On the question of dispersion in the first preliminary seismic waves1

Abstract

By use of the Byerly-Jeffreys travel-time curve for P, and Geiger's method of least-squares adjustment, the epicenter of the Alaskan earthquake of October 24, 1927, was placed at and the time of occurrence was placed at 15^h 59^m 55^s ± 2^s, G.M.C.T. A second solution was obtained using Mohorovičić's multiple travel-time curves for P. The co-ordinates of the epicenter were the same as those given above, but the time of occurrence was found to be 16^h00^m, G.M.C.T. It has been held by some seismologists that anomalous dispersion can be observed in the first preliminary waves; i.e., that shorter periods travel faster than long ones. Investigations of periods were made with a view to testing this hypothesis, with the following results: Late arrivals of P show a tendency to have shorter first periods than do the early ones, in the Alaskan earthquake. If this is caused by dispersion at all, it must be by a normal and not by an anomalous type. In the Chilean earthquake of November 10, 1922, there is no functional relationship between first period and earliness or lateness of arrival of P. In the Alaskan earthquake there is no general tendency for either gradual increase or decrease of successive periods of the P phase at any one station. In both earthquakes, long and short periods often arrive together at the beginning of P. In the Chilean earthquake there is no functional relation between first period of P and epicentral distance. The general conclusion is that observation of periods gives no evidence for dispersion in waves of longitudinal type. It is shown that, if dispersion did exist, the travel time of the beginning would be a continuous function of epicentral distance, and, therefore, Mohorovičić's curves are not evidence for dispersion. The observations of the epicentral distances at which P₂, P₁, and P_n are most frequently recorded first are contrary to dispersion. In the Alaskan earthquake the distribution of first motion (condensation or rarefaction) is very complicated. Dispersion offers no explanation for this fact, and it is believed that complex movements at the source are responsible for the observed distribution.