LOCAL INTENSITY VARIATIONS IN THE CITY OF ROME DURING THE 1997- 1998 UMBRIA-MARCHE (ITALY) SEISMIC SEQUENCE AS INFERRED BY SCHOOL QUESTIONNAIRE SURVEYS

摘要

Two high density macroseismic surveys were performed in Rome during the September 1997-April 1998 Umbria Marche (Central Italy) seismic sequence. This work was developed by the cooperation among Istituto Nazionale di Geofisica (ING), Universita Roma Tre and public high schools of Rome. The study was carried out for the October 14, 1997 (15:23 GMT) earthquake (M_w=5.6; I_0=Ⅷ MCS; h ≈ 10 km), located about 115 km far from Rome, and for the March 26, 1998 (15:25 GMT) earthquake (M_w=5.3; I_0=Ⅶ MCS; h ≈ 50 km), 150 km far from Rome. The aim of the work was to investigate local macroseismic variations by means of school questionnaire surveys, collecting significant data to test historical issues and numerical modelling and to integrate, when available, ground motion and weak motion measurements. A sort of macroseismic network in the urban area was arranged in 27 public high schools, where suitable questionnaires were delivered to students after the two events. A large amount of macroseismic information was synthesized in 669 and 928 observation points, respectively. The sample reliability was checked considering the data distribution versus urban framework discontinuities and the frame of outcropping geological formations. The analysis on seismic effect distribution versus geological conditions outlined a preferential distribution of major effects on Holocene alluvial deposits. Recent alluvium are characterized by a higher MCS intensity (one degree, on average) with respect to volcanic and sedimentary formations, for both earthquakes. In particular, low intensity effects are largely predominant on pliocenic bedrock formations. Macroseismic variations were also investigated as a function of alluvium thickness and distance from the edge of the Tiber valley. A new tight correlation between largest effects and alluvium emerged in correspondence of the minor hydrographic network in modern suburban areas of Rome. All results seem to confirm the role of local geological heterogeneity in the seismic response of the area.