Active tectonics in the central Apennines (Italy) - Input data for seismic hazard assessment

Quaternary tectonics and paleoseismological investigations have defined a reliable framework of active faults in the southern Umbria and Abruzzi Apennines. Two sets of NW-SE to NNW-SSE trending, 16 to 33 km-long, normal and normal-oblique faults or fault systems have caused the displacement of Late Pleistocene-Holocene deposits and landforms within the investigated sector. Available data on vertical offsets indicate that both Late Pleistocene-Holocene and Quaternary (since the later part of the Early Pleistocene; 0.9-1 Ma) slip rates range between 0.4 and 1.2 mm/yr (range 0.6-0.8 mm/yr preferred). Paleoseismological investigations show that recurrence intervals for surface faulting events are always greater than 1,000 years and are usually greater than 2,000 years. Both paleoseismological data and long-term seismicity show that activation of the investigated faults may result in earthquakes of M = 6.5-7.0. The extension rate across the two sets of primary faults ranges between 0.7 and 1.6 mm/yr. Horizontal seismic strain has been calculated to be 0.5-0.6 mm/yr, based on the summation of the seismic moment of M > 5.3 earthquakes which have affected the investigated area since 1200 AD. This value may be lower than that inferred through geological data, probably because the seismological record reliable for the addition of the seismic moments covers a too short time window (about 800 years) to be considered representative of the tectonic activity in the investigated area. This conclusion is corroborated by the large recurrence interval per fault (> 1,000-2,000 years) inferred from paleoseismological analysis. A comparison of the active-fault framework and historical-seismicity distribution indicates that the entire eastern set of active faults has likely not been activated since 1000 AD, thus indicating that the elapsed time since the last activation for several faults of the investigated area may be greater than 1,000 years. In terms of hazard, the highest probability of activation is related to the eastern set faults, due to the observation that the elapsed time for some of these faults may be similar to the recurrence interval. As an example, paleoseismological and archaeoseismological data indicate that the elapsed time for the Mt. Vettore and Mt. Morrone Faults may be greater than 1,650 and 1,850 years, respectively. These data may have significant implications for risk related to a number of towns in central Italy and to the city of Rome. As for the latter, in fact, monumental heritage has suffered significant damage due to earthquakes of M > 6.5 which originated in the investigated Apennine sector.