Lower mantle high-velocity zone beneath the Kurils as inferred from P-wave travel time and amplitude data.

Abstract

We attempt to detect a high-velocity zone in the lower mantle beneath the Okhotsk Sea by analyzing travel time and amplitude data for teleseismic P-waves from intermediate and deep earthquakes in the Kurils. Travel time residuals with respect to the J-B tables are obtained by correcting ISC arrival time data for the Earth's ellipticity, station anomaly, and station elevation and are then inverted to obtain a P-velocity model for depths shallower than 1, 250 km in the Kuril subduction zone. The velocity model exhibits a dipping high-velocity zone down to a depth of 1, 200 km in the lower mantle immediately beneath the Kuril deepest seismicity. The velocity in the dipping zone is 2-3% higher than the ambient lower mantle. Resolution analysis supports the existence of a slablike high-velocity zone, but indicates that the depth extent of the zone cannot be well resolved by travel time data alone. We further constrain the velocity model by analyzing amplitude data, which are more sensitive to fine velocity structure than travel times. After correcting the amplitude data for the source radiation pattern, geometrical spreading, station anomaly, and instrument response, we obtain the amplitude anomaly due to laterally heterogeneous earth structure. The amplitude at a period of 1.5 s decreases by a factor of 2 to 4 at stations in the distance range between 50-65 and with azimuths perpendicular to the Kuril trench. To separate the effects of slab structure from those of large-scale mantle heterogeneity, we model the amplitude anomaly using a hybrid method combining a finite element (FEM) scheme with geometrical optics. The high-velocity zone obtained by the travel time inversion produces an amplitude reduction for the short-period P-waves which is comparable to the observed amplitude reduction. This suggests that the observed amplitude anomaly is primarily due to the lower mantle high-velocity zone. Comparison of the observed and theoretical amplitudes shows that a much larger velocity gradient is required near the upper boundary of the dipping high-velocity zone than near the lower boundary. A high-velocity zone with a maximum depth of 1, 000 km, thickness of 150 km, and velocity contrast of 3.5% gives the best fit to the amplitude data. Similarities in the velocity structure and the dip angle between the lower mantle high-velocity zone found by the present study and the upper mantle slab suggest that the former represents a lithospheric slab penetrating into the lower mantle.