The P-wavespeed structure in the lowermost 700 km of the mantle below the central part of the Indian Ocean

Abstract

Earthquakes from the Indonesian region recorded at distances between 78 and 104° from broad-band stations of the Kaapvaal network were used to investigate the P-wavespeed structure of the lowermost 700 km of the mantle below the central part of the Indian Ocean. Analysis of arrival times indicates a maximum in slowness gradients at a distance of 88°, and wavespeeds in the lowermost 700 km of the mantle that are lower than in the reference model IASP91. These results suggest that the lowermost mantle is relatively hot. Measurements of P-wave amplitudes also show a pronounced maximum at a distance close to 88°. Phase-weighted stacking of waveforms from three deep-focus earthquakes do not show any later arrivals that might identify a triplication in travel times associated with the presence of a discontinuity at the top of D′′. However, synthetic seismograms produced by both the WKBJ and reflectivity methods for models having a small discontinuity at the top of D′′ show the presence of an amplitude maximum, due to constructive interference between the closely spaced signals near the cross-over point of a triplication whose branches are not clearly resolvable visually. The observed amplitude maximum, however, is too large to be explained by the smooth model MIDIND01 fitted to the travel times. Stacking of synthetic seismograms constructed from models with small discontinuities at the top of D′′ gives similar results to those for the real seismograms. The observed amplitudes, travel times, results of phase-weighted stacking and synthetic seismograms, together suggest the presence of a small P-wave discontinuity of 0.4–0.6% that defines the top of D′′ 210 ± 20 km above the core–mantle boundary.