Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009geoji.179.1645t&link_type=abstract
Geophysical Journal International, Volume 179, Issue 53, pp. 1645-1657.
Astronomy and Astrophysics
Astronomy
2
Body Waves, Wave Scattering And Diffraction, Wave Propagation
Scientific paper
Many seismic observations have shown that strong heterogeneities exist in the bottom few hundreds kilometres of the mantle. Among different seismic phases, this region, that is, the D'' layer, can be most globally sampled by diffracted waves along the core mantle boundary. Here, we assess the amplitude and distribution of S-wave velocity variations in the D'' layer of an existing tomographic model. We compare observed SHdiff traveltime anomalies to synthetic ones obtained using (1) the coupled spectral element method (CSEM), which is our reference exact method, (2) non-linear asymptotic coupling theory (NACT) and (3) 1-D ray theory. Synthetic waveforms are calculated down to 0.057 Hz with a corner frequency at 0.026 Hz. In the first part of this paper, we compare the traveltime anomaly predictions from the three different methods. The anomalies from CSEM and NACT are obtained by taking cross-correlations of the 3-D and 1-D synthetic waveforms. Both NACT and standard ray theory, which are used in other recent tomographic models, suffer from biases in traveltime predictions for vertically varying structure near the core-mantle boundary: NACT suffers from saturation of traveltimes, due to the portion in the kernel calculation that is based on the reference 1-D model, while ray theory suffers from wave front healing effects in the vertical plane, exacerbated in the presence of thin low velocity layers. In the second part, we compare observed traveltime anomalies and predictions from CSEM. The data consists of 506 Sdiff traveltime anomalies from 15 events, obtained form global seismograph network records. The tomographic model does a good job at predicting traveltimes of Sdiff phases especially when the path mostly samples fast S velocity regions at the base of the mantle, such as beneath India, China, North America and Northern Pacific. The underprediction of the positive observed traveltime anomalies seems to occur in regions where the paths sample close to the border of the large low shear velocity provinces, such as beneath Southern Pacific, South of Australia, Philippine Sea plate and Northwestern Pacific Ocean. Inspection of the observed seismograms at higher frequencies indicate that (1) the gradient of structure is underestimated and (2) there are post-cursors to the Sdiff which bias the lower frequency traveltime measurements, and which are not taken into account by the tomographic inversion, because of the approximate theoretical framework.
Romanowicz Barbara
To Akiko
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