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

We analyze 44 earthquakes ( M 2.1–4.1) recorded by the Marchesan Seismograph Network during the 1997 Umbria‐Marche sequence. We calculated the spectral decay parameter κ [ Anderson and Hough , 1984] using 33 events ( M 2.2–4.1) and ƒ c < 10 Hz located in the middle of the rupture area. Although the source‐station paths have approximately the same distance (±2.5 km) and azimuth for a given site, the estimates of κ show great variability at all the stations. Even events with small standard deviation on the estimate of κ 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 κ was estimated. With the best estimates of κ we calculated the mean for each site, and we observed an apparent azimuthal variation of the attenuation. We used the observed changes of κ 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 κ is the spatial variation of Q near the seismogenic zone.