Seismic Crustal and Subcrustal Phases Propagation.
Final rept. 1 Jan-30 Sep 80,
PARIS-6 UNIV (FRANCE) LAB DE GEOPHYSIQUE APPLIQUEE
Pagination or Media Count:
Some thirteen earthquakes which occurred around or inside France generated local waves recorded on the French seismic network run by L. D. G. up to 1500 km. A rather qualitative study of these phases Pn, Pg, Sn, Lg was first conducted phase identification, velocities computation, attenuation versus epicentral distance, comparison of the different quakes. Pn and Sn waves are not very sensitive to superficial propagation anomalies and have attenuation factor around gamma 2 with A proportional to K D to the -gamma power D distance. On the contrary Pg and Lg phases propagating through the crust are more sensitive to geological anomalies, and have consequently more dispersed attenuation factors. For one of the thirteen earthquakes off shore Brittany - 01. 15. 78 local phases have been studied in more details. By filtering process, the attenuation factor for each phase was computed versus distance from 0.5Hz to 16Hz. For each local wave, high frequency amplitudes attenuate stronger than low frequency amplitudes, with distance. The maximum energy of Pn phases lies in the 8-16Hz band for distances below 400 km, and shifts to the 4-8Hz band for distances beyond 400 km. Similarly Pg phases maxima shift from 4-8Hz to 2-4Hz for the same distance ranges, Lg maximum energy is found in the 0.5-4Hz band except for very short distances. In fact Lg attenuation factor versus distance increases with frequency, from 1.5 0.5-1Hz to 3.9 8-16Hz. On the other hand, the quality factor Q computed for each local phase also shows a clear increase with frequency.