The process of formation of seismic surface waves1

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