TOMOGRAPHIC STUDY OF UPPER MANTLE ATTENUATION IN THE PACIFIC-OCEAN

The quality factor Q has been measured in the Pacific ocean along many paths using long period seismograms of the Geoscope and IDA networks. The Love and Rayleigh phase velocity and Q(R)-1 have been regionalised in the period range 60s to 200s, and then simultaneously inverted at depth to obtain 3D images of S-wave velocity and attenuation Q(beta)-1 in the depth range 60-300 km. U(p) to 100s, three very attenuating zones appear, one under the East Pacific Rise (E.P.R.), another one around Hawaii, a third one east of Kermadec. By inversion, those three anomalies axe found stable down to 160km. Below 200km, lateral variations are less intense and differently distributed; the maximum attenuation seems to concentrate along a northeast trend in the North Central Pacific.