Abstract
Similar spectral ratio techniques were used for Q-determinations in two regions. In South America, Q P in the (somewhat seismically active) wedge bounded by the seismic plane, the trench and volcanoes, is 1000–2000. Beneath Japan for the corresponding region (which is a seismic) Q P is 400–500; this value is identical to that in the asthenosphere below the trench, i.e., there is no evidence for extra low Q-values beneath the volcanoes. In South America, the high Q P-values (up to 3000) extend to 350 km, but there do not seem to be any paths from deep earthquakes (600 km) with Q P greater than 1000, i.e., there must be a Q-inversion below 350 km. In Japan, however, the high Q P-values in the Benioff zone (≈3000) persist down to the deep earthquakes. Above the deep earthquake zone in Japan and Fiji, very low Q P-values (100) were found, in contrast to a minimum value of 350 in South America. The tectonic implications of these structures are that lithospheric plates have varying thicknesses; continental plates are several times thicker than oceanic plates.
Published Version
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