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Last two decades have proven that remote sensing represents a key tool to improve our knowledge of volcanic systems but also to monitor active volcanoes. Based on the specific case of Piton de la Fournaise, Reunion Island, the most active French volcanoes, we here illustrate how Synthetic Aperture Radar (SAR) data, providing information even in cloudy conditions, make it possible to map eruptive deposits, to quantify their volumes but also to estimate the volcanoes topography (with metric precision) as well as surface deformation fields (with a precision reaching a few millimeters). [ABSTRACT FROM AUTHOR]

Abstract: ASAR-ENVISAT Interferometric Synthetic Aperture Radar (InSAR) data collected over the Lastarria-Cordon del Azufre complex (Chile–Argentina) between March 2003 and May 2005 show the persistence of the large wavelength ground inflation revealed by Pritchard and Simons in 2002 from the analysis of ERS InSAR data [Nature 418 (2002) 167–170]. After reducing the tropospheric contribution in the interferograms using a combination of data network adjustment and analysis of MODIS images, we produced an accurate interferometric time series showing a 2 yr long temporal evolution of the ground displacements patterns. Two distinct inflating signals are detected. The main signal covers an elliptical area with a 45 km NNE–SSW major axis and a 37 km minor axis. It is correlated with a regional topographic dome. We estimated its maximum inflation rate to ∼2.5 cm yr−1. We inverted the InSAR data for a range of source geometries (spherical, prolate ellipsoids, penny-shaped cracks). The inferred source parameters for 2003–2005 period are consistent with an over-pressured reservoir at shallow to intermediate crustal depths (7–15 km), with an average volumetric rate of inflation of about 14×106 m3 yr−1. In addition to this main signal a new feature highlighted by the ASAR data is short wavelength inflation (6 km wide) at the location of Lastarria volcano on the northern margin of the large wavelength signal. We explain this short wavelength signal by a spherical over-pressured source lying 1000 m below the summit of Lastarria volcano. We estimate the average volumetric rate of inflation during the observation period to be ∼35×103 m3 yr−1. It is remarkable that both volumetric variations for the large and small inflations exhibit the same evolution during the 2003–2005 period, suggesting that both processes could be related. On the basis of the inversion results and of arguments provided by field evidences and a morpho-structural analysis of the Digital Elevation Model of the area, we propose that the deep source have a magmatic origin while the shallow source is most likely related to hydrothermal fluids. In our interpretation, the on-going deformation processes observed at Lastarria-Cordon del Azufre volcanic complex could represent an evolving pre-caldera silicic system. Further field geological and geophysical investigations will be required to confirm these hypotheses and refine the proposed model, mostly based on satellite observations. [Copyright &y& Elsevier]

A devastating lahar (volcanic mudflow) occurred at Casita volcano (Nicaragua) in 1998, triggered by excessive precipitation associated with Hurricane Mitch. We investigate here the morphology and drainage structure of the flow deposition area, primarily using satellite optical and radar imagery. Because the lahar destroyed several towns and villages, killing over 2500 people, we also assess the utility of images available at the time of the event for disaster management. We find that SPOT multispectral data are most suited to delineate and characterize the flow field, but show limitations for damage assessment, and problems with cloud contamination. ERS Synthetic Aperture Radar (SAR) imagery largely failed to detect the lahar deposits, with RADARSAT performing slightly better. The relatively coarse-grained deposits, together with a dense cover of wooden debris, made the flow nearly imperceptible to the short wavelength C-Band radar, especially at ERS's steep incident angle. Because of spatial, spectral and radiometric limitations in all types of imagery used, the synergistic potential of optical and radar, as well as high- and low-resolution optical data was explored. The best synergy was found not in merged imagery, but in the incorporation of auxiliary information, such as elevation, map and GIS data. Given the cloud problem, forthcoming radar satellites with variable polarization and incident angles are expected to provide better results in comparable, future situations. [ABSTRACT FROM AUTHOR]

The interaction between volcanic activity and flank instability during the Christmas Eve eruption at Mount Etna in 2018 is explored, using a mechanically consistent inverse model fitting high spatial resolution SAR data. Inversions search for fractures that may be curved and can accommodate co‐eval pressure and shear stress changes. Displacements associated with the eruption result from the interaction between two intrusion sources: a buried dyke and a curved sheared intrusion that fed the eruption. Moreover, we identify that the sheared magmatic intrusion induced the observed eastward slip on the Pernicana fault, while the Fiandaca fault was undergoing stress accumulation, which was suddenly released during a M5.0 seismic event. The Fiandaca fault is determined to be listric, rooting beneath the mobile eastern flank of the volcano. This study highlights the role of curved fractures, acting as sheared intrusions or as faults, in volcanoes exhibiting flank instabilities. Plain Language Summary: In this study, we thoroughly examine how volcanic activity and flank dynamics interacted during the 2018 eruption of Mount Etna. We use high‐resolution satellite SAR data and a model considering complex mechanical aspects of the volcano's behavior. Formal inversions reveal that the eruption is triggered by the interplay between two intrusion sources: a buried dyke and a curved sheared intrusion, consistent with the earthquake pattern. This study emphasizes the significant role of curved sheared intrusions and faults at volcanoes prone to flank instabilities. Furthermore, we quantify how summit magmatic intrusions promote fault slip in the eastern flank of Etna. These findings differ from previous research that used simpler approaches. In summary, our study provides a new insight into how volcanoes like Etna can erupt and trigger flank slip, shedding light on the complex interactions between magma and tectonics. Key Points: Modeling shows that the magma intrusion feeding the 2018 eruption accommodated coeval opening and shear displacementsBoth the summit intrusions and the Fiandaca fault are curved, probably connected to the detachment at the base of the mobile eastern sectorThe Fiandaca fault was previously locked. It released accumulated stress, while the Pernicana fault passively responded to the intrusion [ABSTRACT FROM AUTHOR]

In volcanic domains, magma transport and pressure build-up induce high stress–strain perturbations in the surrounding volcanic edifice that may lead to volcanic flank movements and possible instability. In this study, we focus on the 2007 March–April episode of volcanic activity at Piton de la Fournaise (PdF) Volcano, La Réunion Island. This episode was associated with a large volume of emitted lava (240 × 106 m3) and a 340-m caldera collapse. We present observations of continuous seismic velocity changes measured using cross-correlations of ambient seismic noise over 10 yr at PdF. Overall, we observe a large velocity reduction starting a few days prior to the major 2007 April 2 eruption. Comparison of seismic velocity change measurements with observations of deformation from InSAR and GPS shows that the seismic velocity reduction coincided with a widespread flank movement starting at the time of injection of magma to feed an initial eruption, a few days before the 2007 April 2 eruption. We emphasize the potential of noise-based seismic velocity change measurements, together with geodetic observations, to detect and monitor possibly hazardous slope instabilities. [ABSTRACT FROM AUTHOR]

Abstract: The stratigraphy and age of the Neogene Cappadocia ignimbrites (Central Turkey) have been inferred in previous studies from fieldwork and K–Ar age determinations. The resulting stratigraphic schemes, however, differed from each other, suggesting that further studies were required to produce a reliable succession. In this paper, we examine the chronostratigraphy of mammalian remains recovered in the continental sediments interbedded with the Cappadocia ignimbrites. Using recent advances in mammalian chronostratigraphy, we evaluate selected taxa and faunal associations to place new and independent constraints on the ignimbrite ages. The biostratigraphically bracketed ages concur with some published radiometric dates, but they disagree with others, principally at localities where major stratigraphic discrepancies have arisen in the literature. In order to reconcile these apparent inconsistencies, we combine, at selected sites, our field observations with the biostratigraphic and radiometric age limits, and we compare these with the available geochemical and magnetic data. This allows us to present revised age estimates, and a revised stratigraphy which includes the correlation of the local Sofular deposits with the large-volume Gördeles ignimbrite. The issues faced in this study apply to other ignimbrite provinces in the world. For instance, ignimbrite eruption frequency in Cappadocia is higher than the resolution of many published K–Ar ages. Furthermore, different K–Ar ages have led to the description of individual and distinct ignimbrites that fieldwork and geochemical data allow to merge into a single ignimbrite. Argon loss from pumice samples leading to radiometric “rejuvenation” provides a likely explanation for most stratigraphic discrepancies. Cappadocia is the only documented ignimbrite field in which the chronostratigraphy of vertebrate remains provides better constraints on some ignimbrite ages than scattered K–Ar dates. We further argue that K–Ar dates from the Cappadocia area are too imprecise to establish a reliable magnetostratigraphic scheme for the ignimbrite succession, with the exception of the ∼2.8 Ma Valibaba Tepe ignimbrite. [Copyright &y& Elsevier]

Magma propagation is an unsteady process controlled by magma‐crust interaction. To provide information on its dynamics, we invert complementary ground deformation data spanning the 8 hr preceding the 26 May 2016 eruption at Piton de la Fournaise (PdF) volcano (La Réunion, France). Data are inverted using 3‐D boundary element models combined with a Monte Carlo inversion method. The final geometry of the displacement source is determined based on four interferograms spanning the whole propagation phase while the dynamics of the propagation is inferred from temporal inversion of continuous Global Navigation Satellite System (GNSS) data, using the final geometry as an a priori to constrain the source. The best modeled magma path consists in a 2,700‐m‐long sill located 800 m above sea level and connected to the eruptive fissure by a subvertical dike. The quick opening of the horizontal part of the intrusion could have been favored by limited flank sliding during the early stage of propagation. The intrusion then stalled for ∼5 hr, while pressure increased slightly, until final upward propagation and eruption. Volume budget suggests that the eruption was fed by a single batch of magma quickly disconnected from its source. The delay prior to the eruption may reflect a limited magma supply. Finally, two mechanisms, potentially acting together, might have favored the eruption: a driving role of magmatic gas and/or, as often observed at Piton de la Fournaise, an eastward flank slip. Plain Language Summary: Basaltic magma stored beneath volcanoes reaches the surface by fracturing the Earth's crust. As experienced in May 2018 at Kilauea volcano (Hawaii, USA), magma can travel kilometers from the reservoir and fissure opening may threaten man‐made structures. Anticipating where and when eruptive fissures open requires better understanding of the factors controlling magma propagation. During the 26 May 2016 eruption of Piton de la Fournaise, Réunion Island,the preeruptive crisis spanned 8hr25min from the first signal recorded by the observatory to the eruption onset.We determine the magma paths and propagation timing, which led to this eruption using complementary satellite data of ground surface displacement, combining radar interferometry, which provides high spatial resolution, with GPS, which provides high temporal resolution. We highlight complex magma propagation within the subareal volcano, showing two direction changes, an arrest and an acceleration. Flank slip and magma degassing seem to play a key role in controlling both the geometry and the timing. Based on this scenario, this event was close to turn into a failed eruption as there was a 5‐hr pause in propagation before magma finally reached the surface. Understanding such unusual eruptions is a challenge for observatories as it may lead to repeated "false" alerts. Key Points: Magma feeding the May 2016 eruption propagated laterally as a sill before turning into a dikeThe sill propagation is stepwise with an initial acceleration followed by a 5‐hr pauseThe eruption was fed by a single batch of magma quickly disconnected from its source [ABSTRACT FROM AUTHOR]

Abstract: The use of Synthetic Aperture Radar interferometry (InSAR) in northern Chile, one of the most seismically active regions in the world, is of great importance. InSAR enables geodesists not only to accurately measure Earth’s motions but also to improve fault slip map resolution and our knowledge of the time evolution of the earthquake cycle processes. Fault slip mapping is critical to better understand the mechanical behavior of seismogenic zones and has fundamental implications for assessing hazards associated with megathrust earthquakes. However, numerous sources of errors can significantly affect the accuracy of the geophysical parameters deduced by InSAR. Among them, atmospheric phase delays caused by changes in the distribution of water vapor can lead to biased model parameter estimates and/or to difficulties in interpreting deformation events captured with InSAR. The hyper-arid climate of northern Chile might suggest that differential delays are of a minor importance for the application of InSAR techniques. Based on GPS, Moderate Resolution Imaging Spectroradiometer (MODIS) data our analysis shows that differential phase delays have typical amplitudes of about 20 mm and may exceptionally exceed 100 mm and then may impact the inferences of fault slip for even a Mw 8 earthquakes at 10% level. In this work, procedures for mitigating atmospheric effects in InSAR data using simultaneous MODIS time series are evaluated. We show that atmospheric filtering combined with stacking methods are particularly well suited to minimize atmospheric contamination in InSAR imaging and significantly reduce the impact of atmospheric delay on the determination of fundamental earthquake parameters. [Copyright &y& Elsevier]

Abstract: We present a detailed study of Lascar volcano (Chile) based on the combination of satellite, aerial and ground-based data, in order (i) to better characterize the deformation style of Andean explosive volcanoes, and (ii) to provide new insights on the potential of space techniques to monitor active volcanic deformations on such edifices. Lascar is one of the most active volcanoes in Central Andes characterized by a recent cyclic activity. Additionally, it is located in favourable conditions for radar imaging. Lascar thus offers very good conditions for studying large to small scale ground deformations associated with volcano dynamics. The analysis of InSAR (Synthetic Aperture Radar interferometry) time series data from the European and Japanese satellites (ERS, JERS) acquired between 1993 and 2000, encompassing three eruptive events, confirmed the absence of broad far-field deformation signal. Thus during the recent activity of Lascar we discard significant magmatic input at depth. The following approaches were used to improve the InSAR signal/noise ratio in order to detect possible local deformation. We carried out a quantitative evaluation of the potential tropospheric contribution in INSAR interferograms for the Salar de Atacama–Lascar area using radar (ASAR-ENVISAT) and spectrometer (MODIS) data. We also used an accurate aerial photogrammetric and GPS constrained DEM in our InSAR data reprocessing. We find a co-eruptive ground-deformation confined into the summit crater for the 1995 eruption. This deformation has spatial dimension of 500 by 400 m and relates to a subsidence of crater floor up to 17 mm. We interpret it as pressure or volume decrease at subsurface levels below the active crater. Our study made it possible to image a new near-field volcanic deformation confined within the summit crater of the Lascar volcano. It also demonstrates that the combination of precise photogrammetry DEM and INSAR data can significantly improve our ability to remotely sense subtle surface deformation on these explosive volcanoes. This methodology might contribute to better understand volcano dynamics and to complement their monitoring in remote areas. [Copyright &y& Elsevier]