DIPPING STRUCTURE UNDER DOURBES, BELGIUM, DETERMINED BY RECEIVER FUNCTION MODELING AND INVERSION

Teleseismic broadband P and S waves recorded at the NARS station NE06 (Dourbes, Belgium) are shown to exhibit strong anomalous particle moti on not attributable to instrument miscalibration or malfunction. Azimu thally varying radial and tangential components have been observed on 38 recordings after vector rotation of horizontal P waves into the ray direction. The tangential P waves attain amplitudes comparable to the radial components from the east with negative polarity and west with positive polarity, but tend to be zero in the north and south, suggest ing major discontinuities in the crust dipping southward. The SH wave from the east contains a large SPmP phase, an S-to-P conversion at the free surface and then reflected back to the surface from the Moho. Th e polarity of this SPmP phase presents further evidence for a southwar d-dipping Moho. We employ ray theory for three-dimensionally dipping i nterfaces to compute the P-wave response. Linear inverse theory with s moothness constraints is applied to the simultaneous inversions of P-w ave receiver functions for four different backazimuths. Through the pr ogressive change of interface strike and dip and the inversion of laye r shear-wave velocities, a dipping crustal model that is consistent wi th both the observed waveforms and results of previous local geophysic al surveys has been determined. The results suggest a large velocity c ontrast in the shallow structure near the surface, another major inter face at a depth of 12 km with dip of 10 degrees, and a seismically tra nsparent unit below the interface. The interface at a depth of 12 km r eportedly emerges at the Midi fault 50 km north of the station NE06.