VOLCANIC RISKS AND THE NYIRAGONGO VOLCANO.

On January 17 2002 a series of fractures opened on Nyiragongo's (3470 m) upper southern flanks triggering drainage of magma stored in the crater lava lake, conduit and edifice and the emission of turbulent flows of highly fluid pahoehoe lava. A remarkable network of fractures propagated rapidly down slope up to 16 km from the crater triggering more pahoehoe but chiefly aa lava emissions from numerous dike-fed vents that reached the outskirts of the city of Goma (population 400 000), the airport and stopped within 4 km of Lake Kivu. A small volume of lava entered Lake Kivu but had no significant consequences on the physico-chemistry of its deep gas-charged waters. The eruption of 25 x 106 m3 of magma formed a complex lava flow filed which ultimately fed two main flows that destroyed about 15% of the large town of Goma. The 1669 AD eruption of Etna is the only other historical eruption that produced lava flows that destroyed a large city. Most of Goma's 400 000 inhabitants escaped the advancing lava flows crossing the border into neighbouring Rwanda. The eruption that destroyed the homes of 120 000 people and caused between 70 and 100 victims was accompanied by unprecedented levels of felt high frequency seismicity of volcanic and tectonic origin. This eruption reactivated and formed new N-S oriented fractures, parallel to pre-existing structures of the main rift-related tectonic faults. Tectonic rift-related seismicity with epicenters trending NE-SW between Nyiragongo and Idjwi island has remained at very elevated levels since 2002, as evidenced by the largest earthquake in 20 years which occurred October 24 6.2 Mb earthquake localized on a normal rift fault 40 km SW of Goma. As detected by sudden major rise in the lake level, this eruption was also remarkably associated with perhaps up to 0,6-1 m of ground subsidence in the Goma area along the axis of the rift and thus aligned with the newly formed fractures.

Hazard assessment and adequate risk mitigation must consider :

(1) whether the extensive tectonic-controlled fracture system of Nyiragongo's southern flanks will promote more frequent flank-eruptions near populated centers without the prerequisite of a long-lasting lava lake;

(2) whether future rift-related migration of magma from the Nyiragongo conduit might proceeding faster and further towards the zone of water-saturated ground within 2 km of lake Kivu favorable to phreatomagmatic explosive eruptions, or ultimately below and into the deep gas-charged basin of lake Kivu to trigger subaqueaous volcanic eruptions and potential catastrophic degassing and lake-overturn. Geological evidence that volcanic centers have occurred on the SW and S volcanic rift zones at or below the level of the lake render such considerations fundamental as the volcano and rift integrated multidisciplinary monitoring system is further developed.

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