High-density macroseismic survey in urban areas. Part 2: Results for the city of Rome, Italy

High-density macroseismic surveys were carried out in Rome, following both the 14 October 1997 (M-w = 5,6; I-0 = VIII MCS), and the 26 March 199& (M-w 5.3; I-0 = VI-VII MCS), Umbria-Marche (Central Italy) earthquakes. The aim of the work was to investigate the relationship between intensity and loca l geology for the modern urban area of Rome, an issue yet to be examined in contemporary times. Intensity data for both earthquakes were correlated wi th near-surface geology. The results show a statistically significant ampli fication in Holocene alluvial deposits. In particular, we found one degree of difference in MCS intensity between hard rock and recent alluvium format ions. Furthermore, the distribution of the earthquake effects in alluvium-f illed valleys was studied as a function of the shape ratio (WI) of the allu vial basins. In the Tiber River valley, higher effects were found to be con centrated within a 150-m-wide band along the bedrock-alluvium surface conta ct, suggesting the occurrence of an edge effect. Surprising results were ob served in the minor alluvial valleys, which displayed the strongest effects in terms of shaking. The narrowest valleys (width < 150 m) revealed the hi ghest intensities, particularly where values of the shape-ratio range withi n 0.3 and 0.8. This behavior is in agreement with expected 2D resonance in small-sized alluvial basins. For areas of Rome urbanized after the damaging 13 January 1915 Apennine earthquake, these results contribute to a precise hazard assessment of areas located above recent alluvial deposits. Such a detailed analysis has the advantage of being fast and cheap and could be ea sily applied to other large cities, particularly where instrumental measure ments are difficult to obtain or not available.