Investigating Attenuation and Velocity Structure from Surface-Wave Amplitudes

Investigating Attenuation and Velocity Structure from Surface-Wave Amplitudes We have investigated upper-mantle attenuation (1/Q) structure using a large data set of fundamental-mode Rayleigh-wave amplitudes measured from seismograms recorded by the stations of the IRIS/USGS GSN and other global networks. Historically, the development of Q models has lagged behind that of velocity models in large part because of the difficulties involved in measuring and interpreting seismic-wave amplitudes. Specifically, effects on the amplitude due to source excitation, focusing and defocusing by lateral velocity heterogeneity, and instrument response must be accounted for before the data can be interpreted in terms of anelastic structure. Despite these complications, improving models of seismic-wave attenuation in the mantle is an important objective of seismology, as Q is highly sensitive to temperature and can provide an independent set of constraints on the Earth’s internal structure that is complementary to the results of elastic-velocity tomography. Additionally, large lateral variations in attenuation will cause significant dispersion of waves traveling at different periods and must be considered when constructing and comparing velocity models derived from seismic observations from different portions of the seismic frequency band.
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Our inversion solves for spherical-harmonic maps of attenuation and phase velocity (for periods spanning 50 - 250 seconds) in addition to scalar amplitude correction factors for each source and receiver that provided data for the inversion. The effect of focusing on the wave amplitude is related to the second derivative of phase velocity perpendicular to the ray path; we correct for it using the linear approximation of Woodhouse and Wong (1986). The degree-12 attenuation maps that result from this analysis show a strong correlation with surface tectonics for intermediate periods that becomes less prominent at the longest periods (i.e., for waves sampling the transition zone). The top figure shows the retrieved lateral variations in attenuation for 150-second Rayleigh waves, which are primarily sensitive to structure between 150 - 300-km depth. The East-Pacific Rise, western United States, and Red Sea region are highly-attenuating features, while old continental shields such as the Baltic region, Canada, Antarctica, and the cratons of Africa appear as areas of low attenuation. When attenuation and source and receiver uncertainty are adequately treated, the amplitude data can also provide strong constraints on global phase velocities. The bottom figure shows phase-velocity maps for 150-second Rayleigh waves determined from the amplitude data alone (no travel times) (left) and from phase-delay measurements alone (right). The correlation between the two is quite striking (correlation coefficient = 0.76). It appears that when the extraneous effects on wave amplitude are properly accounted for, an improved image of 3-D anelastic structure can be achieved.

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