Reflections of P′P′ Seismic Waves from 0 to 150 Km Depth Under the Ninety East Ridge, Indian Ocean, and the Atlantic-Indian Rise

  1. John G. Heacock
  1. James H. Whitcomb

Published Online: 15 MAR 2013

DOI: 10.1029/GM014p0211

The Structure and Physical Properties of the Earth's Crust

The Structure and Physical Properties of the Earth's Crust

How to Cite

Whitcomb, J. H. (1971) Reflections of P′P′ Seismic Waves from 0 to 150 Km Depth Under the Ninety East Ridge, Indian Ocean, and the Atlantic-Indian Rise, in The Structure and Physical Properties of the Earth's Crust (ed J. G. Heacock), American Geophysical Union, Washington D. C.. doi: 10.1029/GM014p0211

Author Information

  1. Seismological Laboratory, California Institute Of Technology, Pasadena California 9110

Publication History

  1. Published Online: 15 MAR 2013
  2. Published Print: 1 JAN 1971

ISBN Information

Print ISBN: 9780875900148

Online ISBN: 9781118664049

SEARCH

Keywords:

  • Adams and Randall's GH branch;
  • Atlantic-Indian Rise area;
  • Bolt's AB and DF branches;
  • Largest-amplitude branch;
  • Moho discontinuity;
  • Ninety East Ridge areas;
  • P'P' data;
  • P'P' seismic waves

Summary

P′dP′ phases, that is, reflections of P′P′ seismic waves at depth d, are investigated for 0 < d < 150 km near the Ninety East ridge, Indian Ocean, and the Atlantic-Indian rise, south of the Cape of Good Hope. The P′P′ epicentral range is 55° to 80° Conversion of P′dP′ travel times to depth values strongly depends on the times and relative amplitudes of the main P′P′ branches, which until now have been uncertain. This conversion is made by comparing the observed travel time with the appropriate P′P′ branch and computing the depth of reflection d from the time difference, using a reasonable velocity distribution. The P′P′ data studied here best fit the times of Bolt's AB and DF branches and the time of Adams and Randall's GH branch. The largest-amplitude branch is found to be GH between 55° and 62.5° epicentral distance, AB between 62.5° and 72°, and DF between 72° and 80°. The largest amplitude of the P′P′ phase reflecting in an ocean area can be a reflection from the ocean surface or ocean bottom; reflections from the ocean bottom are the more common. The time separation between these two reflections can be up to 8 seconds for deep oceans. The beginning of the ocean-bottom reflection may be picked earlier if a slightly deeper reflector, such as the Moho, is present. The data indicate that some of the earliest largest-amplitude P′P′ arrivals delineate a discontinuity (possibly the Moho discontinuity) under part of the Ninety East ridge area. A depth for this discontinuity of 23 km is found under the ridge and beneath a shoal feature just to the west of the ridge. Errors in these depth estimates depend mostly on the velocity model used to reduce the data. Reflectors 20 north of the Atlantic-Indian Rise area are at 21 km, possibly shallowing toward the rise, and at 9 km. A deeper reflecting zone is seen in the Ninety East ridge area at about 15°S latitude. Its lower bound is at 102 km 6.5° west of the ridge, deepens to 137 km 3.5° west of the ridge, and shallows to 87 km under the ridge both at 15°S and 7°S latitude. This feature is believed to be related to the tectonics that formed the ridge itself. Magnetic and bathymetric evidence precludes the possibility that the Ninety East ridge is a crustal-spreading feature. Several possibilities could explain the depth variation of the reflecting zone. If the reflection zone is a partial-melting zone, the depth variation could be caused by migration of water or partial melt upward, leaving behind more solid rock and effectively shallowing the zone under the ridge, or thicker sediments at the sides of the, ridge may act as a thermal blanket and raise temperatures underneath, thus lowering the melting zone to the west. If the base of the zone is the base of the lithosphere, the depth variation could be caused by compressive buckling of the lithosphere or by sinking of a lithospheric slab dipping west.