(a) ‘Modified Wadati diagram’ showing for each event, and for each pair of station (i, j), the difference between P-phase (T_Pi − T_Pj, x-axis) and S-phase (T_Si − T_Sj, y-axis) arrival times. In black is the best-fit line (which provides an estimate of an average V_p/V_s ratio of 1.885 ± 0.003), in grey are the theoretical lines for several values of V_p/V_s ratio. (b) Dispersion of the points around the best-fit line.