EVIDENCE OF A LATERALLY VARIABLE LOWER MANTLE STRUCTURE FROM P-WAVES AND S-WAVES

Examples of Gräfenberg‐array data showing anomalous P‐waves which typically arrive 3–5 s after the direct P‐wave and which have a slowness 0.7–0,8 s deg‐1 smaller than direct P are presented. This additional phase is most frequently observed for events located in the NE portion of the southern Kurile Island subduction zone 73°‐80° from Gräfenberg, but systematically disappears for events in the SW portion of this zone. Because of the magnitude of the slowness difference, these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab. Likewise, they may not be accounted for by near‐receiver structure because these phases are not seen for all Kurile events. If present they appear at all stations of the array but they follow direct P too closely to be a multiple from the Mono. Therefore, we conclude they are very likely caused by lower mantle velocity structure. The most likely explanation is the presence of a P velocity jump of about 3 per cent approximately 290km above the core‐mantle boundary, since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional P phase. the distribution of bounce points on this reflector for the Kurile events indicates a lateral extension of this velocity anomaly under northern Siberia of about 150 km by at least 200 km. the best fitting S‐wave model has a reflector in the same depth, but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the P and S velocity in D″. Few events from other regions in this distance range are suitable for a definitive analysis of this kind. From among this group some observations indicate a lower mantle anomaly under the Lomonosow Ridge and under northern Greenland; but since the lower mantle under western Siberia, northern Novaya Zemlya, the Azores Islands region and the USSR‐Afghanistan border region does not produce an additional phase in the Gräfenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature. Copyright © 1990, Wiley Blackwell. All rights reserved