Deep magma accumulation at Nyamulagira volcano in 2011 detected by GNSS observations

- Detected pre-eruption inflation prior to 2011-2012 Nyamulagira eruptions from GNSS.
- Inflation occurred for 6 months prior to 2011-2012 Nyamulagira eruptions.
- Deep magma chamber expansion is the likely cause of pre-eruption inflationary signal.

People in the area of the Virunga Mountains, along the borders of the Democratic Republic of Congo, Rwanda, and Uganda, are at very high natural risk due to active volcanism. A Global Navigation Satellite System (GNSS) network, KivuGNet (Kivu Geodetic Network), has operated since 2009 for monitoring and research of the deformation of Nyamulagira and Nyiragongo volcanoes as well as tectonic deformation in the region. We detected an inflationary signal from the position time-series observed in the network using our detection method, which is a combination of Kalman filtering and principal component analysis. The inflation event began in October 2010 and lasted for about 6 months prior to the 2011-2012 eruption at Nyamulagira volcano. The pre-eruptive inflationary signal is much weaker than the co-eruptive signal, but our method successfully detected the signal. The maximum horizontal and vertical displacements observed are ∼9 mm and ∼5 mm, respectively. A Mogi point source at a depth >10 km can explain the displacement field. This suggests that a relatively deep source for the magma chamber generated the inflationary signal. The deep reservoir that is the focus of this study may feed a shallower magma chamber, which is the likely source of the 2011-2012 eruption. Continuous monitoring of the volcanic activity is essential for understanding the eruption cycle and assessing potential volcanic hazards.