Seismology

Participants :

We study fine scale structure of the earth's deep interior using large-scale high-quality dense arrays to extract generally weak body-wave phases. We address low signal-to-noise ratios and interference with unwanted neighbouring phases introducing scale-dependent slowness filters with improved time-space resolution. We apply these tools to high-quality data from dense arrays located in North America to conduct regional studies of the CMB and D* using* compressional waves with unprecedented resolution. We specifically obtain observations of PcP-P differential travel-times of short-period teleseismic body waves providing essential constraints on the properties of this part of the mantle. Regions currently sampled span from Alaska and the north of Canada, to regions of the Pacific outside of the Pacific large-low shear-velocity province (LLSVP) and on its eastern boarder (near central America). We carefully analyze and, when possible and significant, correct for the main sources of bias, i.e., mantle heterogeneities, earthquake mislocation and intrinsic attenuation. The accuracy of our observations is then limited mainly by the highest frequency of the signals used and the level of noise. Although we focus on body-wave separation, the tools we propose are more general, and they may prove useful in other applications.

The aim of this project is to produce regional tomographic images of the Earth's interior using global seismographic network waveforms. In this objective, we developed a new method that allows to confine wave propagation computations inside the region to be imaged. The principle of the method is as follow:

1) The seismic wavefield induced by a distant Earthquake (i.e outside the region that we would like to image) is modeled globally in a reference Earth model (see bottom panels in figure 1); This is done once for all and the wavefield values are recorded versus time at the surface of the sub-volume that we would like to image.

2) The recordings obtained in the previous step are transformed in a set of seismic sources acting on the boundaries of the region to be imaged. When the model is left unperturbed, the new set of sources regenerates the original wavefield, i.e. as modeled in step 1, in the sub-volume of interest. When the model is perturbed inside the sub-volume, one obtain the same wavefield as the one we would have obtained by modeling seismic wave propagation globally but at a much lower computational cost. The method is illustrated in figure 1 and detailed in Masson et al., (2013), "On the numerical implementation of time-reversal mirrors for tomographic imaging".

Using this approach, we are able to compute 3D global seismograms using regional wave propagation modeling. As shown in figure 2, these seismograms contain a lot more information compare to those usually computed in regional full waveform inversions. These seismograms can be used with any tomographic method based on full waveforms.

Tomographic models have limited resolution and are therefore smoothed representations of the seismic velocity structure of the Earth. Unresolved small-scale structure can nevertheless influence the propagation of seismic waves and notably lead to the development of a seismic coda that consists of incoherently scattered waves. We examine in this project, how we can numerically model these scattered waves combining recent tomographic models of the Earth's mantle with randomly distributed small-scale scatteres, and what information we can extract about the statistical distribution of these scatters. With this procedure, we want to gain insight about the small-scale structure of Earth's mantle and about its influence on the propagation seismic waves.

Anomalous PKP observations as a function of the angle Xi (i.e. the angle between the Earth's rotation axis and the PKPdf path in the inner-core) have been observed for decades and ascribed to inner-core anisotropy. However, outer-core structures like polar caps of tangent cylinder may be responsible for at least parts of the "L-shaped" travel-time anomalies in polar paths. We collect a global dataset of thousands of PKPbc-PKPdf, PKPab-PKPdf and PKiKP-PKPdf differential travel-time residuals and compare the measurements with predictions of travel-time anomalies for models with outer-core structures or inner-core anisotropy.