SCATTERING OF TELESEISMIC WAVES IN THE LOWER CRUST - OBSERVATIONS IN THE MASSIF-CENTRAL, FRANCE

High-frequency coda signals consistently recorded by a temporary seism ic network of 29 short-period stations during a 6-month survey in the French Massif Central are concordant with independent evidence for a h eterogeneous lower crust obtained from wide-angle and near-vertical re flection experiments in the same region. The teleseismic recordings of 22 events have been analysed in record sections rather than as single station seismograms, Following the low-frequency (LF; 0.5-1.5 Hz) fir st P-arrival a high-frequency (HF) coda (2-4 Hz dominant frequency) ex ;:ends over several seconds duration. The HF signals become clearly vi sible after bandpass filtering, but can also be identified in the orig inal seismograms. In event sections the HF coda forms a consistent pat tern of reverberations which is characterized by the following propert ies: (1) the duration is typically longer on the radial (4-15 s) than on the vertical (3-11 s) component; (2) the beginning of the HF coda, referred to the picked LF first P-arrivals varies between 0-2.5 s, wit h a dominance around 1.5-2 s; (3) the amplitude of the HF coda is abou t 3%-10% of the primary LF P-phase amplitude; (4) the signals are inco herent between neighbouring stations and over the network; (5) the HF coda arrives dominantly around the plane of incidence of the teleseism ic wave in the first 2 s to maximum 4 s; afterwards a widening of the particle motion to an elliptical shape is observed, indicating arrival s off the sagittal plane. The data parameters (1) to (5) point to the conclusion that the HF signals are generated by a scattering process. Wide-angle and CDP-reflection experiments in the same region measured a similar reverberating coda pattern and located the origin also in th e lower crust. These observations and the data parameters of the HF co da suggest that the I-IF teleseismic waves are scattered at heterogene ities in the lower crust in the Massif Central. In this contribution w e mainly concentrate on the presentation of the teleseismic recordings and conclude with a preliminary structural model which contains rando mly distributed scatterers in the lower crust. (C) 1997 Elsevier Scien ce B.V.