Seismic Q estimates in Umbria Marche (Central Italy): Hints for the retrieval of a new attenuation law for seismic risk

In the Umbria-Marche (Central Italy) region an important earthquake sequence occurred in 1997, characterized by nine earthquakes with magnitudes in the range between 5 and 6, that caused important damages and causalities. In the present paper, we separately estimate intrinsic and scattering Q(-1) parameters, using the classical multiple lapse time window analysis (MLTWA) approach in the assumption of a half-space model. The results clearly show that the attenuation parameters Q(i)(-1) and Q(s)(-1) are frequency dependent. This estimate is compared with other attenuation studies carried out in the same area, and with all the other MLTWA estimates obtained till now in other tectonic environments in the Earth. The bias introduced by the half-space assumption is investigated through numerical solutions of the energy transport equation in the more realistic assumption of a heterogeneous crust overlying a transparent mantle, with a Moho located at a depth ranging between 35 and 45 km below the surface. The bias introduced by the half-space assumption is significant only at high frequency. We finally show how the attenuation estimates, calculated with different techniques, lead to different peak ground acceleration decay with distance relationships, using the well-known and well proven Boore's method. This last result indicates that care must be used in selecting the correct estimate of the attenuation parameters for seismic risk purposes. We also discuss the reason why MLTWA may be chosen among all the other available techniques, due to its intrinsic stability, to obtain the right attenuation parameters.