Rayleigh wave phase velocities in the Pacific with implications for azimuthal anisotropy and lateral heterogeneities

Abstract

The lateral distribution of fundamental-mode Rayleigh wave phase velocities in the Pacific has been calculated in order to determine the variation in velocity as a function of the age of the oceanic plate, the importance of azimuthal anisotropy, and the presence of secondary lateral heterogeneities. The data set used in this study consists of phase velocity measurements in the period range 20-125 s for 178 paths traversing the Pacific. Both the pure-path and spherical harmonic inversion techniques are used in this investigation with an emphasis placed on determining the effectiveness and resolving power of these methods in calculating the velocity distribution in an oceanic regime.The first-order velocity-age relation which dominantly characterizes surface wave dispersion in an oceanic environment can be adequately modelled by either the pure-path or spherical harmonic techniques. Azimuthal anisotropy is the dominant second-order effect and is best developed in regions less than 80 Myr in age. In this age zone, the faster direction of wave propagation is either in the direction of fossil seafloor spreading or in the direction of present-day absolute plate motion. In the western Pacific, there is a correlation with paleo-relative motion (fossil seafloor spreading) for periods less than 50 s. Lateral velocity heterogeneities which are not related to the age of the oceanic plate are found by pure-path inversions and are correlated with regions associated with shallow residual depth anomalies. This effect (slow velocities with respect to the velocity-age model) monotonically decreases as a function of period and is indicative of a shallow origin. Age independent anomalies were also modelled by the application of the pure-path and spherical harmonic methods in a sequential inversion.

Rayleigh-Love wave coupling in an azimuthally anisotropic medium.

“…This feature is consistent with above-mentioned measurement of ultramafic rocks (e.g., Christensen and Salisbury, 1979), and with petrological modeling of Estey and Douglas (1986). The overall feature of upper mantle anisotropy of KB-Z model is also quite consistent with seismic anisotropy observations in the Pacific Ocean; Pn velocity anisotropy of up to 8 percent (e.g., Shimamura et al 1983), a small azimuthal anisotropy of Sn velocity (e.g., Shearer and Orcutt, 1986), SH-SV polarization anisotropy (e.g., Mitchell and Yu, 1980), azimuthal anisotropy of up to 4 percent of mantle Rayleigh wave velocities (Nishimura and Forsyth, 1988) and a small azimuthal anisotropy of mantle Love wave velocities (Nishimura and Forsyth, 1985).…”

“…We do not take sphericity of the Earth into account, but basic features would hold good for a spherical Earth, since magnitudes of sphericity corrections are less than 0.1 km/s (Biswas and Knopoff, 1970;North and Dziewonski, 1976 for periods between 18 and 36 s. Figure 6 shows depth-profiles of the eigenfunctions of Y1, Y3, and Y5 at the locations of (1)- (8) In Fig. 9, phase and group velocities of 0L(1G) and 1R(2G) for a period of 30 s are enlarged along with those of a corresponding transversely isotropic model KB-T(broken lines), whose elastic moduli _?…”

“…Tanimoto and Anderson (1985) mapped azimuthal anisotropy of mantle Rayleigh wave velocities at periods between 100-250 s and concluded that the retrieved anisotropy pattern could simulate the pattern of return convection flow in the methosphere at depths of 200-300 km, which was derived from kinematic consideration by Hager and O'connell (1979). Nishimura and Forsyth (1988) and Suetsugu and Nakanishi (1987) suggested similar results for the lithosphere and asthenosphere under the Pacific Ocean with regional surface wave dispersion at periods shorter than 100 s. Now, it is widely recognized that upper mantle anisotropy observations could have an ability to map mantle convection flow at various depths.…”

Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model

“…Mckenzie, 1967;Parker and Oldenburg, 1973;Parsons and Sclater, 1977;Stein and Stein, 1992). The increase in shear velocity of the uppermost mantle with increasing age observed with surface wave dispersion is also well described by these simple thermal models that predict deepening of isotherms in proportion to the square root of age in young seafloor (Faul and Jackson, 2005;Forsyth, 1975;Kausel et al, 1974;Maggi et al, 2006;Nishimura and Forsyth, 1988;Ritzwoller et al, 2004).…”

Tracing the Hawaiian Mantle Plume by Converted Seismic Waves

“…Lithospheric thickness is estimated to be ~70 km beneath the island of Oahu from converted body waves (Bock 1991), and 85-100 km from Rayleigh wave dispersion to the northwest of Oahu (Woods et al 1991, Woods andOkal 1996) and between the islands of Hawaii and Oahu (Priestley and Tilmann 1999). This lithospheric thickness differs little from the thickness of 80-90 Ma undisturbed oceanic lithosphere (Nishimura and Forsyth 1988).…”