High-density macroseismic survey in urban areas. Part 1: proposal for a methodology and its application to the city of Rome

The aim of this research is to provide an original contribution to the investigation of local macroseismic variations in urban areas by means of questionnaire surveys. In this paper, we propose a methodology to investigate earthquake effects in large cities. This procedure for a high-density macroseismic survey is here applied to the city of Rome (Italy) during the September 1997-April 1998 Umbria-Marche (Central Italy) seismic sequence. A sort of macroseismic network in the urban area was arranged, thanks to the co-operation of public high schools, where ad hoc macroseismic questionnaires were delivered to students. This method provided us with a large amount of macroseismic information related to the October 14, 1997 (Mw =-5.6; I0 VIII MCS; h ?10 km) and March 26, 1998 (Mw = 5.3; I0 = VII MCS;h ?50 km) earthquakes. In the first survey, 949 useful questionnaires were collected in 10 high schools and related to 669 observation points. For the second event, 1083 useful questionnaires were collected in 27 high schools and related to 928 (+39%) observation points. The mean data density in the urbanized sector reached 3.4 data/km2 in the first survey and rose to 4.7 (+38%) data/km2 in the second one. Such a high density was hardly achieved in previous macroseismic surveys in large cities. The sample reliability was checked considering the data distribution versus urban setting inhomogeneity and the percentage distribution of the main lithological units outcropping in the investigated area. Such reliability was also confirmed by the check of the data density distribution. All results confirm that the data sample is largely representative. Both the applications here shown proved that this method can be successfully performed in a large city