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

Abstract

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 κ [Anderson and Hough, 1984] using 33 events (M2.2-4.1) and fc < 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. Copyright 2000 by the American Geophysical Union.