Group Velocity Distributions of Rayleigh Waves and Two Upper Mantle Models in the Pacific Ocean.

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Group velocity distributions of Rayleigh waves, which are expressed in terms of spherical surface harmonics, are newly calculated for the periods 40, 70 and 90 seconds. Lateral heterogeneity of the upper mantle under the ocean is represented by means of contour lines on the maps of group velocity distributions. Two upper mantle models for the youngest and oldest regions are proposed. It is found from the regional group velocity characteristics that the famous oceanic model 8099 corresponds to the region of approximately 85 million year old ocean-floor east of the Hawaiian Islands. Group velocity is lowest in the youngest region with an age of 0 m.y. and highest in the oldest region of about 150 m.y. The group velocity difference between these two regions amounts as much as 0.5 km/sec (about 13%) at a period of 40～50 seconds. The increase of surface wave velocity with age seems to be attributable to the thickening oceanic lithosphere and partly to the increasing shear wave velocities with age in the lithosphere or in the low velocity zone. The shear velocity in the low velocity zone for the youngest region is estimated to be 4.136 km/sec, and that in the oceanic lithosphere for the oldest region to be 4.728 km/sec. The thickness of the oldest lithosphere is estimated to be 85±5 km.
1978-09-30