PALEOSEISMICITY IN THE GRAN-SASSO MASSIF (ABRUZZO, CENTRAL ITALY)

The upper Maone and Venacquaro valleys, on the northern side of the Gran Sasso Massif (Apennines, Central Italy), have been studied in detail from the geological and geomorphological standpoints to date the activity of the faults that produced spectacular fault scarps in the Late Glacial and Holocene sediments and morphological features. The faults that produced the studied scarps have been active at least four times during the last 18,000 years, and each time, on the basis of the indications emerging from the study of the scarp heights, displacements of similar size were probably produced. The first faulting seems datable to a period slightly more recent than 18,000 years ago. The second faulting occurred after the moraines of the second Apennine Stage. These, preceding a lacustrine deposit dated 11,760 +/- 160 BP, could be put in the older portion of the period between 16,000 and 13,000 years ago. The third faulting must be more recent than 6000-7000 years ago, and slightly older than 5450 +/- 170 and 6110 +/- 180 BP. A fourth phase is appreciably later than 3490 +/- 160 BP and older than the year 1000 A.D. For the Venacquaro valley, the size of the displacements (more recent than 18,000 years ago), indicated by the height of the fault scarps and by geophysical data, is about 16-18 m; for the Val Maone the size of the displacements (more recent than the moraines of the second Apennine Stage) is 10-11 m. The respective slip-rates are 0.88-1 mm/year for the Venacquaro valley and approximately 0.67-0.78 mm/year for the Maone valley. It is very probable that the fault activity occurred during a seismic event or events that took place during a short time. The intervals between the seismic events or seismic periods (although these cannot be defined with precision) appear longer than 2500-3000 years and shorter than 6000-7000 years. It is possible that also in the Gran Sasso, as in other parts of the Abruzzo Apennines, seismic activity is characterized by events of great magnitude with long return periods.