Velocity Gradients in the Continental Crust from Head-Wave Amplitudes

  1. John G. Heacock
  1. David P. Hill

Published Online: 15 MAR 2013

DOI: 10.1029/GM014p0071

The Structure and Physical Properties of the Earth's Crust

The Structure and Physical Properties of the Earth's Crust

How to Cite

Hill, D. P. (1971) Velocity Gradients in the Continental Crust from Head-Wave Amplitudes, in The Structure and Physical Properties of the Earth's Crust (ed J. G. Heacock), American Geophysical Union, Washington D. C.. doi: 10.1029/GM014p0071

Author Information

  1. Seismological Laboratory, California Institute Of Technology, And U.S. Geological Survey, Pasadena 91109

Publication History

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

ISBN Information

Print ISBN: 9780875900148

Online ISBN: 9781118664049

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Keywords:

  • Heat flow province;
  • Pg amplitudes data;
  • Q' values;
  • United States;
  • Velocity gradients

Summary

Small velocity gradients in a refracting horizon have a pronounced effect on the spectral amplitudes of head waves. Negative velocity gradients and anelasticity (Q −1) result in a similar amplitude decay with distance for narrow-bandwidth data. Positive velocity gradients result in a net amplitude gain with distance compared with the head wave from a homogeneous, perfectly elastic refractor. Wave-theoretical expressions for these effects applied to published amplitude data for the major crustal refraction branches, Pg and P*, suggest that the ‘granitic’ crust in the Basin and Range province has either negative velocity gradients of the order of 10−2 km/sec/km or an anelastic Q of the order of 400, whereas the ‘granitic’ crust in the eastern United States and on the California coast has slightly positive velocity gradients. Similarly, the ‘basaltic’ intermediate layer appears to have a negative gradient of the order of 10−2 sec−1 under the Snake River plain and null or slightly positive gradients under Lake Superior and Mississippi. Velocity gradients inferred from laboratory measurements on granite and basic igneous rocks, together with published geothermal gradients, are generally consistent with the gradients inferred from amplitude data.