GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 1.3 231 Monitoring the Campi Flegrei caldera through Passive Image Interferometry L. Zaccarelli 1 , F. Bianco 2 , M. La Rocca 2 , D. Galluzzo 2 1 Istituto Nazionale di Geofisica e Vulcanologia, sezione di Bologna, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli, Osservatorio Vesuviano, Italy Campi Flegrei caldera. The Campi Flegrei caldera is an active volcanic complex located in the southern Italy. The caldera includes a very densely populated area, as well as the western suburbs of the city of Naples. Its formation dates back to the occurrence of two major collapses: the Campanian Ignimbrite (39 ka), and the Neapolitan Yellow Tuff (15 ka) eruptions. In the followings volcanism continued and has been distinguished into 3 epochs of activity. After the last epoch a 3.4 ky of quiescence followed, and then the last eruption took place in 1538, generating the Monte Nuovo crater. The majority of the eruptive events has been characterized by high explosivity and was accompanied by pyroclastic density currents (Orsi et al. , 2009). This kind of eruptive activity means that it is particularly important the understanding of the Campi Flegrei volcanic processes in order to delineate a realistic volcanic hazard in this areas. What makes it even more complex than in other volcanoes is the existence of a very active hydrothermal system inside the caldera, which causes major deformations such as bradiseismic episodes. As sake of example the 1982-84 events generated 180 cm of maximum uplift and was accompanied by more than 16,000 earthquakes (with low-medium magnitudes, the more energetic events was a M = 4), centered in the Pozzuoli town (Aster et al. , 1992). In the last decades the constant subsidence of the caldera have been interrupted by four minor uplift episodes in 1989, 1994, 2000, and 2005-2006. The general pattern is that the resurgence phases are accompanied by seismicity occurrence, while the subsidence is an aseismic phenomenon (Saccorotti et al. , 2001). Thanks to the installation of a broadband seismic network in support of the permanent short-period monitoring stations, during the last ground uplift it has been possible to distinguish and analyse also some Long Period events among the Volcano Tectonic seismic swarms (Saccorotti et al. , 2007). After 2005, the ground started a new uplift phase that has been accelerating ever since, possibly indicating the early signs of a new period of volcanic unrest at Campi Flegrei (Chiodini et al. , 2012). In the three years period between January 2010 and December 2012 (which the present study is referred to), the Campi Flegrei experienced a low velocity uplift of 6 cm and, generally a low rate of seismicity, with some low energy seismic swarms. In particular, on March 30th 2010, a 147 volcano tectonic events swarm was recorded. The entire swarm occurred in the Solfatara area in a depth range between 1 and 2 km, with a maximum magnitude 1.2. During 2011 a low rate of very low energy seismicity occurred again mainly in the Solfatara area. The rate of seismicity increased during 2012. In particular, on September 7th 2012, a seismic swarm of several thousands events (maximum magnitude = 1.7) struck the area westward to Pozzuoli and la Solfatara, affecting the first 4 Km of the crust. It is remarkable that, the higher velocity values for the uplift (in the range [1.5 - 3] cm/month) have been recorded in the period July - August 2012, just before the occurrence of the September swarm.  The compositions of the fluid analysed in the area, shows a magmatic content that appears to be increased in the analysed period, as well as the spatial amount of degassing in the area aof Solfatara and Pisciarelli (just eastward of Solfatara). According to Chiodini et al. , 2012, this phenomenology may be addressed to the occurrence of repeated magmatic fluid injections in the hydrothermal system. Moreover, recently Amoruso et al. , 2014, modeling the 2011 - 2013 accelerated uplift with a two sources model (in agreement with the ones proposed by D������ ’Auria et al. , 2012), suggested a predominantly magmatic unrest in 2011 - 2013. From a seismological point of view, the Campi Flegrei area shows a spot activity. In this picture, it is particularly difficult to distinguish between unrests of magmatic or hydrothermal origin, especially because at this stage the volcano is quiescent and few data are available