Evidence of a laterally variable lower mantle structure from P- and S-waves

• Published in: Geophysical Journal International
• Main Authors: Weber, M., Davis, J. P.
• Format: Text
• Language: English
• Published: Oxford University Press 1990
• Subjects: Articles; Greenland; Novaya Zemlya; Siberia
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/102/1/231; https://doi.org/10.1111/j.1365-246X.1990.tb00544.x

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 290 km 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 ...