SplitLab: A shear-wave splitting environment in Matlab

International audienceWe present a graphical user interface to facilitate the processing of teleseismic shear-wave splitting observations. In contrast to a fully automated technique, we present a manual, per-event approach that maintains user control during the sequence of processing. The SplitLab environment is intended to undertake the repetitive processing steps while enabling the user to focus on quality control and eventually the interpretation of the results. Pre-processing modules of SplitLab create a database of events and link the corresponding seismogram files. The seismogram viewer tool uses this database to perform the measurement interactively. Post-processing of the combined results of such a project includes a viewer and export option. Our emphasis lies in the application to teleseismic shear-wave splitting analysis, but our code can be extended easily for other purposes. SplitLab can be downloaded at http://www.gm.univ-montp2.fr/splitting/

Traveltime dispersion in an isotropic elastic mantle: Strong lower mantle signal in differential-frequency residuals

International audienceWe study wavefield effects of direct P- and S -waves in elastic and isotropic 3-D seismicstructures derived from the temperature field of a high-resolution mantle circulation model.More specifically, we quantify the dispersion of traveltime residuals caused by diffraction instructures with dynamically constrained length scales and magnitudes of the lateral variationsin seismic velocities and density. 3-D global wave propagation is simulated using a spectralelement method. Intrinsic attenuation (i.e. dissipation of seismic energy) is deliberatelyneglected, so that any variation of traveltimes with frequency can be attributed to structuraleffects. Traveltime residuals are measured at 15, 22.5, 34 and 51 s dominant periods by cross-correlation of 3-D and 1-D synthetic waveforms. Additional simulations are performed for amodel in which 3-D structure is removed in the upper 800 km to isolate the dispersion signal ofthe lower mantle. We find that the structural length scales inherent to a vigorously convectingmantle give rise to significant diffraction-induced body-wave traveltime dispersion. For both P- and S-waves, the difference between long-period and short-period residuals for a givensource–receiver pair can reach up to several seconds for the period bands considered here. Ingeneral, these ‘differential-frequency’ residuals tend to increase in magnitude with increasingshort-period delay. Furthermore, the long-period signal typically is smaller in magnitude thanthe short-period one; that is, wave-front healing is efficient independent of the sign of theresiduals. Unlike the single-frequency residuals, the differential-frequency residuals are sur-prisingly similar between the ‘lower-mantle’ and the ‘whole-mantle’ model for correspondingsource–receiver pairs. The similarity is more pronounced in case of S -waves and varies betweendifferent combinations of period bands. The traveltime delay acquired in the upper mantleseems to cancel in these differential signals depending on the associated wavelengths andthe length scales of structure at shallow depth. Differential-frequency residuals may thus proveuseful to precondition tomographic inversions for the lower-mantle structure such as to reducethe influence of the upper mantle for certain length scales. Overall, standard deviations of thediffraction-induced traveltime dispersion between the longest (51 s) and the shortest (15 s)period considered here are 0.6 and 1.0 s for P - and S -waves, respectively. For comparison, thecorresponding standard deviations of the 15 s residuals are 1.0 s and 2.8 s. In the lower-mantlemodel, standard deviations are 0.3 and 0.6 s, respectively, which gives an average lower-mantlecontribution to the total dispersion of 50 per cent for P -waves and 60 per cent for S -waves

Synthetic seismograms for a synthetic Earth: long-period P- and S-wave traveltime variations can be explained by temperature alone

International audienc