Upper mantle attenuation and velocity structure from measurements of differentialSphases

Abstract

SUMMARY Although much progress has been made in determining the 3‐D distribution of seismic wave velocities in the Earth, substantially less is known about the 3‐D distribution of intrinsic attenuation. In this study variations in attenuation and shear velocity of the Earth's mantle are constrained using measurements of differential traveltime and attenuation. The data are broad‐band displacement SH seismograms filtered to have energy in the period range 8–20 s. The seismograms are obtained from over 600 globally distributed earthquakes of magnitude, Mw, 5.5 or greater. Differential traveltimes and differential t* values from multiple S phases are estimated by a waveform‐fitting method, resulting in approximately 4300 measurements for SS–S in the distance range 50°–105° and 1000 measurements for SSS–SS in the distance range 90°–179°. Each measurement consists of a differential traveltime and a corresponding differential t*. The differential traveltimes and t* values are inverted to obtain models of the lateral variation of shear velocity and lateral variation of qμ, where qμ=1/Qμ. The models explain the data well but have limited depth resolution. The velocity models show good correlation with previous studies; in particular, low velocities are observed underlying mid‐oceanic ridges and convergent margins and high velocities are found for continental regions. The qμ models show shield regions to be less attenuating than PREM, with ridges appearing as highly attenuating features. The models have limited depth resolution and to address this problem we also present a shear velocity model obtained from the combination of body wave and surface wave data sets.

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

“…Most body wave attenuation tomographies are based on variants of Equations (11.17) and (11.18) because they allow us to eliminate the contribution of the potentially uncertain seismic moment by measuring amplitude ratios of different seismic phases (e.g. Reid et al, 2001;Cheng & Kennett, 2002;Kennett & Abdullah, 2011). Early studies of Q in the Earth were primarily based on the decay of free oscillation peaks and long-period surface wave amplitudes (e.g.…”

“…As an alternative to surface waves, Reid et al (2001) estimated t * from amplitude ratios of globally recorded S, SS and SSS waves. Also neglecting the effect of focusing, the t * measurements were inverted for a degree-8 model of Q μ in the upper mantle.…”

“…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 Spectral-Element Method, Beowulf Computing, and Global Seismology

“…As much as the establishment of global digital networks of seismometers revolutionized global seismology in the past two decades, we believe that the SEM offers unique opportunities for the development in the next decade of a new generation of highquality mantle models and the determination of better earthquake source parameters (40). Ϫ1 dsϪ͐ S ray (␤Q) Ϫ1 ds distribution from (37). Large values of ␦t SSϪS * are often interpreted in terms of strong attenuation, whereas small values reflect weak attenuation.…”

Synthetic study on SS waveform splitting and complication

“…Among these waveforms, many researchers choose SS wave because of its strong energy. For example, SS wave is used to do some researches on the anisotropy in the crust and upper-mantle at its bounce point (Yang and Fischer, 1994;Wolfe and Silver, 1998), global and large scale mantle tomography of S wave (Grand, 2002), the Q value distribution of S wave in the crust and upper-mantle (Bhattacharyya et al, 1996;Reid et al, 2001). At the same time, together with the arrival time difference of SdS on '410'and '660' discontinuities of upper-mantle, SS wave has been used to the researches on topography of global upper-mantle discontinuities and distribution for the thickness of transition zone (Lawrence and Shearer, 2006;Schmerr and Garnero, 2006).…”