PO/SO SYNTHETICS FOR A VARIETY OF OCEANIC MODELS AND THEIR IMPLICATIONS FOR THE STRUCTURE OF THE OCEANIC LITHOSPHERE

Po/So synthetic seismograms were computed for a variety of oceanic structures in order to model data from an mb= 5.7 earthquake, recorded during the OSS IV seismic experiment. Satisfactory modelling of Po/So waveshapes and frequency content was achieved with no lateral heterogeneity, but with a small random vertical heterogeneity in the mantle, superimposed on a mean velocity structure which is consistent with seismic refraction data. the random heterogeneity in the P and S velocities appears to be about 2 per cent with a scale length of 5 km. This kind of heterogeneity is easily achieved by varying slightly the major mineral components in either the peridotite or eclogite mantle mineralogy and can be another cause of observed upper mantle anisotropy. Using a step‐function source time behaviour, our numerical modelling also indicates that in the upper mantle QP is roughly proportional to f0.22, rising from about 450 at f= 1 Hz to about 800 at f= 15 Hz, and that Qs is roughly proportional to f0.25, rising from about 900 at f= 1 Hz to about 1800 at f= 15 Hz. A slight decrease in Qs from these values in any small zone of the upper mantle reduces So amplitudes drastically and we believe this is why So is sometimes missing from observed seismograms. The computed synthetic record sections were also used to examine the relation between intrinsic Q and the apparent Q inferred from data in earlier studies. Apparent Q was found to be lower than intrinsic Q below about 5 Hz and higher than intrinsic Q above 5 Hz. When these differences are accounted for, the mantle Q's of this study agree with the mantle Q's of Butler et al. (1987) to within 10 per cent. There can no longer be any doubt that oceanic upper mantle Q's are very large, above 5 Hz, and that Qs is about twice as large as QP. Copyright © 1990, Wiley Blackwell. All rights reserved