Summary and Outlook
This study is concerned with the initial motion of the P waves at Pasadena and Huancayo. The results have been given in maps. In order to make the distribution more obvious, some boundary lines have been drawn. These boundaries are generally not to be taken as geometrical lines, but rather as transition zones. The appearance and positions of these lines can clearly be given in more or less detail, depending on the number of earthquakes used. For an earthquake well within an area of either compression or dilatation we can expect a clear first motion of P, whereas for earthquakes within mixed areas and for earthquakes on or in the vicinity of boundary lines we generally cannot.
As a general conclusion, we find that compressions and dilatations have certain definite geographical distributions, indicating that the general tectonics are the same within relatively large areas. Furthermore, these distributions are independent of time, throughout the period for which seismic records are available, and presumably the distributions change only with geological time. The distributions we have obtained in particular for Pasadena and Huancayo are also to be taken as reliable results. On the other hand, it should be strongly emphasized that the efforts made to explain some of the observed distributions in terms of tectonics are highly tentative. In order to arrive at more definite results regarding the general tectonics of the earthquake regions, we need the detailed distributions of compression and dilatation for a large number of stations. The author would like to use this opportunity to suggest strongly that these distributions be determined for the seismic stations over the world. In such work we have to bear in mind that only cases which are certain beyond doubt should be used. Naturally, the notations “compression” and “dilation” refer to the motion of the ground and not to the motion of the seismmograph pendulum. When such distributions have been determined for a large number of stations, they could be brought together to give more definite information. I should also like to suggest that the initial motion of P waves should be given in seismic bulletins whenever it can be determined with reliability, but only then. Directions determined only from records of horizontal seismographs may naturally be used also, once the location of the epicenter is known approximately.