HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) References Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by NIAZI, M. Search for Related Content

SH travel times and lateral heterogeneities in the lower mantle

SEISMOLOGICAL LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA 91109

Abstract

The horizontal long-period seismograms of two shallow earthquakes in Turkey and Iran recorded in selected azimuths are combined for travel-time studies of the SH wave beyond the angular distance of 40°. The observed travel times along two profiles which sample the deep mantle in the vicinity of Iceland and the North Pole show monotonically increasing differences beyond 65°, indicating lateral heterogeneity in the lower mantle. The travel-time difference becomes as large as 7 sec at 95°, implying a variation as much as 0.06 km/sec, or about 1 per cent, in the shear-wave velocity near 2,500 km depth. Inversion of observations, adjusted to surface foci, results in an average lower mantle structure with lower shear velocities than those given by Jeffreys. The difference exceeds 0.1 km/sec at the core boundary.

The arrival time and signature of S waves recorded in Greenland show anomalous features which may be related to deep seated anomalous zones associated with the Mid-Atlantic Ridge system.

Footnotes

^* Present address: University of Mashad, Mashad, Iran.