Ambient noise HVSR measurements in the Avellino historical centre and surrounding area (southern Italy). Correlation with surface geology and damage caused by the 1980 Irpinia-Basilicata earthquake

Abstract

The article deals with the results of extensive surveys conducted in the town of Avellino, in southern Italy, with the aim of studying the site seismic response. Avellino is a town located in the Apennine Chain in Irpinia, which is a region characterized by a high seismic hazard. Several strong earthquakes hit the town in the past, the last of which occurred on 23 November 1980 (Mw = 6.8). In the last decades since that event, background low magnitude seismicity persists, including few moderate earthquakes. In order to analyse the site seismic response, our data set was made up by borehole and downhole records, ambient noise measurements both in free-field and inside buildings, and macroseismic data related to the 1980 Irpinia-Basilicata earthquake. With the intention of ascertaining the occurrence of resonance effects influencing the distribution of the earthquake damage, we performed a correlated analysis of all the data acquired. HVSR free-field peak frequencies, joined with the data obtained from previous surveys [22], agree with the computed 1D resonance frequencies and are in the range between 1.6 and 13.0 Hz. The resonance frequencies for fifteen typical buildings in Avellino, which were partly estimated from ambient noise measurements and partly by applying the Italian regulation code, are in the range between 1.2 and 4.6 Hz, so showing that soil-structure resonance effects can be generated in a wide area of the town. Finally, we drew up a detailed damage map, related to the 1980 earthquake, which affected Avellino seriously. From our research two aspects come to light. The first regards the fact that soil-structure resonance effects can be generated in the town and were possibly associated to some buildings damaged by the 1980 earthquake. The second concerns the circumstance that the amplitudes of HVSR peaks correlate well with the rock/soil velocity contrast at depth, but do not show a relationship with the earthquake damage pattern. The results of this study will be useful in view of putting into the field suitable risk mitigation countermeasures.