P velocity heterogeneities in the lower mantle determined with the German Regional Seismic Network: improvement of previous models and results of 2D modelling

Abstract

P waves recorded at the German Regional Seismic Network (GRSN), the Gräfenberg-Array, the German Experiment Seismic System (GERESS) Mini-Array and two Czech stations are used to analyse the inhomogeneous lowermost mantle (D″) under Northern Siberia and the Nansen Basin. Events with a good signal-to-noise ratio are used to determine the extension of the anomaly. Relative travel time residuals compared with a 1D model with a relfector in D″ give information on the shape of the anomaly. Improved travel time residuals are derived by applying a source or/and station correction. This correction, which was not previously applied, reduces the size of the inferred peak to peak topography at the top of D″ by 20 km. If the sources are near lateral inhomogeneities, e.g. near the bend of a subduction zone, such corrections are necessary, otherwise models of D″ structure can be severely biased. Modelling of waveforms for such laterally inhomogeneous structures in D″ show strong effects on the wave field at dominant periods of 1 to 5 s, as multipathing of PdP (the P wave reflected at the top of D″), travel time differences and strong variations in amplitude, in agreement with the observations. Typical dimensions of such hills and valleys are 400 to 1200 km laterally and 10 to 100 km in topograph.