FIG. R-3. Rayleigh-wave motion. (a) Theoretical Rayleigh wave has retrograde elliptical motion at the surface. (b) Hodograph of particle velocity involved in ground roll. (From Howell, 1959, 80.) (c) Experimental measurements from a small explosion measured with buried geophones, showing the change from retrograde to prograde motion with depth. (After Dobrin, 1951).