1,721,003 research outputs found
Mapping Hydrothermal Alteration at the Fentale-Dofan Magmatic Segment of the Main Ethiopian Rift
Full text linkThe Main Ethiopian Rift (MER) is characterized by extensional tectonics and volcanism, associated with active hydrothermal systems, hydrothermal alteration and fumarolic deposits. The spatial distribution of these hydrothermal products and their link with faults and rock types provides important clues to what controls fluid flow in the subsurface. However, little is known about this in the East African Rift. We address this issue with a multidisciplinary approach in the Fentale-Dofan magmatic segment of the MER, an area characterized by intense volcanic and tectonic activity and a geothermal prospect. Primarily we conduct mapping of hydrothermal alteration and fumarolic deposits, and rock lithologies using a surface feature classification technique of multispectral satellite images. Then we interpret the map using a new database of faults and active hydrothermal manifestations such as hot-springs and fumaroles. We find that the surface hydrothermal alteration and deposits are mainly focused near Fentale and the Dofan Volcanic Complex (DVC). At DVC the hydrothermal products are focused on rhyolites on the western side of the volcano, in an area of intense NNE striking, rift parallel faults. At Fentale volcano the hydrothermal products are mainly associated with ignimbrite and show a circular pattern around the volcanic edifice, but also in places follow the NNE striking faults. At Fentale, the more complex association of hydrothermal products and active manifestations around the edge of the ignimbrite suggests formation contacts may also localize fluid flow in places. At both volcanoes the association between hydrothermal products with either the rhyolites and ignimbrites is likely due to them being relatively easily altered (in comparison to basalt), and also their brittle nature allows for fracturing through which localized fluid flow can occur (as opposed to the sediments). The general pattern of hydrothermal products suggests a stronger structural influence at the DVC with respect to Fentale. The presence of hydrothermal products and active hydrothermal manifestations, along with other lines of evidence such as locus of subsurface dike intrusion at the volcanic centres, suggest that discrete and localized magma reservoirs beneath Fentale and the DVC are the heat source for hydrothermal circulation. Our study also demonstrates that geology, including hydrothermal deposits, can be successfully mapped using automated remote sensing based classification
The 2008–2010 subsidence of dallol volcano on the spreading erta ale ridge: Insar observations and source models
Full text linkIn this work, we study the subsidence of Dallol, an explosive crater and hydrothermal area along the spreading Erta Ale ridge of Afar (Ethiopia). No volcanic products exist at the surface. However, a diking episode in 2004, accompanied by dike-induced faulting, indicates that Dallol is an active volcanic area. The 2004 diking episode was followed by quiescence until subsidence started in 2008. We use InSAR to measure the deformation, and inverse, thermoelastic and poroe-lastic modelling to understand the possible causes of the subsidence. Analysis of InSAR data from 2004–2010 shows that subsidence, centered at Dallol, initiated in October 2008, and continued at least until February 2010 at an approximately regular rate of up to 10 cm/year. The inversion of InSAR average velocities finds that the source causing the subsidence is shallow (depth between 0.5 and 1.5 km), located under Dallol and with a volume decrease between −0.63 and −0.26 × 106 km3/year. The most likely explanation for the subsidence of Dallol volcano is a combination of out-gassing (depressurization), cooling and contraction of the roof of a shallow crustal magma chamber or of the hydrothermal system
Diverse pattern of strain at the Afar triple junction
No full textStrain and seismicity show us the mode by which deformation is accommodated in rifting continents. Here we present a combined analysis of InSAR derived strain maps and seismicity of the Afar triple junction from 2006 to 2010. Our analysis shows that that the plate spreading motion is accommodated in different modes. A dogbone-shaped seismicity and strain distribution dominates the northern branch of the triple junction, likely as a result of repeated dike intrusions 2005-2010. In the eastern branch strain is distributed across the central part of several overlapping rifts while seismicity appears decoupled from strain and it focuses at the rifts tips. A third branch shows a narrow and elongated zone of both high strain and seismicity. The pattern suggests that the history of magmatic intrusions and rifting events creates a diverse triple junction accommodation zone
Seismicity induced by fluid migration in the Main Ethiopian Rift
No full textIn the last years a large amount of geological, geophysical and seismological data have been produced by induced seismicity studies. These studies have given a great contribute to the understanding of the physical processes triggered by the fluid-structure interaction in active fault zones. Frictional laboratory experiments on fault rock samples have shown that the style of deformation (e.g. stick-slip or aseismic creeping) can change in presence of fluids. In addition, high-precision seismicity locations coupled with the interpretation of high-resolution seismic profiles have allowed to understand the time-spatial evolution of the permeability inside complex fault systems. Here we integrate different data available from the literature to model the stress perturbations induced by wastewater injection in the Val d’Agri oil field by coupled fluid flow and geomechanical numerical simulations. We model the behaviour of main faults involved in the induced seismicity process by frictional contacts in a finite element framework. The permeability into the fault zones are changed in order to understand the role of these structures either as fluid pathways or as efficient compartmentalization barriers for the pore pressure diffusion front associated to wastewater injection. To do this a fault zone model that considers the spatial variation of the hydromechanical properties from the damage zone to the fault core has been implemented in the numerical code. Final aim is to have a more complete picture of the induced seismicity in Val d’Agri oilfield as a consequence of the pore pressure increase as well as its relationship with the deformation style of the fault system and the active stress field
Study of the structure of the lithosphere from the continental plateau of western Ethiopia to the active Afar rift (RESIF-SISMOB) [Data set]. RESIF - Réseau Sismologique et géodésique Français.
No full textSeismological data set from the Ethiopian 2017,MARGIN_AFAR continental plateaus temporary experiment. Seismic Stations: 16. Start date: 2017-01-01. End date: 2018-12-31
Evidence of Fluid Induced Earthquake Swarms From High Resolution Earthquake Relocation in the Main Ethiopian Rift
Full text linkFluid overpressure and fluid migration are known to be able to trigger or induce fault slip. However, relatively little is known about the role of fluids on generating earthquakes in some of the major continental rifts. To address this, we investigate the interaction between fluids and faults in the Main Ethiopian Rift (MER) using a large seismicity catalog that covers both the rift axis and rift margin. We performed cross-correlation analysis on four major earthquake clusters (three within the rift and one on the rift margin) in order to significantly improve accuracy of the earthquake relative relocations and to quantify families of earthquakes in which waveforms are similar. We also analyzed variation of seismicity rate and seismic moment release through time for the four clusters. The major results are that for all four clusters the earthquake relocations are 5–15 km deep, aligned to clear N-NNE striking, steeply (>60°) dipping planes. For the three clusters within the rift, the cross-correlation analysis identifies earthquake families that occur in short swarms during which seismic rate and moment release increases. Together, this space and time pattern of the seismicity strongly points toward them being fluid induced, with fluid likely sourced from depth such as mantle derived CO2. In contrast, the seismicity on the rift margin lacks earthquake families, with occurrence of earthquakes more continuous in nature, which we interpret as pointing toward tectonic stress-driven microseismic creep. Overall, our results suggest that deep sourced fluid migration within the rift is an important driver of earthquake activity
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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