Crustal and Mantle Inhomogeneities as Defined by Attenuation of Short-Period P Waves
- John G. Heacock
Published Online: 15 MAR 2013
Copyright 1971 by the American Geophysical Union.
The Structure and Physical Properties of the Earth's Crust
How to Cite
Berg, J. W., Long, L. T., Sarmah, S. K. and Trembly, L. D. (1971) Crustal and Mantle Inhomogeneities as Defined by Attenuation of Short-Period P Waves, in The Structure and Physical Properties of the Earth's Crust (ed J. G. Heacock), American Geophysical Union, Washington D. C.. doi: 10.1029/GM014p0051
- Published Online: 15 MAR 2013
- Published Print: 1 JAN 1971
Print ISBN: 9780875900148
Online ISBN: 9781118664049
- New Mexico and Nevada profiles;
- P waves;
- Ray-tracing program;
- Shoal nuclear explosion
Attenuation was calculated by using the slope of the logarithm of the amplitude spectrum of first-arriving P waves at each epicentral distance. This was done for arrivals to distances of 90° from three nuclear explosions. Seismic arrivals from two profiles recorded in eastern New Mexico and Nevada by the U. S. Geological Survey were used to determine attenuation to depths immediately below the crust-mantle boundary. For regional and teleseismic distances, data from permanent stations recording the first arrivals on Benioff (short period) instruments were used. A ray-tracing program was employed to compute attenuation structure at shallow depths (60–70 km). The shallow attenuation structure is more complex than the velocity structure given for the eastern New Mexico and Nevada profiles. It is suggested that this type of information could possibly augment conventional interpretation of seismic refraction arrivals. An analysis of wave type is important to such an interpretation. Along refraction profiles, the average Q for the eastern New Mexico area was computed to be 169 ± 42 at 5 cps (frequency of peak amplitude) and that for the Nevada area was calculated to be 116 ± 38 at 4 cps. Ten models were fitted to the attenuation data for the mantle. The interpreted best fitting model yields: Q = 200 for depths to 200 km; Q = 400 for depths between 200 and 600 km; and Q = 2000 for depths greater than 600 km. The upper-mantle data apply to the western United States. There is indication that horizontal as well as vertical variations may occur in the attenuation structure of the mantle.