The Rieti (Italy), Central Apennines, 1898 earthquake: how the study of moderate seismic events can help the earthquake hazard reduction
by
Valerio Comerci
Dipartimento di Scienze Chimiche Fisiche Matematiche, Università dell'Insubria, Via Lucini 3, 22100 Como, Italia
Coauthors: Alessandro M. Michetti (Dipartimento di Scienze Chimiche Fisiche Matematiche, Università dell'Insubria, Via Lucini 3, 22100 Como, Italia), Leonello Serva (ANPA (Agenzia Nazionale per la Protezione dell'Ambiente), via V. Brancati 48, 00144 Roma, Italia)

On June 27, 1898, an earthquake struck the Central Apennines at the southeast margin of the Rieti intermountain extensional basin, arguably rupturing a small sector of the Rieti Fault (RF) (the 25 km long NW-trending, SW-dipping normal fault that controlled the Quaternary evolution of the basin). The macroseismic epicentre of the earthquake is, in fact, located a few kilometres east of the Rieti town, and the epicentral intensity was of VIII degree in MCS scale, corresponding to an estimated macroseismic magnitude of ca. 5.5. It is therefore a classic moderate seismic event, however the knowledge of its macroseismic effects makes it possible to prepare ourselves for more catastrophic earthquakes (M 6.5 to 7.0) that, according to available paleoseismological data, ruptured the entire length of the RF during the Holocene, and characterize the seismic potential of the area. Due to several detailed aftershock reports, the study of the 1898 earthquake enables us A) to carry out the microzonation of the Rieti town and B) to calibrate the paleoseismic evidence, substantiating the derived seismic hazard assessment.

A) The careful study of the damage distribution in the Rieti town, and the detailed reconstruction of the local geological conditions, also based on the analysis of several boreholes, allow to identify the stratigraphic features that contributed to the significant damage in the ancient downtown of Rieti and in the area close to the Velino River banks.

B) Moderni (1899) observed near S. Rufina (few kilometres east of Rieti) "seven parallel large and long cracks" and near Castelfranco (few kilometres north-west of S. Rufina, along the RF zone) "five parallel long cracks" (vertical ground displacement is not reported, being probably insignificant). The comparison with the Holocene surface faulting features previously recognized at 3 trench sites across the same fault provides obvious constraints for the evaluation of the magnitude of the strongest earthquakes that the RF is capable to generate.

Date received: April 22, 2002