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Measurements of Mantle Velocities of P Waves with a Large Array


Johnson, Lane Richard (1966) Measurements of Mantle Velocities of P Waves with a Large Array. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/D44Y-6K65.


A large array has been used to investigate the P-wave velocity structure of the lower mantle. Linear array processing methods are reviewed and a method of nonlinear processing is presented. Phase velocities, travel times, and relative amplitudes of P waves have been measured with the large array at the Tonto Forest Seismological Observatory in Arizona for 125 earthquakes in the distance range of 30 to 100 degrees. Various models are assumed for the upper 771 km of the mantle and the Wiechert-Herglotz method applied to the phase velocity data to obtain a velocity depth structure for the lower mantle. The phase velocity data indicates the presence of a second-order discontinuity at a depth of 840 km, another at 1150 km, and less pronounced discontinuities at 1320, 1700 and 1950 km. Phase velocities beyond 85 degrees are interpreted in terms of a triplication of the phase velocity curve, and this results in a zone of almost constant velocity between depths of 2670 and 2800 km. Because of the uncertainty in the upper mantle assumptions, a final model cannot be proposed, but it appears that the lower mantle is more complicated than the standard models and there is good evidence for second-order discontinuities below a depth of 1000 km. A tentative lower bound of 2881 km can be placed on the depth to the core. The importance of checking the calculated velocity structure against independently measured travel times is pointed out. Comparisons are also made with observed PcP times and the agreement is good. The method of using measured values of the rate of change of amplitude with distances shows promising results.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:(Geology)
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geology
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Smith, Stewart W. (advisor)
  • Anderson, Donald L. (advisor)
Thesis Committee:
  • Unknown, Unknown
Defense Date:10 May 1966
Funding AgencyGrant Number
Advanced Research Projects AgencyUNSPECIFIED
Air Force Office of Scientific ResearchAF 49(638)-1337
Record Number:CaltechTHESIS:03252016-135744669
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9634
Deposited By: Benjamin Perez
Deposited On:28 Mar 2016 21:50
Last Modified:28 Feb 2024 18:17

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