The process of formation of seismic surface waves

The results of studies concerning the characteristics of the surface waves of earthquakes, carried on by many investigators, lead to the conclusion that: (a) in the case of near earthquakes, quite short and mostly irregular waves predominate, though at the same time, it is true, there appear as well very long waves; (b) the waves tend, with increasing distance, to become continuously longer and more regular; (c) at a distance equal to about a quarter of the earth's circumference, waves with periods of from twelve to twenty-four seconds contain the maximum energy. At still greater distances from the focus, the short waves are missing altogether; on the other hand, the waves of from thirty to fifty seconds period again become, in general, weaker, and after one complete circuit of the earth or still later (W_3 and W_4) one finds, as a rule, only very long waves (L) and waves with periods of from sixteen to twenty seconds. The two wave trains are propagated independently, the first with a velocity of from 4.4 to 4.5 kilometers per second, the second with approximately 3.3 kilometers per second and less.