Temporal Change in Spectral Coda Attenuation Q^-1_C Associated with the K=13.3 Earthquake of 1983 near Garm, Tadjikistan Region in Soviet Central Asia

Abstract

Coda decay of band-pass filtered seismograms were studied in relation to the occurrence of an earthquake of K=13.3 (M_L=5.2) in Soviet Central Asia. The gradient of coda decay is characterized by coda attenuation Q^-1_C for each frequency band. Examining the temporal variation of log₁₀(Q^-1_Cf) for small earthquakes which occurred in a restricted volume around the main shock focus during the period between 1979 and 1984, we found that the variances of log₁₀(Q^-1_Cf) for forerhocks were significantly larger than those for aftershocks for the frequency bands of 0.62-5Hz and 18Hz. Especially at the 5 Hz band, log₁₀(Q^-1_Cf) took a significantly lower value in the three-year period preceding the main shock compared with the preceding and following time periods. In the earthquake preparation stage, we may expect the formation of crack-clusters, which are composed of new cracks and reopened and connected existing cracks, in a fairly large region compared with the focal region due to the stress accumulation. The gradual appearance of inhomogeneously distributed crackclusters in the heterogeneous crust seems to be the most plausible mechanism by which the coda decay gradients are caused to scatter largely.