Magma sources involved in the 2002 Nyiragongo eruption, as inferred from an InSAR analysis

Abstract

along a 20 km-long fracture network extending from the volcano to the city of Goma. The event was captured by InSAR data from the ERS-2 and RADARSAT-1 satellites. A combination of 3D numerical modeling and inversions is used to analyze these displacements. Using Akaike Information Criteria, we determine that a model with two subvertical dikes is the most likely explanation for the 2002 InSAR deformation signal. A first, shallow dike, 2 km high, is associated with the eruptive fissure, and a second, deeper dike, 6 km high and 40 km long, lies about 3 km below the city of Goma. As the deep dike extends laterally for 20 km beneath the gas-rich Lake Kivu, the interaction of magma and dissolved gas should be considered as a significant hazard for future eruptions. A likely scenario for the eruption is that the magma supply to a deep reservoir started ten months before the eruption, as indicated by LP events and tremor. Stress analysis indicates that the deep dike could have triggered the injection of magma from the lake and shallow reservoir into the eruptive dike. The deep dike induced the opening of the southern part of this shallow dike, to which it transmitted magma though a narrow dike. This model is consistent with the geochemical analysis, the lava rheology and the pre-and post-eruptive seismicity. We infer low overpressures (1-10 MPa) for the dikes. These values are consistent with lithostatic crustal stresses close to the dikes and low magma pressure. As a consequence, the dike direction is probably not controlled by stresses but rather by a reduced tensile strength, inherited from previous rift intrusions. The lithostatic stresses indicate that magmatic activity is intense enough to relax tensional stresses associated with the rift extension.

Historical Volcanism and the State of Stress in the East African Rift System

“…In addition, rift-wide extension, detected by InSAR, together with seismicity was interpreted in terms of a southward propagating shallow dyke and a deeper one, 40 km long (Wauthier et al, 2012). Wadge and Burt (2011) argued that a very similar N-S dyke-driven eruption occurred during the only other historical flank eruption in 1977.…”

Dynamics of the Mount Nyiragongo lava lake

“…In our model, we assume that the convective magma (which could represent between 300 and 1700 × 10 6 m 3 by 2022) is in part emplaced within a magma chamber, while the other part is intruded into dykes. Wauthier et al [2012] suggest that the deep, slowly filling dyke is situated either under the city of Goma or part of another slowly filling dyke system. They suggest that the deep dyke they inferred might be too narrow to cause detectable InSAR displacements.…”

“…The last eruption (January 2002) has shown that while only a small part of the lava lake erupted (less than 10%), a much larger amount of magma intruded into dykes, representing a significant volcanic hazard if a catastrophic fissural eruption similar to 2002 were to be repeated [Favalli et al, 2009;Wauthier et al, 2012]. Thus, even if the lava lake volume is predicted to range within 10 to 15 × 10 6 m 3 for the next decade, it does not reduce the risk of lava flows within the city of Goma.…”

“…Mount Nyiragongo is located in an extensional environment likely to promote rifting and dyking. Based on data from the January 2002 eruptive event, d 'Oreye et al [2008] and Wauthier et al [2012] used interferometric synthetic aperture radar (InSAR) analysis to infer the presence of two dyke intrusions. One formed as a shallow dyke at~2000 m depth on average and associated with a 20 km long eruptive fissure, while the second was 6000 m deep and extended to~40 km.…”

Split-Band Interferometric SAR Processing Using TanDEM-X Data

“…One of the platforms inside the crater (P3), the deepest, is varying of several tens of meters a year due to the presence of a very active lava lake [4]. The flanks of the crater are so steep that classical phase unwrapping always fails connecting P3 to the surrounding, preventing a correct height measurement of the platform to be made.…”