GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 1.3 263 events. An example of array analysis showing the occurrence of a tremor episode is plotted in Fig. 1. Results of array analysis show coherent phases characterized by small incidence angle (slowness in the range 0.1 s/km – 0.4 s/km), indicating a source located at some depth below the crater. The analysis of more than 4 years of array data and the careful inspection of signals previously recorded by the local monitoring network permitted the discovery of at least two dozens of tremor episodes occurred during the last decade. The array offers a further advantage consisting in the signal stacking, which improves the signal to noise ratio of coherent signals, thus permitting a better identification of seismic phases in low amplitude and emergent events like volcanic tremor and LF earthquakes. After the unexpected discovery of volcanic tremor, we committed particular attention and performed detailed analysis to any LF signals detected during the last decade. From such study we realized that pure LF events attributable to sources involving the interaction of fluids with rock are very rare at Vesuvius. In fact the most of LF signals recorded during the last decade revealed to be VT earthquakes with anomalously low frequency (La Rocca and Galluzzo, 2016). An example of such event is shown in Fig. 2a as recorded by three seismic stations installed upon the crater, likely around the epicenter. The predominant role of P and S direct waves in the seismograms is evident at all stations as observed usually for VT earthquakes, while the low frequency content is evident in Fig. 2b. More than 20 earthquakes with these features, hereafter referred to as LFVT, have been identified and studied. All of them are characterized by similar low frequency spectra, but the most important feature is the Ts-Tp > 1.2 s, as described below. Fig. 2 – ����������� �� � ���� ���������� �� ����� ������ �������� ��� ��� ����� ������� ���� � ��� � ������ ����� Seismograms of a LFVT earthquake at three summit stations (a) and their spectra (b). P and S direct waves typical of VT events are highlighted by background colored boxes. Stacked array signals of a tremor with many P-S wave pairs marked by color bar and label (c).