Fourier analysis is made of P and pP waves radiated from a Hindu-Kush earthquake (focal depth; 207km) to study attenuation of the seismic waves in the mantle. The long-period seismograms are supplied by the U.S. Coast and Geodetic Survey.
In order to reduce random error in the observed spectra due mainly to incomplete correction for the effect of crustal layering, the P and pP wave observations are divided into three and two groups, respectively, according to their epicentral distances, and the average spectral structure is obtained for each of the groups. Assuming that the intrinsic Q
α is independent of frequency in the period range involved (3-30sec for P and 3.75-15sec for pP), three independent values of the differential attenuation are derived from the spectral ratios of the average spectra. These observed values agree with the theoretical ones expected for the MM8-Q
β better than with those for the modified Q
α-model by Teng (1966). This implies that Q
α is roughly equal to Q
β in the MM8-model throughout the mantle. To obtain a better agreement with the observations the MM8-model is modified. For the trial Q
α-model thus obtained, we have Q
α=340, Q'
α=140, and Q"
α=2370, where Q
α, Q'
α, and Q"
α are values of the average Q
α throughout the whole mantle, for the upper mantle (h<0.14R), and for the lower mantle (h>0.14R), respectively.
View full abstract