Nicolas d'Oreye

Strain accommodation by slow slip and dyking in a youthful continental rift, East Africa

Continental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions. These rifting crises are rarely observed in thick lithosphere during the initial stages of rifting. Here we show that most of the strain during the July–August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning.

High‐resolution TanDEM‐X DEM: An accurate method to estimate lava flow volumes at Nyamulagira Volcano (D. R. Congo)

Nyamulagira and Nyiragongo are two of the most active volcanoes in Africa, but their eruptive histories are poorly known. Assessing lava flow volumes in the region remains difficult, as field surveys are often impossible and available Digital Elevation Models (DEMs) do not have adequate spatial or temporal resolutions. We therefore use TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) interferometry to produce a series of 0.15 arc sec (∼5 m) DEMs from between 2011 and 2012 over these volcanoes. TanDEM-X DEMs have an absolute vertical accuracy of 1.6 m, resulting from the comparison of elevation with GPS measurements acquired around Nyiragongo. The difference between TanDEM-X-derived DEMs from before and after the 2011–2012 eruption of Nyamulagira provides an accurate thickness map of the lava flow emplaced during that activity. Values range from 3 m along the margins to 35 m in the middle, with a mean of 12.7 m. The erupted volume is 305.2 ± 36.0 × 106 m3. Height errors on thickness depend on the land covered by the flow and range from 0.4 m in old lavas to 5.5 m in dense vegetation. We also reevaluate the volume of historical eruptions at Nyamulagira since 2001 from the difference between TanDEM-X and SRTM 1 arc sec DEMs and compare them to previous work. Planimetric methods used in literature are consistent with our results for short-duration eruptions but largely underestimate the volume of the long-lived 2011–2012 eruption. Our new estimates of erupted volumes suggest that the mean eruption rate and the magma supply rate were relatively constant at Nyamulagira during 2001–2012, respectively, 23.1 m3 s−1 and 0.9 m3 s−1.

Systematic InSAR monitoring of African active volcanic zones: What we have learned in three years, or an harvest beyond our expectations

We present here a brief overview of some findings and preliminary results obtained after almost three years of systematic monitoring of active volcanic areas in Africa by means of differential synthetic aperture radar interferometry (InSAR). With a database rich of more than 400 SAR scenes of Fogo (Cape Verde), Ol Doinyo Lengai (Tanzania), Nyiragongo-Nyamulagira (DR of Congo) and Mount Cameroon volcanoes, we processed more than 2000 interferograms among which we could detect significant and major geophysical processes: the first dyking event ever captured geodetically in a continental rift (Lake Natron; Northern Tanzania), the co-eruptive deformations of the Lengai, Nyiragongo and Nyamulagira volcanoes, the co-seismic displacements associated to the mb 6.1 February 3rd 2008 Bukavu earthquake as well as the identification of atmospheric induced phase delays over Fogo and Mount Cameroon volcanoes to be attributed to the seasonal oscillations of the inter-tropical convergence zone (ITCZ). These results have been reached given the abundance of data that increases the chances to capture unpredictable events, and capture them with the most favorable interferometric conditions as possible (e.g. in terms of geometrical and temporal baselines that minimized the vegetation-induced decorrelation). They provided strong scientific material as well as tools for hazard assessment.

Long-term monitoring of long-period seismicity and space-based SO2 observations at African lava lake volcanoes Nyiragongo and Nyamulagira (DR Congo)

Magma ascent that may lead to an eruption is commonly accompanied by variations of long-period seismic activity and SO2 degassing. Space-based measurements of SO2 emission rates represent a rapidly emerging and highly convenient approach for volcano monitoring; however, combining these long-term remote sensing observations with seismic data is still rare and, in particular, the potential of such a multidisciplinary approach as volcano monitoring tool remains largely unexplored. Here, shallow magmatic activity and magma migration patterns at the two closely located African volcanoes Nyiragongo and Nyamulagira are inferred from a nearly three-years-long SO2 emissions record and seismic observations between April 2014 and February 2017. The discrimination of magma movements into shallow plumbing systems allows for signs of volcanic unrest to be deciphered on a daily time scale, even with limited instrumentation on site.

L-band and C-band InSAR studies of African volcanic areas

Radar interferometry has proven to be a very suitable, low-cost and accurate tool to measure surface displacements. We investigate several data fusion or time-series analysis strategies which aim to mitigate C-band InSAR restrictions for volcano deformation monitoring applications. The focus is on active African volcanic areas. Firstly, data fusion of C-band ENVISAT/ASAR and L-band ALOS/PALSAR sensors helps the determination of a rifting event sequence that took place in summer 2007 in Lake Natron area. The second strategy investigated is a new Wavelet Based InSAR time series applied on ERS-2 data covering the Nyiragongo-Nyamulagira area. It allows new ground displacements identifications outside the local rift valley. Lastly, PALSAR Quad-Pol POLInSAR applicability is explored for La Palma Island.

Split-Band Interferometry-Assisted Phase Unwrapping for the Phase Ambiguities Correction

Split-Band Interferometry (SBInSAR) exploits the large range bandwidth of the new generation of synthetic aperture radar (SAR) sensors to process images at subrange bandwidth. Its application to an interferometric pair leads to several lower resolution interferograms of the same scene with slightly shifted central frequencies. When SBInSAR is applied to frequency-persistent scatterers, the linear trend of the phase through the stack of interferograms can be used to perform absolute and spatially independent phase unwrapping. While the height computation has been the main concern of studies on SBInSAR so far, we propose instead to use it to assist conventional phase unwrapping. During phase unwrapping, phase ambiguities are introduced when parts of the interferogram are separately unwrapped. The proposed method reduces the phase ambiguities so that the phase can be connected between separately unwrapped regions. The approach is tested on a pair of TerraSAR-X spotlight images of Copahue volcano, Argentina. In this framework, we propose two new criteria for the frequency-persistent scatterers detection, based respectively on the standard deviation of the slope of the linear regression and on the phase variance stability, and we compare them to the multifrequency phase error. Both new criteria appear to be more suited to our approach than the multifrequency phase error. We validate the SBInSAR-assisted phase unwrapping method by artificially splitting a continuous phase region into disconnected subzones. Despite the decorrelation and the steep topography affecting the volcanic test region, the expected phase ambiguities are successfully recovered whatever the chosen criterion to detect the frequency-persistent scatterers. Comparing the aspect ratio of the distributions of the computed phase ambiguities, the analysis shows that the phase variance stability is the most efficient criterion to select stable targets and the slope standard deviation gives satisfactory results.

The January 2002 eruption of Nyiragongo volcano (DRC) captured by InSAR

On 17th January 2002, Nyiragongo erupted along an approximately 20 km long fracture network extending from the volcano to the city of Goma and its airport. The event was captured by InSAR data from the ERS-2 and RADARSAT-1 satellites acquired in three different geometries. These data show complex ground displacements, with several overlapping fringe patterns, associated to a combination of sources of magmatic and tectonics origins. A combination of 3D numerical modeling and inversions is used in order to interpret these displacements. Synthetic tests indicate that with one to three INSAR geometries, the best fit and mean models are within the confidence intervals whether the source of displacements is a single dike, a dike combined with a west dipping normal fault or a dike combined with an ellipsoid. Increasing the number of InSAR geometries makes the confidence intervals smaller and the inversions faster. At this stage of the study, only the area close to the eruptive fissures was analyzed assuming displacements were caused by a single dike. The best-fit dike model obtained with a simultaneous inversion of the three InSAR geometries is subvertical, and has a low overpressure. Both characteristics are consistent with the rift context.

Multidimensional Small Baseline Subset (MSBAS) for Two-Dimensional Deformation Analysis: Case Study Mexico City

ABSTRACT Time series of ground deformation are used to describe motion produced by various natural and anthropocentric processes, such as earthquakes, volcanic eruptions, landslides, subsidence due to resource exploitation, and uplift due to fluid injection. Presented here, the multidimensional small baseline subset (MSBAS) technique simultaneously processes multiple ascending and descending DInSAR datasets and produces 2-D, horizontal east-west and vertical, deformation time series with combined temporal resolution over overlapped area. The set of linear equations that comprises MSBAS is usually rank deficient and is solved in the least-square sense by applying the singular value decomposition (SVD) and the zero-, first-, or second-order Tikhonov regularization. The MSBAS source code is written in C++ and is linked to the linear algebra package (LAPACK) library that provides SVD support. For demonstration of capabilities, MSBAS is used to compute 2-D deformation time series of Mexico City by simultaneously processing ascending and descending RADARSAT-2 data acquired during October 2008–December 2012. This area is known to subside due to excessive groundwater extraction that produces pore water pressure drop and compaction of highly compressible clays. During the studied period we observed subsidence with rates over 35 cm/year and horizontal motion of up to 5 cm/year. The MSBAS software can be downloaded from http://insar.ca/.

Single-Station Seismo-Acoustic Monitoring of Nyiragongo's Lava Lake Activity (D.R. Congo)

Since its last effusive eruption in 2002, Nyiragongo has been an open-vent volcano characterized by the worlds largest persistent lava lake. This lava lake provides a unique opportunity to detect pressure change in the magmatic system by analyzing its level fluctuations. We demonstrate that this information is contained in the seismic and infrasound signals generated by the lava lake’s activity. The continuous seismo-acoustic monitoring permits quantification of lava lake dynamics, which is analyzed retrospectively to identify periods of volcanic unrest. Synchronous, high-resolution satellite SAR (Synthetic Aperture Radar) images are used to constrain lava lake level by measuring the length of the SAR shadow cast by the rim of the pit crater where the lava lake is located. Seventy-two estimations of the lava lake level were obtained with this technique between August 2016 and November 2017. These sporadic measurements allow for a better interpretation of the continuous infrasound and seismic data recorded at the closest station (~6 km from the crater). Jointly analyzed seismo-acoustic and SAR data reveal that slight changes in the spectral properties of the continuous cross-correlated low-frequency seismo-acoustic records (and not solely the single LP events) can be used to track fluctuations of the lava lake level on a daily and hourly basis. We observe that drops of the lava lake and the appearance of significant LP “lava lake” events are a consequence of deep magma intrusion, which induces changes in the shallow magmatic system. This study highlights the potential to continuously monitor Nyiragongo’s lava lake activity (and subsequent information about pressure changes within the magmatic system) using a single seismo-acoustic station located several kilometers from the vent.

Natural and anthropogenic ground deformation monitored using high spatio-temporal resolution MSBAS time series method

Multidimensional Small Baseline Subset (MSBAS), methodology is used for integration of multiple InSAR data sets for computation of two or three dimensional time series of deformation. The method is applied to monitor ground deformation with high spatio-temporal resolution. The MSBAS approach allows combination of all possible air-borne and space-borne SAR data acquired with different acquisition parameters, temporal and spatial sampling and resolution, wave-band and polarization. The method has four main advantages: (i) it achieves combined temporal coverage over an extended period of time when data from many different sensors with different temporal coverages are available; (ii) temporal resolution of produced time series increases since it includes the combined sampling from all data sets, which helps to observe the signal in more detail and also to improve the quality of post-processing (i.e. filtering); (iii) two or three components of ground deformation vector are computed, which helps in interpretation of observed ground deformation and further modeling and inversion; (iv) various sources of noise (i.e. tropospheric, ionospheric, topographic, orbital, thermal, etc.) are averaged out during the processing improving a signal-to-noise ratio. Performing double difference between time series of carefully chosen pixels allows reducing noise from common sources such as atmosphere and eliminating the influence of the reference area taken for the time series. For demonstration purposes we apply MSBAS methodology for mapping volcanic ground deformation in Virunga Volcanic Province in Congo and mining deformation along the French-German border.