Abstract

Upper mantle velocity structure in the Kurile Islands, Kamchatka, and the Sea of Okhotsk regions of the northwest Pacific has been studied in detail by analysis of P- and S-wave travel-time data of 137 mantle earthquakes. Wave velocities were obtained at the depths of foci of earthquakes, in the inclined seismic zones in these regions, by using Kaila's (1969) analytical method. The P- and S-wave velocity functions obtained for the inclined seismic zones beneath the southern Kuriles, northern Kuriles, and the Kamchatka regions each show a linear increase of velocity to a depth of about 180 km. However, there are significant lateral variations of the velocity structure in the inclined seismic zones beneath these regions. The velocities determined for the southern Kuriles seismic zone increase from 8.15 km/sec at 40 km depth to 8.63 km/sec at 185 km depth for P waves, and from 4.59 km/sec at 40 km depth to 4.80 km/sec at 170 km depth for S waves. These velocities are 3 to 5 per cent higher than those found at comparable depths in the seismic zones beneath the island arcs of Japan, the northern Kuriles, and the Kamchatka regions. The velocity models obtained for the inclined seismic zones have been suitably modified such that the theoretical travel-time curves, calculated by using the modified velocity models, match well with the observed travel-time data in various regions. The modified velocity models thus derived, both for P and S waves, using different estimated velocity models for the inclined seismic zones in various regions obtained in the present study, are quite close to each other and represent the velocity structure in the surrounding normal mantle in the Kurile Islands-Kamchatka regions. Both P- and S-wave velocities in the inclined seismic zones are found to be relatively higher than those in the surrounding normal mantle, by 4 to 6 per cent in the southern Kuriles, and by 2 to 3 per cent in the northern Kuriles and the Kamchatka seismic zones.

The velocity functions for P and S waves, determined in the depth range from 290 to 640 km in the Sea of Okhotsk region, both reveal a sharp first-order velocity discontinuity at a depth of 390 km with an associated velocity jump of 9.1 per cent for P waves, and 5.4 per cent for S waves. The P- and S-wave velocities below this 390-km discontinuity increase linearly, however, with much smaller velocity gradients than those of Jeffreys (1939) velocity models to a depth of 640 km. The relatively lower than normal S-wave velocities obtained to a depth of 640 km may imply relatively higher than normal upper mantle temperatures at those depths in the Okhotsk Sea region.

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