Detailed Identification of Large Seismogenic Sources in Calabria, Southern Italy, Based on Recent Shallow Seismicity and Crustal Tomographic Structure vs. Local Geology

Although Calabria is perhaps the most earthquake-prone region of the Italian peninsula and one of the most active areas in the entire Mediterranean, the understanding of the geometry, kinematics and exact location of its main seismogenic sources is still at a rather early stage. We tackled this problem by performing a joint reinterpretation of (i) hypocentral locations and focal mechanisms of the local earthquakes recorded between 1978 and 2003 (about 7,000 events), (ii) the crustal tomographic structure of Vp and Vs seismic velocities derived from the same dataset, (iii) the distribution of potential sources of M5.5+ earthquakes recently hypotesized for the study area, and (iv) selected geologic and geomorphic information. We specifically focused on the portions of the study region affected by significant seismic sequences or swarms during 1978-2003. Then, we analyzed with different methods the hypocentral trends in the separate space-time clusters and performed separate tomographic inversions of earthquake data in the respective sectors. This step was aimed at obtaining the greatest accuracy of the velocity structure in the crustal volumes affected by the sequences. Given the complexity of the study region, during the investigation we encountered different anomalous or unexpected situations, including (a) spots of recent low-magnitude seismicity that do not correspond to any identified potential earthquakes source, and (b) large seismogenic faults that were partially or totally quiescent during the past 25 years. These were interpreted in the frame of the existing geologic and tectonic knowledge and to constrain current seismotectonic hypotheses.