the time-frequency and spatial response characteristics of the disturbance signal,and the records of surface wind speed and ocean buoy,it is considered that the low-frequency disturbance signal recorded by the component strain gauge has a good correlation with the offshore wave height,and the typhoon disturbance signal recorded by the strain gauge is generated by the infragravity waves from the offshore area.
