Global surface wave phase velocity variations

Global Love and Rayleigh wave phase velocity variations from 85 to 250 s are determined using about 30,000 seismograms from earthquakes with M ≥ 6.0. The phases used are G1, R1, G2, and R2 arrivals. The data were obtained from the Chinese Digital Seismic Network, GEOSCOPE, and Global Seismic Network archives for the time period January 1980 to June 1992. All seismograms underwent careful quality control in the time and frequency domains. We consider the inversion damping and model parameterization issues, examining the data statistics and assuming that errors in data and model spaces are Gaussian random distributions with zero mean. It is not stable to invert for a model with resolution higher than about spherical harmonic degree, l = 22, given the path sampling and scatter of our data set, and the low frequency data only warrant expansion up to about l = 12. We find that a block model parameterization, with its local basis functions, has fewer artifacts than a high order spherical harmonic inversion which must be heavily damped. However, spherical harmonic functions provide intrinsically superior recovery of the long-wavelength structure. Our final models involve a hybrid parameterization, in which an initial iteration retrieves the low order spherical harmonic components that are used as an aspherical reference model for performing a final block model inversion. The optimal block size decreases with frequency. This approach exploits the separate attributes of global and local basis functions for resolving different components of the heterogeneity, allows for variable resolution for different frequencies, and provides a natural framework for embedding localized high resolution regional inversions into the global model. We perform inversions both with and without corrections for shallow crustal structure. At the shorter periods the phase velocity maps strongly reflect the surface tectonics, correlating with continental shields, mid-ocean ridges, and tectonically active areas. The tectonic correlation diminishes gradually with period.

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