Spatiotemporal variation of ambient noise wavefield in Japan

We develop a simple method to estimate back azimuth of ambient noise using a single station, which is a branch of widely used polarization analysis, and applied to continuous record of all 788 Hi-net stations to reveal spatiotemporal variation in directionality of ambient noise wavefield in Japan. For the development, we derived a relationship between source distribution of ambient noise and cross spectra of vertical and horizontal components at a single station by representing ambient noise wavefiled as a superposition of uncorrelated plane waves. The cross spectra of vertical and horizontal components correspond to the first order terms of a Fourier series expressing noise source distribution. Using the relationship, we estimated the directionality of ambient noise wavefield in whole Japan. In a period band of 4-8 s, the back azimuths showing spatially and temporally coherent pattern well point to the track of low pressure migrating through the sea around Japan. In contrast, in a shorter period band of 1-2 s, back azimuth of each station is oriented to the direction to the nearest sea. In a middle period band of 2-4 s, the spatial variation of back azimuth shows clear difference between southeast and northeast Japan separated by a major tectonic boundary, suggesting that ambient noise wavefield is also controlled by structural boundary.