GNGTS 2016 - Atti del 35° Convegno Nazionale

266 GNGTS 2016 S essione 1.3 Focal mechanisms of recent seismicity at Campi Flegrei M. La Rocca 1 , D. Galluzzo 2 1 Università della Calabria, Cosenza, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia – Osservatorio Vesuviano, Napoli, Italy Introduction. Campi Flegrei caldera is a complex volcanic field characterized by two nested calderas and tens of craters produced by the eruptive activity of the last 15 ka. Many structures are recognized from the ground morphology and many other have been inferred from a number of studies carried out during the last decades (Orsi et al. , 2004). The most interesting phenomenon of the area is the bradyseism, an alternation of very slow ground subsidence and uplift. In the past the uplift phases have been accompanied by intense seismic activity, while subsidence phases are usually aseismic. At present a very slow uplift (two cm per year on average, http://www.ov.ingv.it/ov/en/campi-flegrei.html ) is ongoing since 2000, after the subsidence of more than 1 meter that followed the bradyseismic strong uplift occurred in 1982- 84. After that event, characterized by more than 16000 earthquakes of magnitude up to M4.2 (Aster et al ., 1992), the seismicity rate was extremely low for about 15 years. Then the number of earthquakes increased again, roughly following the uplift trend, with several swarms of VT events during the last years. The magnitude or recent earthquakes is rarely greater than 2, but the shallow location (often less than 1 km) makes those events felt as ground shaking and audible rumble as well. The source mechanisms of earthquakes is an important information to investigate the stress field in the area. Therefore the analysis of focal mechanisms of VT earthquakes is worth of attention from the seismologists. In this work we show the focal mechanisms of 36 VT earthquakes with 0.6 < Ml <2.5 occurred during the last decade. Data analysis and results. Computation of focal mechanism and/or moment tensor is a routine analysis on earthquake data. For medium to large events it is computable automatically by evolute software. On the contrary, the estimate of focal mechanism becomes less reliable and more difficult as the magnitude decreases. This happens because small earthquakes are recorded only at short distances, thus an adequate coverage of the focal sphere is difficult or impossible to achieve. Moreover, small earthquakes produce mostly high frequency signals that are very difficult to fit by synthetic waveforms. In such cases the estimation of the moment tensor becomes challenging, while the computation of fault plane solution through the polarity of direct waves remains the unique effective method. In volcanic environment, usually characterized by high heterogeneous structures, the problem is further complicated by the amount of scattered signal that can mask the polarity of direct S wave. The seismic monitoring network in the Campi Flegrei area has been greatly improved during the last 15 years, from 10 analog stations in 2000 (Castellano et al ., 2002) to about 30 stations at present (La Rocca and Galluzzo, 2015). Current operative seismic stations are shown in Figure 1 by blue triangle (broad band) and circle (short period), while diamond represent stations no longer available but used in this work. With such a high number of stations, during the last years even earthquakes as small as M1 located in the central part of the caldera may have been recorded by more than 15 stations around the epicenter and in a range of distance up to 6-8 km. The most of VT events are located at depth between 1 km and 3 km bsl. The shallow location constitutes an advantage for the computation of focal mechanisms because the focal sphere is better covered by direct ray paths and even the lower hemisphere may be sampled by the farthest stations. We selected about 100 VT earthquakes with 0.1 < M < 2.0 recorded during the last 15 years and tried to estimate their focal mechanisms. We have used the software FOCMEC (Snoke, 1984, 1989) which allows the use of polarity of P, SV and SH waves. The first step of this analysis, after the source location, is the rotation of horizontal components in the radial and transverse directions. While determining the polarity of P waves is straightforward, the cases of SV and SH often is far from obvious. In many cases the observation of integrated signals (ground displacement)