Seismic Crustal and Subcrustal Phases Propagation. Attenuation of Local Phases in Western Europe

摘要

The purpose of this study is to evaluate attenuation of local seismic phases in France. Using a set of six earthquakes which occurred in France or in its vicinity, the short period data recorded by a homogeneous seismic network (vertical component) have been used, within a range of epicenter to station paths of 1 deg to 10 deg. Global attenuation versus distance has been determined for each phase Pn, Sn, Pg and Lg and the influence of frequency analysed. For Pn and Sn waves they are of the order of 2, and 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 factor is of the order of 1/0.2 deg at 1Hz, a result to be compared with the values of 1/0.7 deg (Eastern US), 1/0.15 deg. and 1/0.35 deg (respectively Northern USSR and South Caspian Sea). The quality factor, computed for each of these local phases, shows a clear dependence on frequency, increasing from some 100 at 1Hz to some 1000 at 10 Hz probably due to scattering effects.