GNGTS 2016 - Atti del 35° Convegno Nazionale

268 GNGTS 2016 S essione 1.3 than 30 degrees are only two. Rake angle is negative for 32 out of 36 solutions, indicating a predominance of normal mechanism (88% of the total amount). The analyzed earthquakes are all very small (M<– 2.5) and their location is spread in a wide area of the caldera. Therefore it is difficult to associate their sources with the main geological structures recognized in the area. It seems more likely that the studied seismicity is associated mostly with many secondary, small faults, rather than with few larger faults. Discussion and conclusions. Focal mechanisms at Campi Flegrei were computed for the strongest earthquakes of 1982-84 bradyseismic episode (Gaudiosi and Iannaccone, 1984; De Natale et al. , 1985), and revised recently through a joint seismological - deformative approach (D’Auria et al. , 2015). In any cases the results show the predominance of normal mechanisms, the same indication given by our analysis on low magnitude recent earthquakes. The predominance of normal mechanisms suggest near horizontal orientation of the tensile stress field likely responsible of the observed seismicity. This feature, the spread epicenter distribution and the shallow location (less than 3 km) seem compatible with a small horizontal elongation of the shallow crust produced by the ongoing uplift phase. On the other hand, the horizontal elongation reduces the normal stress acting on dip and nearly vertical faults, thus favoring the occurrence of dip slip and strike slip earthquakes, of which type we have observed at least 10 out of 36. Our simple interpretation fit very well the observation that VT earthquakes occur only during ground uplift and not during subsidence periods. In fact during subsidence the shallowest part of the crust undergoes a shortening, thus the normal stress on existing dip and nearly vertical faults increases and this condition reduces the occurrence of earthquakes. Another implication of our simple model is that the role of regional stress field is negligible in the generation of Campi Flegrei VT earthquakes. References Aster, R.C., Meyer, R.P., De Natale, G., Zollo, A., Martini, M., Del Pezzo, E., Scarpa, R., Iannaccone, G. (1992). Seismic investigation of the Campi Flegrei Caldera, in “Volcanic Seismology”, Proc. Volcanol. Ser. III (Springler Verlag, New York). Castellano M., Buonocunto C., Capello M., La Rocca M. (2002), Seismic surveillance of active volcanoes: the Osservatorio Vesuviano Seismic Network (OVSN – Southern Italy). Seism. Res. Lett., Vol. 73, 2, pp. 168-175. D’Auria L., B. Massa, E. Cristiano, C. Del Gaudio, F. Giudicepietro, G. Ricciardi, C. Ricco (2015). Retrieving the Stress Field Within the Campi Flegrei Caldera (Southern Italy) Through an Integrated Geodetical and Seismological Approach. Pure Appl. Geophys. 172, 3247–3263. Del Gaudio C., I. Aquino, G. P. Ricciardi, C. Ricco, R. Scandone (2010). Unrest episodes at Campi Flegrei: A reconstruction of vertical ground movements during 1905-2009. J. Vol. Geoth. Res., 195, 48-56. doi:10.1016/ j.jvolgeores.2010.05.014. De Natale G., A. Zollo, A. Ferraro, J. Virieux (1985). Accurate fault mechanism determinations for a 1984 swarm at Campi Flegrei caldera (Italy) during an unrest episode: Implications for volcanological research. J. Geophys. Res., 100 B12, 167-185. Gaudiosi G., G. Iannaccone (1984). A preliminary study of stress pattern at Phlegraean Fields as inferred from focal mechanisms. Bull. Volcanol. 47-2. La Rocca M., Galluzzo D., 2015. ������� ���������� �� ����� ������� ��� ��� �������� �� ����� ����� ������������ Seismic monitoring of Campi Flegrei and Mt. Vesuvius by stand alone instruments. Annals of Geophysics, 58, 5, S0544; doi: 10.4401/ag-6748. Orsi G., M. A. Di Vito, R. Isaia (2004). Volcanic hazard assessment at the restless Campi Flegrei caldera. Bull. Volcanol., 66, 514-530, doi: 10.1007/s00445-003-0336-4. Snoke J.A., J.W. Munsey, A.C. Teague, G.A. Bollinger (1984). A program for focal mechanism determination by combined use of polarity and SV -P amplitude ratio data. Earthquake Notes, 55, #3, 15. Snoke J.A. (1989). Earthquake Mechanisms, Encyclopedia of Geophysics (D. E. James Ed.). Van Nostrand Reinhold Company, New York, 239–245.