The spectral decay parameter κ in the region of Umbria-Marche, Italy

We analyze 44 earthquakes (M2.1.-4.1) recorded by the Marchesan Seismograph Network during the 1997 Umbria-Marche sequence. We calculated the spectral decay parameter kappa [Anderson and Hough, 1984] using 33 events (M2.2-4.1) and f(c) < 10 Hz located in the middle of the rupture are:a. Although the source-station paths have approximately the same distance (+/-2.5 km) and azimuth for a given site, the estimates of kappa show great variability at all the stations. Even events with small standard deviation on the estimate of kappa can have a value 2-3 times different from that estimated for other events located within the 2.5 km. This observation suggests that the attenuation near the source can change significantly in a small distance range. We separated source and path effects by performing a generalized spectral inversion. We dismissed events with source functions having corner frequencies between 10 and 20 Hz, the frequency band where kappa was estimated. With the best estimates of kappa we calculated the mean for each site, and we observed an apparent azimuthal variation of the attenuation. We used the observed changes of kappa to estimate the near-source attenuation, finding that in a distance of 2 km, Q may vary between 16 and 52, depending on the source-station direction. The results of the analysis suggest that the most likely cause of the observed variability of kappa is the spatial variation of Q near the seismogenic zone.