Seismological Notes

Abstract

IN the Bulletin de V Academie Imperiale des Sciences de St. Petersbourg, VI S?rie, October 1, Prince B. Galitzin adds another valuable contribution to seismometry. In No. 14, 1909, of the same Bulletin he pointed out that the azimuth of an earthquake epicentre can be determined from observations made at a single station. This direction is that of the first longitudinal wave, and is obtained by taking the resultant of amplitudes recorded in two directions at right angles to each other. Because horizontal pendulums are usually oriented north-south and east-west, the displacements are given in these directions. If, for example, one instrument recorded 10 mm. of north-south motion and the other 10 mm. of east-west motion, we see that the direction of motion was N.E.—S.W., but we do not know if it came from the north-east or from the south-west. This is the question that Prince Galitzin answers. If the front of the first wave is dilatational in character this motion is towards and down to the epicentre, but if it is condensational it is away and up from the same. This distinction is made clear by the records of a seismograph recording vertical motion. Observations have shown that sometimes the first movement of this instrument is upwards and sometimes it is downwards. The upward motion indicates a condensation, and the latter a dilatation. Between July 2, 1909, and June 8, 1911, Prince Galitzin has determined the position of forty-two epicentres by methods in which these rules have been followed.