LAPSE-TIME DEPENDENT QC-SPECTRA OBSERVED IN THE DINARIDES REGION (YUGOSLAVIA)

The paper presents results of the coda quality factor (Q(c)) measurements in the central Outer Dinarides and the central Adriatic Sea regions. The main objective of this study was to analyse quantitatively the frequently observed Q(c) increase with lapse time, t. 108 earthquakes were chosen for analysis, with lapse times between 30 and 130 s. After computing the coda amplitude spectra A(f) for 0.6 less-than-or-equal-to f less-than-or-equal-to 5.0 Hz, the curve ln(At) = a + bt(c) was fitted to observed data for each earthquake and for each frequency. Denoting its slope by p(t), Q(c) was then estimated as a function of t as Q(c)(t\f) = -pi-f/p(t). In this way coda decay rate measurements were done in a consistent way, regardless of the lapse time window used. These individual values were then weighted and averaged in order to calculate the regional Q(c)(f,t). The results indicate that in the region studied Q(c) increases both with frequency and lapse time, while the degree of frequency dependence decreases with lapse time. If Q(c) is expressed as Q(c)(t) = Q0(t) f(n)(t) then for lapse times between 30 and 130 s Q0 increases from 45 to 275 while n decreases from 1.0 to 0.6.