SCATTERING AND ANELASTIC ATTENUATION OF SEISMIC ENERGY IN CENTRAL AND SOUTH-CENTRAL ALASKA

The characteristics of seismic energy attenuation in central and south-central Alaska are described. The data analyzed are short-period seismograms of local earthquakes recorded digitally in 12 bit at 120-Hz sampling rate by permanent stations of the regional network in the study area. First, the coda amplitude decay rate was compared for two depth ranges, 0-22 km and 63-133 km, using the single-scattering model. The results show dependence of Q(c)-1 on earthquake focal depth in the frequency (f) interval of 1 < f < 16 Hz; the deeper events yield consistently lower values for Q(c)-1 than the shallower events, similar to the results obtained by others for the Kanto district in Japan and for the Hindu-Kush region in Afghanistan. Next, the data were analyzed to separate the effects of intrinsic and multiple scattering on total S-wave energy, by applying the method of Wu to the hypocentral distance dependence of total seismic energy obtained from local earthquakes, which were normalized to a common source by the coda method of Aki. The results show the presence of strong scattering effects - the values of scattering attenuation (Q(s)-1) are an order of magnitude greater than the intrinsic attenuation (Q(i)-1) with the extinction distance in the range of 20-23 km. We found that Q(c)-1 is about a half of Q(s)-1 for the above frequency range, but is much larger than Q(i)-1.