The Times of P and S in Northeastern America

For a study of Lg, the wave discovered by Press and Ewing (1952),tlir records of some North American earthquakes were used. Inthe course of this study it became elear that it was essential for thennderstanding of the nature of the Lg waves to know at what depththe earthquakes in which tliey were recorded originated, especiallyvihether their foci were above or below the Mohorovicic discontinuity.One of the shocks was the Helena shock of October 31, 1935 whichis considered to Ite shallow. But the opinions vary as to one of theothers, tlie strong Timiskaming earthquake of Noveniber 1, 1935. Itwas, therefore, attempted to redetermine the epicentre and depth ofthis earthquake. The tables of Jeffreys and Bullen, (J.-B.) (1940) wereused, but it was soon found that there was a systematic deviation fromthein. For distanees up to about 13", where ali the stations except onewere contained witliin a sector of 120", the slope of tlie P time-curvewas smaller than that of any J.-B. curve that would conform to theobservations at great distanees.This as a inatter of fact could not cause surprise, for in later yearsvelocitics of P waves greater tlian those corresponding to the J.-B.tables bave repeatedly been found for the layer immediately below tlieMohorovicic discontinuity. Well observed blasts and rockbursts bavefurnished data that are much more accurate than those obtainablefrom earthquakes.