Report on Nyamulagira (DR Congo) — August 2003
Bulletin of the Global Volcanism Network, vol. 28, no. 8 (August 2003)
Managing Editor: Edward Venzke.
Nyamulagira (DR Congo) Rumbling and explosion sounds April-June, but no confirmed eruptions
Please cite this report as:
Global Volcanism Program, 2003. Report on Nyamulagira (DR Congo) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 28:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200308-223020
1.408°S, 29.2°E; summit elev. 3058 m
All times are local (unless otherwise noted)
According to reports from the Goma Volcano Observatory, since late October 2002 tectonic and magmatic seismicity at Nyamuragira has continued. Some of this seismicity was thought to be related to the refilling of a magma chamber emptied by a previous eruption. No eyewitness accounts of activity were reported until 26 February 2003, when a seismic crisis occurred. From 30 April into June 2003, local villagers reported rumblings and sounds of explosions coming from the volcano.
Activity during 27 October-14 December 2002. During 27 October-2 November, seismic data were collected at eight operational stations (Katale, Kibumba, Bulengo, Rusayo, Luboga, Goma, Lwiro, and Kunene). During this time volcanic seismicity was masked by aftershocks from a tectonic earthquake on 24 October 2002. Some rare magmatic events had hypocenters 10-25 km deep. During November 2002 epicenters of magmatic seismicity were concentrated in the NE area where the last eruption took place, an observation consistent with refilling of the magma chamber.
During 3-23 November, magmatic seismicity was more prevalent than tectonic seismicity, the latter dominated by aftershocks of the 24 October earthquake. The distribution of the magmatic earthquakes covered a zone 0-22 km deep, with an earthquake-free zone between approximately 3 and 7 km depth. The latter was interpreted as the location of a magma chamber, the same position as the chamber that fed the 27 July 2002 eruption. The tectonic earthquakes had depths of 0-30 km with an aseismic zone between 12 and 17 km. During the week of 24-30 November both tectonic and magmatic earthquakes were more frequent. Magmatic earthquakes increased at Katale to 348 from 239 during the previous week. High-frequency earthquakes in the E disappeared during the period. High-frequency earthquakes appeared in an isolated area in Virunga and densely NW of Lake Kivu in the area of Kalehe, where a landslide in late April 2003 killed ten people.
During 8-14 December the number of long-period earthquakes fell from to 169 at the Katale station from 239 the previous week, though the number of high-frequency earthquakes increased from 92 to 120. This increase was thought to be related to rifting in the area of the Large African Lakes. In general, volcanic tremors remained omnipresent. The epicenters of these long-period earthquakes were mainly concentrated in the NE of the central crater between 0 and 7 km depths. High-frequency earthquakes were concentrated in the aftershock zone of the 24 October 2002 earthquake in the Territory of Kalehe, NW of Idjwi Island.
Activity during February-March 2003. A seismic crisis started the night of 26 February and continued through the next morning. Seismicity increased greatly at the Goma seismic station; it was mainly tremor, but not at the same high levels of July 2002. Seismograms indicated clear increases in the numbers of both long-period and tectonic earthquakes and an increase in tremor amplitude. Visual observations were limited to the E flank, where the eruption of July 2002 started, but clouds obscured the summit crater.
Although seismic activity and warning phone calls occurred at the same time, there was no visible eruptive activity. Some very minor and brief activity (possibly witnessed) might have occurred in the central crater, which was not visible from the Rumangabo site. Seismic activity in late February included fracturing earthquakes, mainly on the N and NE sides of the volcano. Persistent long-period earthquakes were associated with magma movement. Short-period earthquakes associated with fracturing were observed for the first time.
A fresh outbreak of long-period earthquakes was noted in the NE quadrant during the week of 1-8 March, along with the growing presence of short-period events. Many long-period earthquakes occurred during the week of 16-22 March, including frequent fracture-related earthquakes.
Activity during April-June 2003. From 30 March through 27 April long-period earthquakes were concentrated beneath the NE flank, along with some short-period events. Although the number of long-period earthquakes decreased appreciably in late April and early May, similar seismicity continued through 21 June. The long-period events were distributed along a NW-SE trend, corresponding to the fracture zone towards Nyiragongo.
From 30 April until 1 May, it seemed that there was some renewal of activity, but no eruption was detected. Residents of Katale and Tongo, the closest villages to the volcano, reported some rumblings on 30 April between 1730 and 2130, plus clear sounds of individual explosions. The closest seismic station (Katale) recorded at the same time ~18 distinct explosion signals, directly followed at 1927 by a tectonic earthquake centered under the volcano. Later, seven type-C events followed until 2232. Another tectonic earthquake occurred at 2338.
Residents of Kunene (~12 km SW), Katale (~10 km NE), and Tongo claimed to have heard explosions and growling noises on 9 May. Local tectonic earthquakes from late May through late June were 0-27 km deep, with an aseismic zone at 3-7 km. Seismicity during 22-28 June was dominated by long-period earthquakes concentrated in the NE, in which there was an apparent increase compared to the previous week.
Geological Summary. Africa's most active volcano, Nyamulagira (also known as Nyamuragira), is a massive high-potassium basaltic shield about 25 km N of Lake Kivu and 15 km NE of the steep-sided Nyiragongo volcano. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from the numerous flank fissures and cinder cones. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Recent lava flows extend down the flanks more than 30 km from the summit as far as Lake Kivu; extensive lava flows from this volcano have covered 1,500 km2 of the western branch of the East African Rift.
Information Contacts: Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo.