Abstract
We present a globally distributed data set of ∼400 000 frequency-dependent SH-wave traveltimes. An automated technique is used to measure teleseismic S, ScS and SS traveltimes at several periods ranging from 10 to 51 s. The targeted seismic phases are first extracted from the observed and synthetic seismograms using an automated time window algorithm. Traveltimes are then measured at several periods, by cross-correlation between the selected observed and synthetic filtered waveforms. Frequency-dependent effects due to crustal reverberations beneath each receiver are handled by incorporating crustal phases into WKBJ synthetic waveforms. After correction for physical dispersion due to intrinsic anelastic processes, we observe a residual traveltime dispersion on the order of 1–2 s in the period range of analysis. This dispersion occurs differently for S, ScS and SS, which is presumably related to their differing paths through the Earth. We find that: (1) Wavefront-healing phenomenon is observed for S and to a lesser extent SS waves having passed through very low velocity anomalies. (2) A preferred sampling of high velocity scatterers located at the CMB may explain our observation that ScS waves travel faster at low-frequency than at high-frequency. (3) A frequency-dependent attenuation q(ω) ∝q0×ω−α, with α∼ 0.2, is compatible with the globally averaged dispersion observed for S waves.
Highlights
Seismic tomography is a standard tool for constraining the structure of the Earth’s interior
Global body wave tomography, based on ray theory (RT), has revealed a variety of subducting slabs, some remain stagnant around the 660 km discontinuity, whereas others penetrate into the lower mantle
In our time windows selection and seismic phases isolation scheme (Section 2.1 and Appendix A), broadband seismograms recorded at the Global Seismological Networks (GSN) are first bandpass filtered between 7 and 85 s with a non-causal Butterworth filter, whose short- and long-period corners are denoted by T 1 (7 s) and T 2 (85 s), respectively
Summary
Seismic tomography is a standard tool for constraining the structure of the Earth’s interior. The effect of wave diffraction phenomena is to make traveltime anomalies dependent on Earth structure in the entire 3-D region around the geometrical ray path, rather than only on the infinitesimally narrow ray path itself Because this is not taken into account in RT, it seems progress toward obtaining higher-resolution images of small heterogeneities in the mantle requires a movement away from RT. The global S-wave FF tomographic model obtained by Montelli et al (2006b) is based on (S, ScS–S and SS–S) traveltimes measured in a single-frequency band (∼20 s), and does not benefit from the increased spatial resolution afforded by sensitivity kernels for a range of frequencies. We are especially interested in pointing out in our global data set frequency-dependent effects associated to wavefront-healing, scattering and attenuation
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