Anisotropy in the oceanic lithosphere — theory and observations from the Ngendei seismic refraction experiment in the south‐west Pacific

Summary. P-wave travel-time data from a seismic refraction experiment on the 1983 Ngendei expedition to the south Pacific indicate anisotropy at two levels in the oceanic lithosphere. In the upper mantle, P-wave velocities vary between 8.0 and 8.5 km s-1 with the fast direction at N30°E. Crustal anisotropy within layer 2 is characterized by azimuthal P-wave velocity differences of about 0.2–0.4 km s-1, with the fast direction at N120°E, orthogonal to the upper mantle anisotropy. The observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust. We examine a model of P-wave propagation from a point source in an isotropic layer above an anisotropic half-space and show that under these conditions anisotropy can cause seismic ray-paths to deviate from the vertical plane connecting source and receiver. This deviation has a negligible effect on seismic travel times but must be taken into account in modelling P-wave polarization anomalies. P-wave polarization anomalies within the anisotropic half-space are the sum of the particle motion deviation from the ray-path, and ray-path deviation from the source-receiver azimuth, and are typically range-dependent. P-wave polarization anomalies will also occur in the overlying isotropic layer, purely as a result of the ray-path deviation, with greater deviations at long ranges. A simple model of upper mantle anisotropy (6kms-1 crust above 8.0–8.6 km s-1 anisotropic mantle) produces surface polarization anomalies of 1.5–3.5°. In practice, P-wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels. We examine the implications of anisotropic ray-path deviations on travel times and P-wave polarizations for the Ngendei data as well as some previous marine seismic refraction experiments.

[1]  M. Mottl,et al.  In situ electrical resistivity and bulk porosity of the oceanic crust Costa Rica Rift , 1982, Nature.

[2]  R. Carlson,et al.  An experimental test of P-wave anisotropy in stratified media , 1984 .

[3]  H. D. Garbin,et al.  The compressional modulus of a material permeated by a random distribution of circular cracks , 1973 .

[4]  S. Crampin,et al.  The polarization of P-waves in anisotropic media , 1982 .

[5]  Stuart Crampin,et al.  A review of wave motion in anisotropic and cracked elastic-media , 1981 .

[6]  G. Sutton,et al.  An overview and general results of the Lopez island OBS experiment , 1981 .

[7]  D. Bamford Pn velocity anisotropy in a continental upper mantle , 1977 .

[8]  N. Christensen,et al.  The seismic velocity structure of a traverse through the Bay of Islands Ophiolite Complex, Newfoundland, An exposure of oceanic crust and upper mantle , 1978 .

[9]  R. Stephen Seismic anisotropy observed in upper oceanic crust , 1981 .

[10]  J. D. Eshelby The determination of the elastic field of an ellipsoidal inclusion, and related problems , 1957, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences.

[11]  G. Backus Long-Wave Elastic Anisotropy Produced by Horizontal Layering , 1962 .

[12]  D. Bamford,et al.  Pn anisotropy studies in northern Britain and the eastern and western United States , 1979 .

[13]  H. D. Garbin,et al.  The shear modulus of a material permeated by a random distribution of free circular cracks , 1975 .

[14]  N. Christensen,et al.  The magnitude, symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites , 1984 .

[15]  Don L. Anderson,et al.  The effect of oriented cracks on seismic velocities , 1974 .

[16]  G. Backus Possible forms of seismic anisotropy of the uppermost mantle under oceans , 1965 .

[17]  N. Christensen,et al.  Structure and constitution of the lower oceanic crust , 1975 .

[18]  P. Spudich,et al.  A New Look at the Seismic Velocity Structure of the Oceanic Crust (Paper 80R0584) , 1980 .

[19]  T. Francis,et al.  Anisotropy of the Pacific upper mantle , 1969 .

[20]  Paul G. Richards,et al.  Quantitative Seismology: Theory and Methods , 1980 .

[21]  N. Christensen Seismic Anisotropy in the Lower Oceanic Crust , 1972, Nature.

[22]  N. Christensen,et al.  Geology and seismic structure of the northern section of the Oman ophiolite , 1981 .

[23]  Stuart Crampin,et al.  Seismic-wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic , 1978 .

[24]  R. S. White,et al.  An investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45°N, Mid-Atlantic Ridge , 1984 .

[25]  H. D. Garbin,et al.  Elastic moduli of a medium with liquid-filled cracks , 1975 .

[26]  John A. Orcutt,et al.  Petrology and porosity of an oceanic crustal site: Results from wave form modeling of seismic refraction data , 1980 .

[27]  J. Honnorez,et al.  DSDP Hole 504B, the first reference section over 1 km through Layer 2 of the oceanic crust , 1982, Nature.