Constraints on global maps of phase velocity from surface-wave amplitudes

Abstract

We demonstrate that global maps of phase velocity can be obtained from a data set of fundamental-mode Rayleigh wave amplitudes in the period range 150–250 s. Although the maps are constructed without any information about the wave phase, they are highly correlated with phase-velocity maps derived entirely from measurements of phase delay. We consider there to be four factors that contribute to the observed amplitude anomalies: focusing by lateral velocity heterogeneity along the ray path, variable attenuation along the ray path, uncertainty in the strength of excitation and uncertainty in the response at the station. In our first analysis, we desensitize the data to attenuation, source uncertainty and receiver uncertainty by combining the amplitudes of four consecutive surface-wave arrivals. The resulting quantity is inverted for even-degree spherical-harmonic maps of phase velocity, which explain 50 per cent of the variance in an independent data set of great-circle phase-delay measurements. In our second analysis, the amplitude measurements are inverted simultaneously for global maps of phase velocity expanded to degree 20, global degree-12 maps of attenuation and source and receiver correction factors. The velocity maps obtained from the simultaneous inversion exhibit strong agreement with published phase-velocity maps. The success of our analysis, which is based on linearized ray theory, suggests that the underlying assumptions about the smoothness of heterogeneity in the mantle may be reasonable, and illustrates the utility of amplitude data for constraining elastic heterogeneity in the Earth.