P- and S-wave reflections from anomalies in the lowermost mantle

• Published in: Geophysical Journal International
• Main Author: Weber, M.
• Format: Text
• Language: English
• Published: Oxford University Press 1993
• Subjects: Articles; Kara Sea; Nansen; Nansen Basin; Siberia
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/115/1/183; https://doi.org/10.1111/j.1365-246X.1993.tb05598.x

Seismic P and S waves recorded at the GRF array in Germany are used to study the inhomogeneous structure of the boundary layer D″ at the base of the mantle. The use of the seismic array allows the detection of small-scale anomalies in the lowermost mantle. The lateral resolution attainable is about 100 to 400 km, i.e. more than 10 times better than with tomographic methods. The analysis of 13 years of GRF broad-band array data yields 255 events with high signal-to-noise ratio which are used to map the lowermost mantle. 74 of these events show anomalous P waves (PdP) which arrive 3–6 s after the direct P wave and have a slowness 0.7–0.8 s/° smaller than the slowness of direct P. Other events close to the events with PdP do not have such an anomalous phase. Using slowness, traveltime, amplitude and waveform information it is demonstrated that PdP is caused by an anomalous lower mantle velocity structure below the turning point of the P wave. If the pp phase (i.e. a P wave first reflected at the free surface near the source) is used together with the P phase, distinct and well-sepaarted P-velocity anomalies can be determined under the Nansen Basin, the Kara Sea and northern Siberia. The areas of the bounce points of PdP in the lower mantle have a lateral extension of about 100 by 200 km, but this is not the size of the anomaly, since the resolution of P waves at 1 Hz in this depth is 130 km by 260 km (Fresnel zone). The accuracy to which the depth of the reflector (2612 km under the Nansen Basin and 2605 km under northern Siberia respectively) can be determined for 1-D models is ±10 to 20 km. The velocity contrast at the lower mantle discontinuity is about 3 per cent ±1 to 1.5 per cent. Areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas. 2-D models of the anomalies reveal the range of adequate models and possible trade-offs. If the lateral extension of the anomaly is about 7° the reflector has to be 40 km deeper than in the 1-D model. If the dip of the ...