Constraining 6C-observed seismic anisotropy from seasonal ambient seismic noise

Our recent theory shows that the 6C ground motion (three-component translation and three-component rotation) of ambient seismic noise is capable of measuring the local seismic anisotropy using azimuth-dependent 6C-based cross-correlation functions. However, seasonal variations in ambient seismic noise result in large uncertainties in local velocity measurements due to inaccurate corrections in the azimuth of wave propagation. Here, we show that the time-dependent small azimuth variation of ambient seismic noise can be visualized using horizontal rotation-based cross-correlation functions, which can be applied to constrain the local seismic anisotropy of Rayleigh waves. We apply this approach to a small seismic array (deployed to retrieve the rotational motions of seismic ambient noise) of Pinon Flat Observatory in Southern California. The estimated anisotropy is compatible with results calculated based on azimuth-dependent 6C cross-correlation functions from multiple pairs of stations, demonstrating the applicability of the proposed method.