Seismic monitoring of Lake Nyos, Cameroon

Abstract

On 21 August 1986 a cloud of dense gas was emitted from Lake Nyos in Cameroon causing death, by asphixiation, of 1700 people in the nearby villages. An international relief effort was initiated and scientific teams from a number of countries were sent to Lake Nyos in order to determine the cause of the gas release. It was agreed that the carbon dioxide, which was the predominant gas in the cloud, had a deep-seated magmatic origin and that the lake was charged with it prior to the gas outburst. Uncertainty remained, however, about the mechanism of its eruption from the lake; in particular, whether the lake overturned in response to a small external trigger or whether a phreatic volcanic eruption occurred. In an attempt to resolve this problem and following the report of a second ‘eruptive event’ on 30 December 1986, the British Geological Survey installed a network of hydrophones and geophones in and around the lake during February 1987. During the following six months, regional earthquakes and small local events, interpreted as rockfalls, were detected but no significant crustal earthquakes were found beneath or close to the lake. No clear evidence of magma movement or other signs of volcanic activity were found, suggesting that direct volcanic activity was not involved. The lack of tectonic seismicity at the lake during the monitoring period argues against an earthquake trigger for the gas release. In the wider volcanic province, however, many earthquakes were detected and such a trigger cannot be completely ruled out.