112 OLIVER AND DOHMAN [CHAP. 8 



the longitudinal-vertical plane. In the general case of the layered earth, both 

 Love and Rayleigh waves are dispersive as a result of wave-guide propagation. 

 For studies of phenomena involving dispersion, it is convenient to use the 

 concei)ts of phase and group velocity. Both phase and group velocity are, in 

 general, functions of wave period. Group velocity (the velocity with which 

 the energy travels) is measured in the conventional way, i.e. once the arrival 

 time of waves of a given period has been determined for a station, the group 

 velocity for waves of that period is simply the distance from the epicenter of 

 the shock divided by the travel time. Phase velocity is somewhat more difficult 

 to measure, requiring either information about the nature of the source or data 

 from two or more recording stations. Phase velocity refers to the velocity of the 



|P 16 28 16 30 fS |LQ ILR 



u — ■ — ^^-^ — ^^ ■^_^,.^^'-^.Y /y\/\/\/'\/\'yVvvwM//^AA ■ 



E 



N — — — ^ — ■^'^\'\f\\!\h^iA!r^j\r,\y^ 



Fig. 1. Three-component Palisades long-period seismograms of the Mid-Atlantic Ridge 

 .shock of 8 June, 1960, at 16-19-48 G.C.T. The epicenter at 3.5°N, 35°W is 31.1° or 

 3460 km east of Palisades. The body-wave phases, P and S, are followed by Love 

 (LQ) waves recorded on the N-S (transverse) component and Rayleigh (LR) waves 

 on the vertical and E-W (longitudinal) components. The long duration of the oceanic 

 Rayleigh-wave train is best seen on the vertical component. Much of the dispersion 

 of the Love-wave train is due to the continental portion of the path between the edge 

 of the continental shelf and Palisades. Clock correction is -|- 2 sec. Short-period waves 

 preceding LR and LQ and following S may correspond to higher modes. Symbol at left 

 indicates direction of ground motion corresponding to upward trace motion. 



individual Fourier component wave of a given period. In a dispersive wave 

 train, it is the velocity associated with a certain feature (say a peak or trough) 

 at the particular time that that feature is associated with a particular wave 

 period. Thus, as the dispersive wave packet j^ropagates, peaks and troughs 

 move through the ]jacket at a velocity different from that of the packet. This 

 velocity varies with ])osition in the packet, i.e. in association with waves of 

 different periods within the packet. 



Once the wave trains on a seismogram are identified, usually on the basis of 

 particle motion and velocity, it is possible to obtain the period as a function of 

 arrival time and in this way, to determine precisely group velocities for each 

 kind of wave. If adequate data are available, phase velocities may also be 

 determined. 



