Dispersion and attenuation of mantle waves through waveform inversion

Abstract

Summary. We propose a new approach to the determination of elastic and anelastic parameters of the Earth’s structure from seismic data. Instead of measuring such functionals of the medium response as the phase delay or spectral ratios, we perturb parameters of the model to satisfy the observed waveforms. The advantage of our waveform inversion technique (WIT) is that it uses the properties of the Earth as a smoothing filter. Also, because the Earth’s structure is the common denominator, the method allows simultaneous interpretation of different functionals of this structure; for example, the waveforms of Rayleigh and Love waves for the same source-receiver pair. Following extensive testing of the method on synthetic and actual data, we subject results of our analysis of 37 recordings for various sources and stations to ‘pure path’ decomposition. The period range of analysis extends from 160 to 630s. We distinguish four types of regions: stable continents, areas tectonically active within the last 400Myr, ocean floors younger than 38 Myr, and old ocean floors older than 38 Myr. The results indicate significantly different responses of the oceanic and continental areas at long periods. Stable continental and tectonic regions have nearly the same dispersion for periods greater than 300-35Os, and then diverge rapidly, with stable continents showing higher velocities. The young and old oceans, on the other hand, become distinct at periods as long as 500 s; the old oceans are faster. In terms of shear velocity models, the data are consistent with a difference of about 4 per cent between the young and old oceans in a depth range from 400 to 670 km, while the continental regions are similar and close to the global average. At shallower depths our structures are conceptually similar to those inferred from previous ‘pure path’ analyses and short-period surface wave studies. While we cannot, yet, establish statistically significant differences in Q for the four regions, such differences are obtained if one distinguishes only between continents and oceans: the Q for the latter is 10-20 per cent lower in the period range of the analysis.