Towards new research strategies: silent seismogenic areas or silent sources?

Abstract

Some earthquakes, particularly the strongest ones, can re-occur within hundreds or thousands of years. Therefore, the areas whose "seismic history" seems to be totally lacking in information are indeed a problem. In the past, these "silences" were interpreted in the simplest way, as an indicator of a low degree of seismicity. More recently, the results of historical research, some geological observations and the limits imposed by the physics of the seismic cycle suggest that this interpretation is wrong and must be overcome by new multi-disciplinary strategies. These strategies will involve the use of both the general pattern offered by the knowledge on the historical seismicity, and the little, though valuable information gleaned from land geology. Similar to these "silent" or "missing" earthquakes are the cases of the "missed" earthquakes which occurred in such historical and territorial conditions that they went completely unnoticed. A third case regards "underrated" earthquakes, reported as events of moderate energy. "Missing", "missed" and "underrated" earthquakes call for some reflection on the problem of completeness in the catalogue, and require innovative research projects. In recent years the Istituto Nazionale di Geofisica and SGA have developed three of such projects. These concern: 1) Sicily in the period between the ninth and thirteenth centuries; 2) the Pollino area (Northern Calabria) before the nineteenth century; 3) the Velino-Sirente massif in the period between the eleventh and fourteenth centuries.