abstract

The purpose of this study is to evaluate the attenuation of local seismic phases in France. We use the 25 short-period vertical stations of the French seismic network and select a set of six earthquakes which occurred in France or in its vicinity, and within a range of epicentral distances from 1° to 10°. We have determined the attenuation versus distance for Pn, Sn, Pg, and Lg phases and analyzed the influence of frequency. Attenuation coefficients for Pn and Sn waves are of the order of 2. They are slightly higher for Pg (2.3) and Lg (2.5) waves, but the amplitude of these waves is largely dependent on the azimuth of propagation. Lg/Pg ratios present a large scatter between 1 and 10 with a mean trend around 3 to 4. We found that the Lg anelastic attenuation coefficient is of the order of 0.2 deg−1 at 1 Hz, a result to be compared with the values of 0.07 deg−1 (Eastern United States), 0.15 deg−1 (Northern USSR), and 0.35 deg−1 (south of the Caspian Sea) obtained by Nuttli (1973, 1981). The quality factor, computed for each one of these local phases, shows a clear dependence on frequency, increasing from around 100 at 1 Hz to around 1000 at 16 Hz.

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