Ambient seismic noise wavefield in Japan characterized by polarization analysis of Hi-net records

Abstract

The polarization of seismic waves can be used to infer the directionality of the ambient noise wavefield. We demonstrate a simple relationship between the vertical–horizontal cross-spectra at a single station and the azimuthal energy distribution of incident waves. The imaginary parts of the cross-spectra are related to the azimuthal energy distribution of elliptically polarized waves (Rayleigh waves), and the real parts are related to linearly polarized waves (P waves). The relationship can be used to estimate the dominant backazimuth and directional intensity of the incident waves. We apply our polarization method to records of length 1 yr observed by Hi-net to investigate the characteristics of the ambient noise wavefield in Japan. Rayleigh waves in secondary microseisms in the period range 4–8 s are sensitive to ocean wave activity in the adjacent sea, showing seasonal variations in directionality that are related to a pressure source region estimated by an ocean wave model. Compared with Rayleigh waves, the sources of the P wave energy are distant. The relative contribution of P wave energy in ambient noise increases when the adjacent sea is calm. At periods of 2–4 s, the directionality of the ambient noise wavefield seems to correspond to major tectonic boundaries. In addition to spatial variations of directionality due to the source distribution of microseisms in 2–4 s, strong crustal heterogeneities along tectonic boundaries may produce the abrupt change in the directionality. Trapping and scattering in thick sediments and around those boundaries may reduce the ambient noise directionality originating from their source distribution. The backazimuths at 1–2 s periods point towards the nearest coasts from each station without remarkable seasonal variations, which is attributed to strong path effects, wave scattering and attenuation. As polarization analysis is applicable through a wide frequency range our technique provides a means of characterizing the broad-band ambient seismic wavefield.