GNGTS 2016 - Atti del 35° Convegno Nazionale

262 GNGTS 2016 S essione 1.3 frequency between 1 Hz and 5 Hz, emergent onset, shallow location, and not well defined P-S direct phases. On the other hand, typical features of volcanic tremor are the emergent onset and long duration (from minutes to months) compared with normal earthquakes, while the spectrum is usually rich of low frequency, with the most of energy in the 1 – 5 Hz range (Konstantinou and Schlindwein, 2002), the same of LF events. In the most of documented cases, both LF events and volcanic tremor are obviously related with eruptive activity, occurring before and during eruptions. Therefore volcanic tremor and LF events are the most important precursor phenomena in case of volcanic unrest. The features of seismic signals are well matched by source models based on the interaction of fluids (both magma and gas) with the feeding system (Chouet, 1996, 2003). The resonance of a fluid filled cavity excited by some triggering perturbation is often adopted as source model of LF events. The interaction of magmatic gas with the hydrothermal system is also believed to play an important role in the generation of volcanic tremor and LF earthquakes. At Mt Vesuvius for many decades after the last eruption of 1944, only VT earthquakes of low magnitude have been recorded (Del Pezzo et al. , 2004; D’Auria et al. , 2013). LF earthquakes are very rare (Cusano et al. , 2013), while LP signals related with the volcanic activity have never been observed. Short bursts of volcanic tremor were first recognized in the seismic wavefield in 2012, after the installation of a seismic array named VAS (La Rocca et al. , 2014, 2015, 2016). In this work we compare volcanic tremor and LF events with VT earthquakes to gain some insight about their source features. Data analysis and results . Seismic activity at Mt Vesuvius is monitored by a network of more than 20 stations and one array of 10 short period stations (La Rocca and Galluzzo, 2014, 2015). Array data are analyzed in several narrowband frequencies in the range 1 Hz – 5 Hz with the aim of identifying coherent phases of low amplitude in the background signal. This analysis permitted the discovery of many short bursts of volcanic tremor and some small LF Fig. 1 – ������� �� ����� �������� ���� ��� ������ ���������� ������� ��� ���������� �� � ������ ������ ����������� Results of array analysis with the method Semblance, showing the occurrence of a tremor burst. Backazimuth and slowness of windows characterized by semblance greater than 0.6 are plotted by bold dark symbol to emphasize the most important phases.