Average seismic travel time. (A) Measured average travel time (blue points) using N_skip = 40 and the filtered data (red line). The vertical arrow defines the amplitude of the 11 yr cycle measured between active and quiet years. The VIRGO data were divided into segments of length T = 90 days with a sampling time of 90 days. (B) Residuals between and . (C) Power spectrum of (blue curve) normalised to P₁₁ = P (period = 11 yr). The red curve shows the Butterworth filter with a cutoff at 1.5 yr.