Signatures of typhoons on strain records of four-component borehole strain meter at Sheshan station

In order to analyze the temporal and spatial signatures of typhoons on strain records, we processed the strain data of four-component borehole strain meter (FCBS for short) at Sheshan station during the impacting periods of typhoons "Neoguri" and "Haikui". We integrated methods of wavelet decomposition, continuous spectral analysis and predominant polarization direction to investigate the influence of ocean storms on low-frequency noise. The results show that the process of typhoon development had significantly influenced spectrum amplitudes, which showed an "increase--peak--decrease" pattern, especially over the frequency band 2--4 min. This pattern was highly correlated with the distance between the station and the center of typhoon. And with the increases of the periods, the amplitude of the spectra decreased. When the storm center was approaching to the station, the predominant polarization directions of the strain records were concentrated on the southeast of 160°, which indicates that local coastlines may play an important role in the noise excitation mechanism. The comparison of the mean spectra amplitudes with the wind speed showed that the wind was not the major excitation factor. Finally we come to the conclusion that during the typhoon development, the energy of ocean waves greatly increases, and the waves with higher energy are continuously lapping at the shore, which results in low-frequency disturbances on continents.