The Q structure of the upper mantle: Constraints from Rayleigh wave amplitudes

Abstract

[1] The three-dimensional shear attenuation, or Q m , structure of the upper mantle is still poorly understood despite the increasing accuracy and sophistication of velocity models of the same part of the Earth. Here we present a set of three-dimensional Q m models of the upper mantle based on amplitude measurement of minor and major arc Rayleigh waves in the period range 70 -170 s. The models show that areas of low attenuation underlie continents to a depth of around 300 km and areas of high attenuation are associated with oceans and plate boundaries, particularly in the top 200 km. Below $350 km, the sensitivity of the data to attenuation structure decreases rapidly. It is well known that the amplitudes of surface waves can be strongly influenced by factors other than attenuation, in particular, focusing due to elastic structure and errors in source mechanisms. We consider the effects of focusing by discussing the implications of the results of Selby and Woodhouse [2000]. Comparisons with focusing predictions using phase velocity maps allow us to identify features due to focusing in maps of attenuation distribution. The influence of the source on the observed attenuation distribution is investigated by including a source term in the model inversion. We find that successfully combining the data at each frequency into one threedimensional model requires that this source term must vary with frequency, suggesting that a simple correction to the scalar moment of each event cannot explain the observations. Although the effects of focusing and the source can be significant, we find that the observed attenuation pattern is robust up to degree 8; however, shorter-wavelength structure can be strongly influenced by other controls. Finally, we use one of the models to predict the degree 2 attenuation pattern for 250-s Rayleigh waves. The observed pattern suggests that existing normal mode observations may be explainable in terms of structure above 350 km depth.INDEX TERMS: 7255 Seismology: Surface waves and free oscillations; 7218 Seismology: Lithosphere and upper mantle; 5144 Physical Properties of Rocks: Wave attenuation; 7260 Seismology: Theory and modeling;

The imprint of crustal density heterogeneities on regional seismic wave propagation

“…It follows that the apparent variations in attenuation induced by 3-D density structure can be on the order of tens of percent, thus being comparable to attenuation heterogeneities found on regional and global scales (e.g. Mitchell, 1995;Romanowicz, 1995;Haberland and Rietbrock, 2001;Selby and Woodhouse, 2002;Dalton et al, 2008;Kennett and Abdullah, 2011;Trampert and Fichtner, 2013;Zhu et al, 2015). This result highlights that 3-D density structure should be taken into account when using fullwaveform techniques to invert for 3-D variations in attenuation.…”

Global Imaging of the Earth's Deep Interior: Seismic Constraints on (An)isotropy, Density and Attenuation

“…Dalton et al (2008) compared their global Q μ model to those of Romanowicz (1995), Reid et al (2001), Warren & Shearer (2002), Selby & Woodhouse (2002) and Gung & Romanowicz (2004). When truncated at degree 8, the correlation of these models was found to be mostly below 0.4 throughout the upper mantle.…”

“…The contribution from focusing to the amplitudes of Rayleigh waves was found to be considerable, even for long periods between 70 s and 170 s. This motivated Billien et al (2000) to jointly invert phase and amplitude measurements of Rayleigh waves for degree-20 maps of phase velocities and attenuation. However, being concerned that focusing may not be predicted with sufficient accuracy by current velocity models, Selby & Woodhouse (2002) decided not to account for elastic effects in their inversion for 3D shear attenuation. A similar approach was taken by Gung & Romanowicz (2004) who neglected focusing as well as source/receiver corrections, because these factors seemed to have little effect on their degree-8 model in synthetic tests.…”

Can We Improve Estimates of Seismological Q Using a New “Geometrical Spreading” Model?

“…I note that in Q measurements using longperiod surface wave data, much improved laterally heterogeneous phase velocity models are nowadays available, enabling first attempts to approximately calculate the focusing and defocusing effects (SELBY and WOODHOUSE, 2002;DALTON and EKSTROM, 2006;ROMANOWICZ, 2009). These calculations lead to improved Q measurements and tomographic Q models.…”

“…Individual measurements of path-specific Q, or tomographic inversions of laterally varying Q models, are typically published with discussions of the measurement error or uncertainty (e.g., MITCHELL, 1995;XIE et al, 2004;PHILLIPS et al, 2005;ROMANOWICZ and MITCHELL, 2007;PASYANOS et al, 2009). The vast Q measurements and tomographic Q models collectively reveal two main features of Q in the Earth's crust and mantle: it is highly laterally variable (by a factor of 10 as compared to the velocity variation which is typically less than 20-30%), and often frequency dependent (e.g., DER et al, 1986DER et al, , 1987ANDERSON, 1989;MITCHELL, 1991;XIE and MITCHELL, 1990;MITCHELL and XIE, 1994;WIENS 1994, 1998;SOBOLEV et al, 1996;XIE, 1998;XIE et al, 2004XIE et al, , 2006GUNG and ROMANOWICZ, 2004;SELBY and WOODHOUSE, 2002;WARREN and SHEARER, 2002;SHITO et al, 2004;PHILLIPS et al, 2005;DALTON and EKSTROM, 2006;LEKIC et al, 2009;PASYANOS et al, 2009, also see summaries by ANDERSON, 1989;FLANAGAN and WIENS, 1994;MITCHELL, 1995;ROMANOWICZ, 1998;ROMANOWICZ and MITCHELL, 2007;SATO and FEHLER, 2009). These features have led to inferences of variations of temperature and occurrence of melting in the crust and upper mantle, variations of pore fluid content and other forms of small-scale crustal structure heterogeneities.…”