|Title||Velocities of longitudinal waves in the upper part of the Earth's mantle|
|Source||Publications of the Dominion Observatory vol. 19, no. 10, 1959, 26 pages, https://doi.org/10.4095/311150 (Open Access)|
|Publisher||Canada Department of Mines and Technical Surveys (Ottawa, Canada)|
|Media||paper; on-line; digital|
|Related||This publication is reprinted from ANNALES DE GÉOPHYSIQUE, t. 15, pp. 93-118, 1959.|
|Subjects||geophysics; structural geology; mantle; seismology; geophysical interpretations; seismic interpretations; seismic waves; p waves; seismic velocities; earthquakes; epicentres; refraction; earthquake
|Illustrations||tables; graphs; cross-sections|
|Released||1959 01 01; 2018 11 13|
The investigation deals with Europe only and recalls the complexity of its structure. Because the P time-distance curve is now taken to be nearly a straight line up to about 15º
epicentral distance, the velocity at depth cannot be derived from it by the direct method, but by trial and error a possible solution is obtainable. The velocity gradient has to be taken quite small or zero in an upper layer. This was taken to extent
to 220 km depth and an abrupt increase of velocity and velocity gradient to set in at this depth. From about 15° onwards the P curve becomes associated with waves refracted in the deeper layer.
The velocities given in table 2 were adopted. Up to
22º the corresponding P curve is in good agreement with JEFFREYS' revised 1954 curve.
The properties of P and pP curves of shocks having their foci at some depth in the upper layer were considered. Some deep Rumanian earthquakes all from the same
focus were examined and their P times compared with those calculated. Some deviations seemed explainable only as due to local differences of structure. A deep earthquake in the Tyrrhenian Sea was also examined.
The adopted solution seems a
possible one, but it is not unique and more observations are required. More precise determinations of amplitude variation would be particularly useful.