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Nicola Mauro Pagliuca, INGV (Italy)
Lucio Badiali, INGV (Italy)
François Lukaya , Goma Volcano Observatory (Congo, the Democratic Republic of the)
Arianna Lisi, INGV (Italy)
Alessandro Marchetti, INGV (Italy)
Tiziana Sgroi, INGV (Italy)
Dario Tedesco, Università di Napoli 2 (Italy)
Giuliana Mele, INGV (Italy)
Alex Garcia, INGV (Italy)
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Following the catastrophic eruption of Volcano Nyiragongo on January 17, 2002, a great effort has been devoted to the seismic surveillance of volcanoes Nyiragongo and Nyamuragira located at the North of Lake Kivu in the western branch of the East African Rift. The 2002 eruption destroyed half of the town of Goma, and tens of thousands of refugees have fled to adjacent Rwanda. In the 1977, a similar outpouring of basaltic lava killed almost 2000 people in less than half an hour.
Furthermore, the volcano erupted intermittently for most of 2004. To monitor seismic activity at Nyiragongo volcano for scientific and civil defence purposes, INGV (Istituto Nazionale di Geofisica e Vulcanologia) in cooperation with GVO (Goma Volcano Observatory) installed a new seismic network of six digital stations on the volcano edifice and created a real time acquisition system during the period November 2003 - May 2004. The present study is divided in two steps: 1) deployment of a telemetered seismic network with a given geometry able to efficiently monitor the two local active volcanoes, Nyiragongo and Nyamuragira; 2) analysis of the pattern of seismicity and crustal structure. (1) The network is composed by enlarged-band or broadband 3 component sensors, 24-bit A/D converters, GPS synchronization at the remote station, radio modem link on the 444-447 MHz frequency band, solar panels and batteries for power supply at the remote sites, a PC-based acquisition and analysis system. The network has been designed by INGV based on its previous experiences in digital seismic monitoring during volcanic crisis occurred in the last years in Italy. The safety conditions of the area have forced to install the whole network in safety areas mainly along the south flank of the volcano. The digital telemetered seismic network is currently operating and seismic signals are continuously recorded at the GVO. (2) In this study we selected, from the available dataset, the events occurred in 2004. Based on the waveforms and spectral analysis, volcanic-tectonic earthquakes (VT), long-period events (LP, between 1 and 3 Hz), and very long period events (VLP < 0.1 Hz) have been recognized. For the analysed period we located more than 100 events, using a simple two-layer model, already adopted in previous works in the Rift region (Mavonga et al., 2006) and which incorporate gravity data results obtained from a survey in the Western part of the Virunga volcanic complex (Zana et al., 1992). Using P waves arrivals time of local earthquakes, we constructed the 1D velocity model for the crust beneath the volcano using the VELEST code (Kissling et al., 1995). These results will be then compared with those available from the historical seismicity that hit this region. We will present these results and discuss their implications in defining volcano plumbing system and its relationship with the regional tectonic of the area.
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