THE TIMES OF P AND S IN NORTHEASTERN AMERICA
Main Article Content
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.
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.
Article Details
How to Cite
LEHMANN, I. (1955) “THE TIMES OF P AND S IN NORTHEASTERN AMERICA”, Annals of Geophysics, 8(4), pp. 351–370. doi: 10.4401/ag-5680.
Issue
Section
OLD
Open-Access License
No Permission Required
Istituto Nazionale di Geofisica e Vulcanologia applies the Creative Commons Attribution License (CCAL) to all works we publish.
Under the CCAL, authors retain ownership of the copyright for their article, but authors allow anyone to download, reuse, reprint, modify, distribute, so long as the original authors and source are cited. No permission is required from the authors or the publishers.
In most cases, appropriate attribution can be provided by simply citing the original article.
If the item you plan to reuse is not part of a published article (e.g., a featured issue image), then please indicate the originator of the work, and the volume, issue, and date of the journal in which the item appeared. For any reuse or redistribution of a work, you must also make clear the license terms under which the work was published.
This broad license was developed to facilitate open access to, and free use of, original works of all types. Applying this standard license to your own work will ensure your right to make your work freely and openly available. For queries about the license, please contact ann.geophys@ingv.it.