The rate of transmission of the Guatemala earthquake, April 19, 1902

Abstract

In 1900 I showed,* by a critical examination of the records of earthquakes, obtained at a distance from their origin, that three distinct forms of wave motion could he recognized, to which I applied the terms first, second and third phase, and that these travelled along different paths and at different speeds. I t was suggested that the first and second phases represented the outcrop of condensational and distortional mass waves, which had travelled through the earth, and that the third phase was due to waves, partly elastic and partly gravitational, which had travelled along or near the surface. These explanations have not been universally accepted, and alternative suggestions have been made, but the distinction of the three phases has been generally recognized, the nomenclature adopted, and the first two phases accepted as mass waves travelling through the earth. This last conclusion has been borne out by the time-curves published by Professor Milne, who, using data whose greater abundance compensated for a lesser degree of precision, deduced a set of time-curves essentially identical with mine, in that they showed a curvature in the first two phases which is only compatible with the supposi tion that they belong to mass waves.fIn Japan these conclusions have never been formally traversed, but in the more recent publications of that country we find that no less than eight phases are recognized, and designated by the symbols P1} P3 . . . P8; of these Pj and P3 correspond to the first and second phases of the last paragraph, while the remainder constitute the third phase. The nature of these third phase waves is still a very open question, and it is doubtful whether there is any real difference in the character of the wave motion of P3, P4, P 5, etc., or whether we are not dealing with waves of essentially similar nature, whose rate of propagation is a function of their period; in any case it is acknowledged that these waves are propagated along or close below the surface of the earth. The same conclusion is, however, also adopted for the first two phases, and the * ' Phil.