The 2007 rifting event in Northern Tanzania studied by C and L-band interferometry

peer reviewe

InSAR Applied to the Study of Active Volcanic and Seismic Areas in Africa

Two active volcanic areas situated in immature portions of the East African Rift System (EARS): the Lake Kivu (Dem. Rep. of Congo) and Lake Natron (Tanzania) regions, which are located in the western and eastern branch, respectively, are still poorly known. Both areas indeed lack of ground-based networks, due to security problems or difficult field accessibility. The Lake Kivu area includes two volcanoes erupting frequently: the Nyamulagira and Nyiragongo volcanoes. The SAR database covering these volcanoes in- cludes data from JERS, ERS-1&2, ENVISAT, ALOS, RADARSAT-1&2 satellites back from year 1996. SAR Interferometry (InSAR) is thus applied to study the ground de- formations. MT-InSAR approaches, such as the ”‘StaMPS”’ method, are used to give us new complementary information to better constrain the previous established eruption models, or gain new insights on eruptions missed by the conventional InSAR, as well as on magmatic and tectonic activity. When enough constrains are available, the ground displacements are modeled using a 3D-Mixed Boundary Elements Method combined with a neighborhood algorithm. Hence, the 1996, 2002, 2004 and 2010 eruptions of Nyamu- lagira are modeled. The modeling results, coupled with the StaMPS MT-InSAR results, bring new insights concerning the magma plumbing system of this poorly known volcano and its eruptive mechanisms. The collapse of the eastern flank of Nyamulagira, along the NNW-trend fractures network linking Nyamulagira and Nyiragongo volcanoes, can also be identified. InSAR also captures the ground displacements associated with the January 2002 Nyiragongo eruption. The modeling of this major event evidences a deep magma intrusion beneath the Lake Kivu. Such intrusions should be taken into account for hazard assessment. The magma could indeed finds its way to the Lake Kivu floor, as evidenced by the presence of several old phreato-magmatic cones, and causes a lake overturn. The low dikes overpressures found in the North Kivu and Lake Natron areas indicate that, although the rift is considered as immature, the rift extension is driven by the supply of magma from depth, rather than by the tectonics. A new criterion to identify the rifting stage is found to be the stress state. In the southern part of the rift, tectonic activity dominates, indicating that the tectonics is probably driving the rift opening there

Magma Pathways and Their Interactions Inferred from InSAR and Stress Modeling at Nyamulagira Volcano, D.R. Congo

A summit and upper flank eruption occurred at Nyamulagira volcano, Democratic Republic of Congo, from 2–27 January 2010. Eruptions at Nyamulagira during 1996–2010 occurred from eruptive fissures on the upper flanks or within the summit caldera and were distributed along the ~N155E rift zone, whereas the 2011–2012 eruption occurred ~12 km ENE of the summit. 3D numerical modeling of Interferometric Synthetic Aperture Radar (InSAR) geodetic measurements of the co-eruptive deformation in 2010 reveals that magma stored in a shallow (~3.5 km below the summit) reservoir intruded as two subvertical dikes beneath the summit and southeastern flank of the volcano. The northern dike is connected to an ~N45E-trending intra-caldera eruptive fissure, extending to an ~2.5 km maximum depth. The southern dike is connected to an ~N175E-trending flank fissure extending to the depth of the inferred reservoir at ~3.5 km. The inferred reservoir location is coincident with the reservoir that was active during previous eruptions in 1938–1940 and 2006. The volumetric ratio of total emitted magma (intruded in dikes + erupted) to the contraction of the reservoir (rv) is 9.3, consistent with pressure recovery by gas exsolution in the small, shallow modeled magma reservoir. We derive a modified analytical expression for rv, accounting for changes in reservoir volume induced by gas exsolution, as well as eruptive volume. By using the precise magma composition, we estimate a magma compressibility of 1.9–3.2 × 109 Pa−1 and rv of 6.5–10.1. From a normal-stress change analysis, we infer that intrusions in 2010 could have encouraged the ascent of magma from a deeper reservoir along an ~N45E orientation, corresponding to the strike of the rift transfer zone structures and possibly resulting in the 2011–2012 intrusion. The intrusion of magma to greater distances from the summit may be enhanced along the N45E orientation, as it is more favorable to the regional rift extension (compared to the local volcanic rift zone, trending N155E). Repeated dike intrusions beneath Nyamulagira’s SSE flank may encourage intrusions beneath the nearby Nyiragongo volcano

Modeling of InSAR displacements related with the January 2002 eruption of Nyiragongo volcano

peer reviewe

Magma Pathways and Their Interactions Inferred from InSAR and Stress Modeling at Nyamulagira Volcano, D.R. Congo

International audienceA summit and upper flank eruption occurred at Nyamulagira volcano, Democratic Republic of Congo, from 2–27 January 2010. Eruptions at Nyamulagira during 1996–2010 occurred from eruptive fissures on the upper flanks or within the summit caldera and were distributed along the ~N155E rift zone, whereas the 2011–2012 eruption occurred ~12 km ENE of the summit. 3D numerical modeling of Interferometric Synthetic Aperture Radar (InSAR) geodetic measurements of the co-eruptive deformation in 2010 reveals that magma stored in a shallow (~3.5 km below the summit) reservoir intruded as two subvertical dikes beneath the summit and southeastern flank of the volcano. The northern dike is connected to an ~N45E-trending intra-caldera eruptive fissure, extending to an ~2.5 km maximum depth. The southern dike is connected to an ~N175E-trending flank fissure extending to the depth of the inferred reservoir at ~3.5 km. The inferred reservoir location is coincident with the reservoir that was active during previous eruptions in 1938–1940 and 2006. The volumetric ratio of total emitted magma (intruded in dikes + erupted) to the contraction of the reservoir (rv) is 9.3, consistent with pressure recovery by gas exsolution in the small, shallow modeled magma reservoir. We derive a modified analytical expression for rv, accounting for changes in reservoir volume induced by gas exsolution, as well as eruptive volume. By using the precise magma composition, we estimate a magma compressibility of 1.9–3.2 × 109 Pa−1 and rv of 6.5–10.1. From a normal-stress change analysis, we infer that intrusions in 2010 could have encouraged the ascent of magma from a deeper reservoir along an ~N45E orientation, corresponding to the strike of the rift transfer zone structures and possibly resulting in the 2011–2012 intrusion. The intrusion of magma to greater distances from the summit may be enhanced along the N45E orientation, as it is more favorable to the regional rift extension (compared to the local volcanic rift zone, trending N155E). Repeated dike intrusions beneath Nyamulagira’s SSE flank may encourage intrusions beneath the nearby Nyiragongo volcano

Magma storage and diking revealed by GPS and InSAR geodesy at Pacaya volcano, Guatemala

GPS measurements from a campaign network at Pacaya volcano, Guatemala, occupied from 2009 to 2015 were combined with InSAR data from 2013 to 2014 to model deformation sources for two eruptive time periods: 2009–2011 and 2013–2014. The GPS data for both of these time periods show downward vertical and outward horizontal deformation greater than 25 cm at several stations surrounding the volcano, while InSAR data shows up to 15-cm line-of-sight displacement. To better understand the dynamics of the magma storage system and sources of deformation, we inverted available geodetic data for these two periods. Our analytical modeling suggests that horizontal deformation was dominated by inflation of a shallow, subvertical dike, high within the volcanic edifice, while deflation of a deeper, spherical source embedded below the NW flank of the volcano occurred during at least part of the observation period. The source parameters for the dike feature are in good agreement with the observed alignment of recent eruptive vents, while parameters for the deeper, spherical source accommodate the downward vertical deformation observed at stations on and around the volcano

State of stress and stress rotations: Quantifying the role of surface topography and subsurface density contrasts in magmatic rift zones (Eastern Rift, Africa)

In rift settings, the crustal stress field is dominated by extension, which leads to rift-parallel topography and basin alignments. However in some continental rift systems, some observables of the orientation of principal stresses show substantial deviations from these patterns. Such stress field rotations are currently poorly understood and could reflect the critical role of rift magmatism in the creation of topography, the plate state-of-stress, and volcanic and tectonic processes. Yet the role of magma intrusions, crustal thinning, and rift basin and flank topography on rift zone stress field rotations remain poorly quantified. The seismically- and volcanically-active Magadi-Natron-Manyara region of the East African Rift shows a 60 degrees local stress field rotation with respect to regional extension. Here, we test the hypothesis that such rotation is due to the cumulative effects of surface and subsurface loads (lateral subsurface density contrasts). We use analytical and calibrated numerical models of magmatic rift zones to simulate lithospheric deformation in the presence of magma bodies, crustal thinning, and topography to quantify their effect on intrusions and fault kinematics in a rift setting. Our 3D static models of a weakly extended rift suggest that surface topography influences shallow stress localization, whereas subsurface density contrasts play a larger role in lower crustal stress localization. Both patterns suggest a preferred region for melt storage beneath the rift valley. We show that the interaction between topography, crustal thinning, extension, and a pressurized magma reservoir could generate principal stress orientations consistent with the local stress rotation observed from earthquake focal mechanisms. Our results demonstrate how rift topography and the geometry of crustal thinning can guide magmatism and strain localization, highlighting the need for a three-dimensional treatment of rift kinematics