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LABORATOIRE DE GÉOPHYSIQUE INTERNE, E.R.A. C.N.R.S. NO. 603, 38041 GRENOBLE CEDEX, France
Abstract
Twenty-four seismic stations were installed for 1 month in October and November 1978 to study the seismicity and fault plane solutions of the Arette region (Pyrénées). Six of these stations were three-component magnetic tape recording instruments. We recorded about 115 events which could be located well, having magnitudes varying from 0.6 to 2.9. Nineteen of these earthquakes were recorded on more than two tape recording stations.
Spectra of a given event recorded on the three components of one station are similar, but there are differences in corner frequencies or high-frequency decays between stations and earthquakes.
We first studied Q factor (by assuming it is independent of frequency and that the source spectra are proportional to f–2 at high frequencies). We find a strong correlation of Q with hypocentral distance, which can be attributed to an increase of Qp from 120 to 170 and Qs from 180 to 350 in the top 10 km of the crust. The mean value of Qs 
1.6 Qp.
The coverage of stations and quality of datas allow us to correct spectra for site amplification, geometrical spreading, free surface reflection, and radiation pattern. The mean ration of P- and S-wave corner frequencies for a given event is 1.43, close to that predicted by Madariaga's model.
Madariaga's model was used to calculate source parameters. The highest stress drop calculated (170 ± 80 bars) is significantly higher than the mean stress drop (35 bars) of all the events and was obtained for the earthquake which was located where an earthquake of magnitude 5.1 occurred 15 months later, 29 February 1980, in a place where the seismicity had been below average.
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