Quarter-wavelength representation of the velocity profiles. See Poggi et al. (2012). Characterizing the Vertical-to-Horizontal Ratio of Ground Motion at Soft-Sediment Sites. Bulletin of the Seismological Society of America, 102(6), 2741–2756.

Z0.8 is the depth where the average of the profiles reach the velocity of 0.8 km/s

Z1 is the depth where the average of the profiles reach the velocity of 1 km/s,

Z1.5 is the depth where the average of the profiles reach the velocity of 1.5km/s