GNGTS 2016 - Atti del 35° Convegno Nazionale

260 GNGTS 2016 S essione 1.3 by considering all available data. During the past years many events have been classified as LF events. They are characterized by low amplitude, emergent onset, frequency content between 1 Hz and 5 Hz, and shallow source location. An example of LF event recorded at the array ARF is shown in Fig. 2. Signals with the same features but much longer duration, classified as tremor, were first detected in January 2015. The results of array analysis of data recorded on January 30, 2015, show the presence of many coherent bursts of energy for some hours during the day, particularly from 12:00 to 15:00. These coherent signals identify wavefronts with the same backazimuth from SE (between 140 and 150 degrees) and similar apparent velocity of about 0.7 km/s. The results of array analysis for one hour of such signals are shown in Fig. 3. The vertical component of the seismic signals recorded at one of the array sensors, bandpass filtered around 1.75 Hz, is shown at the top of Fig. 3. The coherence of the seismic wavefield among the array stations, backazimuth and slowness estimated by the array analysis are also shown in Fig. 3. Many signal bursts are characterized by coherence higher than 0.6, while their backazimuth is stable between 135° and 150° (clockwise starting from north). The average slowness of coherent signals is about 0.7 s/km, corresponding to about 1.4 km/s, which indicates a shallow source. By taking into account the raw arrival time at network stations with the best signal- to-noise ratio, we estimate a localization of the tremor source in an area located southwest of Solfatara crater (La Rocca and Galluzzo, 2015). The spectral content of tremor signals shows the most of energy in the frequency range 1.0 – 5 Hz, with the main peak at 2 Hz, the same observed for many small seismic events classified as LF earthquakes. In Campi Flegrei area LF earthquakes recorded during the last 15 years are located at shallow depth and their source is believed to be tightly related with the hydrothermal system. After a full comparison of tremor signals with many LF earthquakes, taking into account their frequency contents, waveform, amplitude, and propagation features observed at the array ARF, we conclude that the tremor Fig. 3 – ����� �������� �� � ���� �� ������ ����� �������� �� ����� ���� ���� ��� �� ������ �� ��� ������� ��� �������� Array analysis of 1 hour of tremor burst recorded at array ARF. From top to bottom of the figure, the filtered seismic signal at 1.75 Hz, the coherence between signals and backazimuth and slowness are shown. Blue symbol indicate the windows of signals with coherence > 0.6. Blue and magenta symbols for slowness indicate the results obtained with Beam Forming and High Resolution methods respectively.