Seismic Hazard Increase in Intramountain Basins: the Case Study of Colfiorito, Central Italy

During the Umbria-Marche seismic sequence (September 1997 to April 1998) two small-aperture array experiments were performed in the Colfiorito basin. The diameter of the basin is 3 km, approximately. The array sites, which are 700 m from each other, are in the middle and at the eastern edge of the basin. Aftershock recordings show a completely different behavior between the two sites. In the middle of the basin, a strong focalisation of energy is observed around 1 Hz. S-wave synchronism lasts for few seconds, then low-apparent-velocity quickly-varying-backazimuth wavetrains cross the array for minutes even at magnitudes as low as 3. The energy increase exceeds a factor of 1,000 compared with a rock reference site. Seismograms of the array at the basin edge show the same predominat frequency, approximately, but smaller amplifications and durations; the low apparent velocity even during direct S waves suggests that the incoming wave field is strongly distorted by interference at the basin edge. Persistent backazimuths from E-SE indicate that, close to the edge, the largest amplitude ground motions are composed of edge-diffracted surface waves. An attempt is made to interpret observations in terms of source-to-receiver geometry and bedrock topography structure as inferred through seismic and geoelectrical surveys. Since weak and strong motions recorded in the basin indicate a tremendous local increase of the shaking level, the research efforts are now addressed to understand wheter the effects found in Colfiorito are a peculiarity of this basin or can be recognized as a possible common behavior of other intramountain basins.