Application of BSM to  a),  c),  b), and  d). In each panel, the running difference of the time-distance plot is overlaid with the contour (white dashed line) of the model function Acos(ωt − kx + φ) with the best-fitted parameters. The best-fitted parameters are follows: a) for , P = 180.0 ± 1.0 s, V_p = 48.0 ± 1.3 km s^-1; c) for , P = 180.0 ± 1.0 s, V_p = 50.0 ± 2.6 km s^-1; b) for , P = 180.0 ± 1.0 s, V_p = 48.0 ± 1.3 km s^-1; d) for , P = 180.0 ± 0.9 s,V_p = 50.0 ± 2.3 km s^-1.