Amplitude and phase data inversions for phase velocity anomalies in the Pacific Ocean basin

Abstract

Rayleigh wave phase velocities at periods 30-80 s in the Pacific Ocean are calculated by inverting phase and amplitude anomaly data using the paraxial ray approximation and the Gaussian beam method. The region is divided into 5"x 5" blocks, and approximately 200 source-receiver pairs from 18 well-studied events around the Pacific Ocean are used. First, we assume phase anomalies for the lithospheric age-dependent model. Next, conventional phase data inversions are conducted assuming great circle paths so that the phase discrepancies are reduced to less than T . This procedure is essential for later inversions using amplitude data. We then determine the residuals of both amplitude and phase terms by calculating ray-synthetic seismograms. Using the Born approximation for a 2-D wave equation, a nonlinear iterative inversion for phase velocities is performed with both residuals. FrBchet derivatives for the inversion consist primarily of two wavefields: (1) the wavefield at the model point from the source, and ( 2 ) the Green's function from the model point to the receiver. These wavefields are also calculated by the paraxial ray approximation and Gaussian beam methods. In the inverse formulations, the simple use of the conventional Backus-Gilbert approach yields undesirable results in the non-linear iterative case and an extra term is necessary to control the model perturbations in order to minimize departures from the a priori model. The use of this additional term guarantees that we are able to obtain a fairly reliable phase velocity model even in the present non-linear problem. In most cases residual variances are significantly reduced after two or three iterations as far as the starting model is fairly correct. Compared with the phase data inversions, this inverse scheme gives more reliable resolution and most of the inverted features in phase velocities are significantly larger than the uncertainty level while some features obtained by the great circle phase data Present address: Seismological Laboratory, California Institute of Technology, Pasadena, CA 9 11 25, USA. 6 162 K. Yomogida and K. Aki inversions are suspicious. The resulting model displays some interesting deviations from the lithospheric age-dependent model. For example, low velocity regions are correlated with the Hawaii, Samoa, French Polynesia and Gilbert Islands hotspots.

Time- and frequency-dependent polarization analysis: anomalous surface wave observations in Iberia

“…Using ray theory, Woodhouse & Wong (1986) showed that anomalous surface wave amplitudes can be caused by focusing or defocusing due to lateral refraction. Similar results were obtained by Yomogida & Aki (1987) for an inversion of Rayleigh wave across the Pacific Ocean using the Gaussian beam method. The ray tracing results of both studies indicate that ray paths can significantly deviate from the great circle.…”

Modelling curved surface wave paths: membrane surface wave synthetics

“…Such an approach was used, for example, by Yomogida & Aki (1987) recently. However, this approach is not quite correct according to our formulation in the previous section, although the solution cannot be grossly wrong.…”

Minor-arc and major-arc global surface wave diffraction tomography

“…Ray-theory is a high frequency approximation, however, which is not justi ed in the presence of heterogeneities whose length-scale is comparable to the wavelength of the wave e.g., Woodhouse, 1974;Wang & Dahlen, 1995. For the ray approximation to be valid, the rst Fresnel zone must be smaller than the scale-length of the heterogeneity, which places limitations on the lateral resolution of seismic models based on ray-theory. The Born or Rytov approximation for surface wave scattering e.g., Woodhouse & Girnius, 1982;Yomogida & Aki, 1987;Snieder & Romanowicz, 1988;Bostock & Kennett, 1992;Friederich et al, 1993, Friederich 1999Meier et al, 1997;Spetzler et al, 2001Spetzler et al, , 2002Yoshizawa & Kennett, 2002;Snieder, 2002 models the nite width of the surface wave sensitivity zone. Ritzwoller et al 2002 discussed the use of this approximation in the context of global surface wave tomography, calling the resulting method global di raction tomography.…”