MICROTREMOR MEASUREMENTS IN PALERMO, ITALY: A COMPARISON WITH MACROSEISMIC INTENSITY AND EARTHQUAKE GROUND MOTION

摘要

The city of Palermo is an appropriate test site where the efficiency of microtremors in predicting ground motion properties during earthquakes can be checked. Palermo is a densely populated city with important historical heritage and was object of previous studies. Areas of local amplification of damage were identified in downtown Palermo using historical macroseismic data. Moreover, aftershocks of the September 6, 2002, earthquake (Mw 5.9, 40 km offshore) provided a dataset of seismograms that quantify spatial variations of ground motion. The availability of more than 2000 boreholes in the city allowed a reconstruction of the 3D structure of surface geology, indicating that all the higher damage zones correspond to sediment-filled valleys. The high variability of the surface geology is mostly due to the presence of two filled river-beds of about 150 m width. In the framework of the SESAME project (Seismic EffectS assessment using Ambient Exctations, funded by the European Union), 90 microtremor measurements were performed across several profiles crossing the soft sediment bodies. The measurement points were intensified close to the valley edges (every 20 m), according to our geological reconstruction. H/V spectral ratio on ambient noise (HVSR) show significant variations along each profile: as soon as the transition stiff to soft is crossed, a typical spectral peak exceeding a factor of 3 in amplitude appears in the HVSR. The peak falls between 1 and 2 Hz and, along each profile, the peak disappears as soon as the other edge of the valley is crossed. These results indicate that microtremors are sensitive to the presence of large impedance contrasts of deep soft soil, at least in the Palermo area, with an important implication: the HVSR method seems to be able to recognize conditions potentially favourable to the occurrence of higher damage even when local geological characters are masked by the urban growth. However, we were not able to establish a quantitative correlation between microtremor properties and ground motion (or damage) amplification.