P- and S-wave attenuation in the region of Marche, Italy

We have found empirical functions to describe the amplitude decay with dist ance of P- and S-waves in the Marche region of Italy. These functions resul t from the analysis of spectral amplitudes of 161 three-component digital r ecords from 26 earthquakes recorded by the Marchesan Seismograph Network (R SM) jointly run by the Italian Seismological Survey of Rome (SSN) and the G eophysical Observatory of Macerata (OGSM). The events have magnitudes in th e range 1.8 < M < 3.6 and were recorded at hypocentral distances between 9 and 125 km. In order to study the possible variations of the quality factor Q with the direction of wave propagation (Q anisotropy), we made independe nt estimates of the attenuation for P-, SV- and SH-waves using the longitud inal, vertically polarized and transversely polarized components, respectiv ely. For the whole distance range and in the frequency band analyzed (1 < f < 25 Hz), the frequency dependence of Q can be approximated as Q(P) = 108. 4 x f(0.70) for the P-waves, Q(SV) = 105.6 x f(0.65) for SV-waves and Q(SH) = 86.1 x f(0.70) for SH-waves. The small difference between the SH and SV spectral decay is within the statistical uncertainties of the estimates, su ggesting that Q anisotropy is negligible. However, when only deeper wave pa ths are used in the analysis, we observed that at low frequencies (f < 4 Hz ) the values of Q(SV) are significantly above the estimates of Q(SH). This observation provides additional evidence of the seismic anisotropy reported by Margheriti et al. (Geophys. Res. Lett. 23, 2721-2724, 1996) for the reg ion. The low Q anisotropy is possibly related with the weakly deformed Adri atic foreland. The high attenuation observed for both P- and S-waves can be explained by the double low-velocity layer reported by Ponziani et al. (Te ctonophysics 252, 391-418, 1995), since the seismic energy can be trapped b etween these layers. In addition, the low values of Q obtained for distance ranges between 5 and 50 km can be due to the presence of hot material in t he lower crust beneath the Northern Apennines, as suggested by Mele et al. (J. Geophys. Res. 102, 11863-11875, 1997). (C) 1999 Elsevier Science B.V. A ll rights reserved.