Seismic response from microtremors in Catania (Sicily, Italy)

A survey of environmental seismic noise (microtremors) was carried out within the Catania municipal area in May 1999. The aim of the data acquisition was to improve our prediction of the seismic ground motion locally, using Nakamura’s approach. The seismic noise was recorded at 39 different sites. These sites were chosen according to the following criteria: 1) significance in terms of geological, geotechnical characteristics; 2) presence of a lithological transition (e.g.: the transition from lava to sediments); 3) presence of other geotechnical or geophysical measurements; 4) alignment with some of the transects along which the ground motion had been simulated numerically. The data were acquired independently by two teams, who used different instrumentation. Measurements overlapped at seven locations. We have found that more than one half of the sites exhibit weak or no amplification. These sites are located either on lava (e.g., sites in downtown Catania and in the northern part of the municipal area) or on wellconsolidated sedimentary soils (Western districts of the city). The only sites which bear evidence of some amplification are located either on soil fill lying over lava, or on the fine alluvial deposits of the Catania Plain. Some sites displayed anomalous, though relatively small in most cases, H/V peaks, in the low frequency band 0.2-1 Hz. Our analysis, performed at some sites, comparing the measured H/V spectral ratios (HVSR) to those simulated numerically, supports the interpretation raised recently by other authors of an effect of the deep structure. Finally, the numerical simulations also predict an appreciable lateral variability of the HVSR. This implies that seismic noise measurements made at single, isolated points should be used with care in the interpretation.