THE TIMES OF P AND S IN NORTHEASTERN AMERICA

I. LEHMANN

Abstract


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

References


DOI: https://doi.org/10.4401/ag-5680
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X