Fig. 16

a The first term of the phase kernel \(K_p\) for \(B=\cos \theta +1\). The first term has an elliptical shape. The yellow triangles show the station location. b The second term of the phase kernel \(K_p\). The second term has a hyperbolic shape. c The whole kernel of the phase kernel \(K_p\). The background phase velocity is 3 km/s. The frequency range is from 0.1 to 0.2 Hz. We also apply a cosine taper for averaging in the frequency range