Active Stress Field Analysis: Examples from Central and Southern Apennines (Italy)

San Francisco (California, USA)

We present two examples of active stress field analysis conducted by integrating geological, geophysical and statistical data. The areas selected are Cittá di Castello-Sansepolcro in central-northern Apennines and the Val d'Agri-Melandro-Pergola in southern Apennines. Although low seismicity is recorded since the instrumental era (1980 to present), the areas have been repeatedly struck by moderate to large earthquakes in the historical time (since 461 B.C.). This suggesting that they may contain earthquake sources but are in a “quiescent” period. Our studies are focused on the characterization of the active stress field in these regions and on the definition of the spatio-temporal distribution of the earthquakes. Then, our data contribute to improve the knowledge of the “seismogenic behavior” of the areas and provide useful information for seismic hazard evaluation.
In order to detect the pattern of the active stress field, we analyzed -the earthquakes recorded by national and local seismic networks in the period 2001-2002, with particular attention to the sequences; - ten deep wells (down to 5.5 max depth) for borehole breakouts analysis.
At last, we have used two non-parametric statistical procedures (Tanner and Wong, 1984; Faenza et al., 2003) to characterize the spatio-temporal distribution of large historical earthquakes and to account tectonics-physics parameters that can influence the spatio-temporal variability.
The results show that the areas are characterized by:
i. tectonic structures favorably oriented with the active stress field oriented N44°+-18° in the southern Apennines and N50°+-17° in the central Apennines;
ii. stress regime with a mainly extensional kinematics;
iii. cluster distribution of seismicity;
iv. the probability that an earthquake with the M≥5.5 will occur in the next 10 years is about 40%.