Extending shear-wave tomography for the lower mantle using S and SKS arrival-time data

Seismic tomography using S wave travel limes faces the difficulty imposed b y the interference between S and SKS phases near 83 degrees epicentral dist ance, as the SKS phase overtakes the S waves in the mantle. If the cross-ov er is avoided completely by excluding S data beyond 82 degrees then no reso lution is available below 2200 km in the lower mantle. A partial solution i s to try to pick up the S phase beyond the cross-over which improves covera ge and resolution in depth. However, a much larger improvement can be made by following the first arrival with S character and including SKS informati on with S. Arrival times for both S and SKS phases and the event hypocentres have been taken from the reprocessing of data reported to international agencies. Ea ch event has been relocated, including depth phase information, and later p hases re-associated using the improved locations to provide a set of travel times whose variance is significantly reduced compared with the original d ata catalogues. S travel-time tomography including SKS information out to 105 degrees, prov ides tomographic images with improved rendition of heterogeneity in the low er mantle. The three-dimensional models of SV wavespeed relative to the ak1 35 reference velocity model show a significant increase in heterogeneity at the base of the mantle which marches the behaviour seen in results derived from waveform inversion. For most of the mantle there is a considerable similarity between the patte rns of heterogeneity in the S wave images and recent P wave tomographic res ults, but greater differences develop in the lowermost mantle. In the D " r egion the SV wavespeed patterns also show some differences from recent SH w avespeed results which mostly correlate with regions of recognised structur al complexity.