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The subsurface condition of Addis Ababa City is heterogeneous and distinctly varies from place to place, ranging from very soft soil formation to competent rock mass units. This study characterized subsurface variations and estimated site-specific average shear wave velocity changes within the upmost 30 m (Vs30) geological medium at representative sites. We implemented combined active-source Multichannel Analysis of Surface Waves (MASW) and passive-source Microtremor Array Measurements (MAM) surface wave methods to effectively characterize and estimate the shear wave velocity (Vs) of shallow and deep subsurface materials at nine selected sites in the city. The Vs30 of geological profile at the selected sites was attained by joint dispersion inversion analysis of active and passive surface wave data. The result shows that the Vs30 calculated at the selected sites in the city ranges from 256 to 1787 m/s. The study results were used for site classification of near-surface geological materials according to the Vs30 classification guideline proposed by the National Earthquake Hazards Reduction Program (NERHP) and Eurocode-8 (EC-8) classification. Based on Vs30 results, the near-surface materials at the representative sites of the study area are categorized as site types C and D as per NEHRP and as site types B and C as per EC-8. Exceptionally, the rocky Arat Killo site is classified as a site type A in accordance with both NEHRP and EC-8. In addition, we validated acquired shear wave velocity profiles with local geological and engineering geological mapping and geotechnical boreholes proximate to measured sites to control the final inversion quality of Vs values to the corresponding layers. Since this study is the first in Addis Ababa City, the findings will be used for preliminary local site characterization and site specific seismic response analysis. [ABSTRACT FROM AUTHOR]

In magma‐rich rifts, normal faulting is commonly thought to be induced by dike intrusions. However, whether fault slip occurs purely tectonically is unclear. An earthquake sequence starting with a Mw 5.5 earthquake occurred in December 2022 in northern Afar, a continental rift near breakup. InSAR measurements show that seismicity was caused by normal faulting alone, without involvement of magma movements. Our best‐fit InSAR models show that conjugate faults ruptured during the seismic sequence with mainly normal dip‐slip and total deformation corresponding to a Mw 5.7 event, in agreement with local seismic recordings. Our models show that tectonic faulting accommodates 26 cm of extension corresponding to ∼30 years of plate spreading without any link to magma. Our observations point toward significant along‐rift variation in the proportion of extension from faulting, potentially caused by along‐rift variations in rate of extension and/or from a spatially and temporally segmented supply of magma. Plain Language Summary: The Earth's continents move away from each other forming a rift valley between the two separating tectonic plates. In mature stages of the continental rupture process, it is commonly accepted that plate separation occurs from the migration of molten rock (magma) toward the Earth's surface in narrow zones beneath the rift valley. Brittle faulting of the plate was thought to be less important. In this study we analyze the recent earthquake sequence of 26–28 December 2022 in the volcanically active northern Afar rift of Ethiopia. We show that, while the sequence occurred within the rift valley, it was caused by pure faulting without any involvement of magma migrations. These observations are unexpected and show that faults alone can assist plate separation even in mature and magma‐rich rift valleys. Key Points: A seismic sequence with a Mw 5.5 mainshock occurred in the Bada region of northern Afar rift between 26 and 28 December 2022InSAR models and seismicity revealed co‐seismic deformation along conjugate faults with no involvement of magma motionsOur observations show that extension can be achieved through purely tectonic processes in magma‐rich continental rifts [ABSTRACT FROM AUTHOR]

Over the past two decades, multidisciplinary studies have unearthed a rich history of volcanic activity and unrest in the densely-populated East African Rift System, providing new insights into the influence of rift dynamics on magmatism, the characteristics of the volcanic plumbing systems and the foundation for hazard assessments. The raised awareness of volcanic hazards is driving a shift from crisis response to reducing disaster risks, but a lack of institutional and human capacity in sub-Saharan Africa means baseline data are sparse and mitigating geohazards remains challenging. New observations of volcanic and magmatic activity in Africa are changing our views of continental rifting and raising awareness of the associated hazards. However, despite a shift from crisis response to reducing disaster risks, limited capacity means mitigating geohazards remains challenging. [ABSTRACT FROM AUTHOR]

On April 3, 2017, an earthquake of magnitude Mw 6.5 ruptured in Botswana in a region where there was no significant recent tectonic activity and where present-day deformation is believed to be negligible. The event was followed by several aftershocks distributed along NW-SE direction with NE-SW extension direction. We focused on the determination of reliable source parameters for the Mw 6.5 main shock using moment tensor inversion, in both time and frequency domains from regional, broadband waveform data. We retrieve the source depth at 38.4 km, probably the deepest earthquake observed in continental Africa. The estimated hypocentral depth of this earthquake is roughly about the Moho depth beneath the region, reflecting a deep source that is relatively rare in stable continental regions. The result may suggest that the seismogenic depth is as deep as the average global Moho thickness indicating the upper mantle and lower crust region is actively deforming due to a reactivation of the preexisting fault oriented in the NW-SE direction. The resulting focal mechanism of the event shows normal faulting with NE-SW extension direction. The result may provide useful information for contemporary geodynamic investigation of the area. [ABSTRACT FROM AUTHOR]

Studying the mechanisms of interaction between rift segments is key to understanding the kinematics of plate boundaries in continental rifts. However, the spatial and temporal evolution of deformation at rift linkage zones is rarely observed directly. Here, we combine InSAR data spanning 2005–2010 and 2014–2019 from ENVISAT and Sentinel‐1 satellites, respectively, with local seismicity from the Afar rift to investigate the plate‐boundary kinematics of the Afrera linkage zone, the junction between the Erta Ale and Tat Ali magmatic segments in Northern Afar (Ethiopia). We obtain time‐series of cumulative InSAR Line‐Of‐Sight (LOS) displacements that show deformation is accommodated by a series of active en‐echelon faults striking ∼NS and characterized by normal slip associated with a left‐lateral strike‐slip component. Additionally, we observe spatial variation in fault behavior with stick‐slip and creep. The faults in the center of the linkage zone behave primarily in a stick‐slip mode (with abrupt fault displacements up to ∼40 mm) and fault motions are associated with earthquakes of ML > 5. Conversely, faults at the edge of the linkage zone, near the magmatic segments, show creep and some stick‐slip behavior (with cumulative LOS displacement up to ∼30–40 mm over a ∼5‐year period) accompanied by low‐level seismicity. Some of the creeping faults are also spatially associated with hydrothermal springs. We interpret that the temporal behavior of the faults in the linkage zone is controlled by the interplay between tectonic extension, high heat flows, and fluid circulation near the magmatic segments where creeping of some faults is favored. Plain Language Summary: The exterior of the Earth is torn apart along mid‐ocean ridges where magma rises and new oceanic crust is created. Mid‐ocean ridges have an unmissable zig‐zag pattern because while some portions called ridge segments are being torn apart others called offsets, slide past each other. However, how the Earth deforms in these offsets is today poorly understood. The Afar region is the perfect place to address this open question as it is one of the few pales where a mid‐ocean ridge is emerged and the segments and offsets can be observed on‐land. In this study, we combine satellite and earthquakes measurements to film how the surface of the Earth moves in an offset. We show that the network of fractures at the surface of an offset can either move with sudden motions generating major earthquakes or slip continuously over years and produce many smaller earthquakes. Our results provide one of the few direct observations of the different type of motion of the Earth in an offset of the mid‐ocean ridge system. Key Points: InSAR time‐series and seismicity analyses of deformation at the Afrera Plain linkage zone in Northern Afar, EthiopiaTime‐series show fault slip accommodated through stick‐slip and creep along en‐echelon NS‐striking faultsVarying fault behavior could be influenced by elevated heat flows and hydrothermal circulation near the magmatic segments [ABSTRACT FROM AUTHOR]

During the evolution of continental rift systems, extension is thought to progressively focus in‐rift to the future breakup boundary while faults along the rift margins progressively deactivate. However, observational constraints on how strain is partitioned between rift axis and rift margins are still lacking. The Afar rift records the latest stages of rifting and incipient continental breakup. Here, we analyzed the recent MW 5.2 earthquake on the Western Afar Margin on March 24, 2018 and the associated seismic sequence of >500 earthquakes using 24 temporary seismic stations deployed during 2017–2018. We show seismicity occurring at lower crustal depths, from ∼15 to ∼30 km, with focal mechanisms and relocated earthquakes highlighting both west‐dipping and east‐dipping normal faults. We tested earthquake depth using InSAR by processing six independent interferograms using Sentinel‐1 data acquired from both ascending and descending tracks. None of them shows evidence of surface deformation. We tested possible ranges of depth by producing forward models for a fault located at progressively increasing depths. Models show that surface deformation is not significant for fault slip at depths greater than 15 km, in agreement with the hypocentral depth of 19 km derived from seismic data for the largest earthquake. Due to the localized nature of deep earthquakes near hot springs coupled with subsurface evidence for magmatism, we favor an interpretation of seismicity induced by migrating fluids such as magma or CO2. We suggest that deep fluid migration can occur at the rifted‐margin influencing seismicity during incipient continental rupture. Plain Language Summary: The Earth's continents are thinned and broken by extensional forces along rift valleys. Rift valleys are bounded by big fractures (called border faults) that form at the inception of extension and that slip causing earthquakes. As thinning proceeds, molten rock (magma) can rise making its way through the crust. It is not well understood where and how the molten rocks migrate through the crust, and whether, e.g., the large border faults are exploited as pathways. The migration of magma, and the gasses and fluids it releases, can fracture rock causing earthquakes. In this study, we analyzed earthquakes occurring along border faults of the Afar rift of Ethiopia. We found that they occur deep in the crust where previous studies indicate the presence of magma. Our results could suggest that border faults could keep slipping and causing earthquakes as a result of the migration of magma into the deep parts of the crust. Key Points: We studied fault activity and kinematics at the Western Afar Margin using seismicity and InSARWe observed a seismic sequence occurring in the lower crust along both west‐dipping and east‐dipping faultsDeep seismicity could be caused by fluid migration in the lower crust [ABSTRACT FROM AUTHOR]

Lower‐crustal seismicity is commonly observed in continental rift zones despite the crust at such depths being ductile enough to prohibit brittle failure. The source of such deep seismicity across the East African Rift remains an outstanding question. Here we present analysis of an isolated cluster of lower‐crustal earthquakes located on the eastern border faults of the Main Ethiopian Rift, near the Corbetti caldera. Lower‐crustal earthquakes have not previously been observed in this area. Phase arrival times were determined using an automated picking approach based on continuous wavelet transform and statistical changepoint detection methods. We overcome misinterpretations from large hypocenter depth errors by considering mixture distributions for all events and their associated uncertainties. These mixture distributions represent probability density functions of any event occurring at a given depth. The mixture distribution mode for a variety of different velocity models and error parameters remained stable at a depth of 28–32 km, with the vast majority of maximum likelihood estimates for individual hypocenters located at depths of 25–35 km. Most events occur over a 2 month period, with 90% of cumulative seismic moment occurring during March and April 2012. The ephemeral and localized nature of this seismicity, combined with low event magnitudes and regional hydrothermal/magmatic activity, suggests that these lower‐crustal events are likely related to fluid or magmatic processes. Plausible mechanisms include the movement of magma and/or exsolution of volatiles at depth causing transient high strain rates and pore fluid pressures that induce seismicity. Key Points: A sequence of lower‐crustal seismicity is identified 30 km east of Corbetti caldera on the eastern border faults of the Main Ethiopian RiftMixture distributions are used to overcome misinterpretations from large uncertainty in individual event locationsSpatial and temporal characteristics indicate that the source of this seismicity relates to fluid or magmatic processes [ABSTRACT FROM AUTHOR]

The Boset magmatic segment (BMS) of the northern Main Ethiopian Rift (MER) is an ideal natural laboratory to investigate the kinematics, interaction, and rates of activity within a fault network in a magma‐rich rift. In this paper we take advantage of the availability of (1) high‐resolution remote sensing data (LiDAR, ASTER); (2) absolute age chronology on offset reference surfaces; and (3) well‐exposed active normal fault arrays to place new constraints on rift kinematics and strain distribution, and to quantify the architecture and fault slip rates at different temporal scales within a magmatic segment. We found that the rift border faults strike approximately NE, while the younger faults in the rift segments strike NNE. Analyses of geometric rift parameters show that the axial active part of the rift is transtensional with an increase of the shear component northward. The fault displacement analyses and displacement:length ratios increase toward the segment tips, suggesting a significant contribution of fault growth by linkage. In contrast, magmatism is focused on the segment center and localized to a narrow zone. Estimated fault slip rates vary, with rates of up to ~0.37 mm/year in ~0.3 Ma old rift floor deposits, whereas higher rates of up to ~4.4 mm/year are observed for faults cutting through ~6 Ka lavas. The difference in slip rates indicates short‐term variability or a very active recent episode compared to long‐term low average slip rates. Key Points: Large‐scale obliquity in the northern MER increases toward Afar with transtensional kinematicsThe Boset magmatic segment is characterized by two main NNE orientated fault zones, bounding a nested grabenMaximum slip rates are ~0.37 mm/year in ~300 Ka volcanic deposits and ~4.3 mm/year offsetting ~6 Ka lavas of Boset [ABSTRACT FROM AUTHOR]

We provide insights into the tectonics of the Asela‐Langano margin, at the eastern boundary of the central Main Ethiopian Rift, East Africa, by combining field structural data, remote sensing, analysis of the distribution of faults, new dating of faulted material, and analysis of seismicity. The area is characterized by the occurrence of two fault sets: boundary and axial (Wonji) faults, oriented NE‐SW and NNE‐SSW, respectively. Boundary faults show a complex arrangement in the lake Langano area where they display a rhomboidal pattern, due to the presence of NW‐SE‐trending structures, likely related to preexisting transverse fabrics (the so‐called Langano Rhomboidal Fault System). Overall, our analysis supports a structure of the margin characterized by a monocline affected by a series of minor horsts and grabens; this confirms a marked asymmetry of the central Main Ethiopian Rift, with a master fault system on the opposite, western margin. Cumulative paleostress analysis indicates an overall N115°E extension across the Asela‐Langano margin; however, a local variation of the extension direction from axial (Wonji) faults (∼N90°E) to the boundary faults (∼N115°E) can be observed, pointing to a stress reorientation at rift margins. New 14C dating of faulted material and seismicity analysis indicate Late Pleistocene–Holocene and current activity on several normal faults affecting the margin. This suggests that extensional deformation in the central Main Ethiopian Rift is accommodated at the rift margins, with subordinate activity on axial faults, supporting that this rift sector reflects an intermediate stage of rift evolution. Plain Language Summary: The motion of tectonic plates away from each other (extension) in rift valleys, such as in East Africa, causes earthquakes, fissuring and faulting of Earth, and volcanic activity. As a rift valley grows, the regions of most intense earthquake and volcanic activity typically changes through time. In this contribution, we combine several different approaches (analysis of satellite images and surface topography, field observations of geology and faults, and analysis of recent earthquakes) to understand the tectonics of the East African rift in Ethiopia. We identify which faults are most active using the geology and location of earthquakes and also measure the direction in which the faults move. Recent earthquakes include a magnitude 5.3 earthquake in January 2017. We also show that the youngest and currently active faults are associated with volcanoes in the rift valley. Overall, our results improve the knowledge of the deformation, seismicity, and volcanism of the rift valley in Ethiopia, which can help scientists understand how a rift valley develops and which regions are currently hazardous. Key Points: We provide new insights into the architecture and the recent and active tectonics of the Asela‐Langano margin, central Main Ethiopian RiftArchitecture of the margin characterized by a monocline affected by a series of minor horsts and grabens confirms a marked rift asymmetryLate Pleistocene–Holocene and current activity on several normal faults affects the margin, where stress reorientation is observed [ABSTRACT FROM AUTHOR]

Magmatic intrusions play a vital role not only in accommodating extensional stresses in continental rifts but also in feeding volcanic systems. The location, orientation, and timescale of dike intrusions are dictated by the interaction of regional and local stresses, the effect of pre‐existing weaknesses, and the composition of magma. Observing active intrusions can provide important information regarding the interaction between magmatic processes and the tectonic stress field during continental rifting. We focus on a seismic swarm that occurred in 2015 to the northeast of Fentale volcano, in the Main Ethiopian Rift (MER), and use radar interferometry to study surface deformation associated with the seismic swarm. Interferograms show a pattern of dike‐induced deformation, with a model estimate of volume change of 33×106±0.6×106m3 at a depth range of 5.4 to 8 km. We use a small baseline subset algorithm to calculate line of sight time series and find that the displacements decay exponentially with a decay constant of ∼83 days. Coupled source‐sink models suggest that such slow dike intrusions require a high viscosity rhyolitic magma. The difference in behavior between Fentale and other caldera systems in the MER, which show multi‐year cycles of inflation and deflation, suggests fundamental differences in magma composition and architecture of the plumbing system. This is the first direct observation of a dike intrusion in the MER and provides new constraints on the temporal‐spatial patterns of stress and strain that occur during continental rifting. Whether this activity is transient or a long‐term feature associated with rift evolution is an open question. Key Points: We report the first directly detected dike intrusion to occur in the Main Ethiopian Rift (MER)The dike‐induced deformation decayed exponentially (τ= 83 days) consistent with a relatively high viscosity peralkaline rhyolite magmaMagmatism in the northern MER is currently tectonically controlled in contrast to caldera‐wide deformation in the Central MER [ABSTRACT FROM AUTHOR]

The Bora‐Tullu Moye (TM) volcanic field is a geothermal energy prospect in the central Main Ethiopian Rift, but little is known about the seismicity of the region. Here we document seismic activity between February 2016 and October 2017, locating more than a 1,000 seismic events of local magnitude 0 to 2.7. This provides new insights into fluid movement and deformation beneath what we only now realize is a complicated volcanic system. A discrete cluster of events lies beneath TM, but, surprisingly, most of the seismicity lies in two clusters that are beneath neither the Bora or TM edifices. In these regions, we use earthquake cluster orientations, fault plane solutions, and fast seismic shear‐wave orientations to show that seismicity is triggered by hydrothermal circulation of fluids along preexisting fractures. The fractures trend in multiple directions and are, in general, not parallel to rifting related structures. Instead, the fractures are parallel to structures created during previous caldera forming eruptions at both Bora and TM. Highly fractured regions such as this could be attractive targets for geothermal power generation. We estimate a minimum depth for a magmatic body beneath TM to be 6.5 km using the mapped brittle‐ductile transition. Frequency analysis of the earthquake waveforms reveal the bulk of the events to be volcano tectonic, but some low‐frequency seismicity is present at a depth of 5 km beneath the TM edifice triggered by high pore fluid pressures. Key Points: More than 1,000 earthquakes are detected and located around Tullu Moye, a volcano in the Main Ethiopian RiftEarthquakes locate to depths less than 6.5 km and cluster onto multiple orientations of faults where hydrothermal fluids are channeledShear‐wave‐splitting observations and fault plane solutions indicate a rift parallel stress field locally affected by volcanic structures [ABSTRACT FROM AUTHOR]

Abstract: Caldera systems are often restless and experience pulses of uplift and subsidence, with a weak, but significant link to eruption. Characterizing the spatial and temporal patterns of deformation episodes provides insight into the processes responsible for unrest and the architecture of magmatic and hydrothermal systems. Here we combine interferometric synthetic aperture radar images with data from Global Positioning System and a network of seismometers at a continental rift caldera Corbetti, Ethiopia. We document inflation that started mid‐2009 and is ongoing as of 2017, with associated seismicity. We investigate the temporal evolution of the deformation source using a Hastings‐Metropolis algorithm to estimate posterior probability density functions for source model parameters and use the Akaike information criterion to inform model selection. Testing rectangular dislocation and point sources, we find a point source at a depth of 6.6 km (95% confidence: 6.3 − 6.8 km) provides the statistically justified fit. The location of this source is coincident with a conductive anomaly derived from magnetotelluric measurements. We use a joint inversion of two geodetic data sets to produce a time series, which shows a volume input of 1.0 × 107 m3/year. This is the first observation of a prolonged period of magma reservoir growth in the Main Ethiopian Rift and has implications for hazard assessment and monitoring. Corbetti is < 20 km from two major population centers and has estimated return periods of ∼500 and ∼900 years for lava flows and Plinian eruptions, respectively. Our results highlight the need for long‐term geodetic monitoring and the application of statistically robust methods to characterize deformation sources. [ABSTRACT FROM AUTHOR]

Abstract: Continental rifting is a fundamental component of plate tectonics. Recent studies have highlighted the importance of magmatic activity in accommodating extension during late‐stage rifting, yet the mechanisms by which crustal thinning occurs are less clear. The Red Sea rift in Northern Afar presents an opportunity to study the final stages of continental rifting as these active processes are exposed subaerially. Between February 2011 and February 2013 two seismic networks were installed in Ethiopia and Eritrea. We locate 4,951 earthquakes, classify them by frequency content, and calculate 31 focal mechanisms. Results show that seismicity is focused at the rift axis and the western marginal graben. Rift axis seismicity accounts for ∼64% of the seismic moment release and exhibits a swarm‐like behavior. In contrast, seismicity at the marginal graben is characterized by high‐frequency earthquakes that occur at a constant rate. Results suggest that the rift axis remains the primary locus of seismicity. Low‐frequency earthquakes, indicative of magmatic activity, highlight the presence of a magma complex ∼12 km beneath Alu‐Dalafilla at the rift axis. Seismicity at the marginal graben predominantly occurs on westward dipping, antithetic faults. Focal mechanisms show that this seismicity is accommodating E‐W extension. We suggest that the seismic activity at the marginal graben is either caused by upper crustal faulting accommodating enhanced crustal thinning beneath Northern Afar or as a result of flexural faulting between the rift and plateau. This seismicity is occurring in conjunction with magmatic extension at the rift axis, which accommodates the majority of long‐term extension. [ABSTRACT FROM AUTHOR]

We present a synthesis of diverse observations of the first recorded eruption of Nabro volcano, Eritrea, which began on 12 June 2011. While no monitoring of the volcano was in effect at the time, it has been possible to reconstruct the nature and evolution of the eruption through analysis of regional seismological and infrasound data and satellite remote sensing data, supplemented by petrological analysis of erupted products and brief field surveys. The event is notable for the comparative rarity of recorded historical eruptions in the region and of caldera systems in general, for the prodigious quantity of SO emitted into the atmosphere and the significant human impacts that ensued notwithstanding the low population density of the Afar region. It is also relevant in understanding the broader magmatic and tectonic significance of the volcanic massif of which Nabro forms a part and which strikes obliquely to the principal rifting directions in the Red Sea and northern Afar. The whole-rock compositions of the erupted lavas and tephra range from trachybasaltic to trachybasaltic andesite, and crystal-hosted melt inclusions contain up to 3,000 ppm of sulphur by weight. The eruption was preceded by significant seismicity, detected by regional networks of sensors and accompanied by sustained tremor. Substantial infrasound was recorded at distances of hundreds to thousands of kilometres from the vent, beginning at the onset of the eruption and continuing for weeks. Analysis of ground deformation suggests the eruption was fed by a shallow, NW-SE-trending dike, which is consistent with field and satellite observations of vent distributions. Despite lack of prior planning and preparedness for volcanic events in the country, rapid coordination of the emergency response mitigated the human costs of the eruption. [ABSTRACT FROM AUTHOR]

In seismically and tectonically active regions, the present-day strain field tends to bias interpretation of the geological record. This is usually reasonable, but in areas such as triple junctions, the orientation of stress and the locus of strain can evolve abruptly in space and time. We present deformation measurements using satellite radar interferometry (InSAR) and seismicity that together capture the intrusion of a ∼6 km long, ∼1.5 m wide dike into the upper crust of the Ethiopian rift in southern Afar during May 2000. Dike-induced volcano-tectonic seismicity suggests that the intrusion was injected laterally during a period of ∼4 days. Seismic moment release accounts for only 5% of the total 1.6 × 1018 Nm geodetic moment, showing that diking accommodates the majority of strain. The intrusion intriguingly strikes at N122°E, perpendicular to the trend of the present-day East African rift. The geometry and age constraints on faulting and volcanic activity in southern Afar, combined with plate reconstructions, suggest that the dike likely intrudes an ∼ESE-SE striking magmatic system that localized strain during Oligo-Miocene rifting in the Red Sea and Gulf of Aden. We also identify the southerly extent of the Arabian Plate in Afar during the Oligocene in mantle seismic tomographic images: an abrupt increase in seismic velocity in southern Afar is coincident with a stepped increase of ∼20 Myr in the time elapsed since the onset of plate stretching. The anomalous orientation of the May 2000 intrusion implies that African-Arabian tectonics still influences the stress field in southern Afar and is at least partly accommodated by magma intrusion. [ABSTRACT FROM AUTHOR]

Although fault and magmatic processes have achieved plate spreading at mid-ocean ridges throughout Earth's history, discrete rifting episodes have rarely been observed. This paper synthesizes ongoing seismic, structural, space-based geodetic, and petrologic studies from the subaerial Red Sea rift in Ethiopia where a major rifting episode commenced in September 2005. Our aims are to determine the length and timescales of magmatism and faulting, the partitioning of strain between faulting and magmatism, and their implications for the maintenance of along-axis segmentation. Most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the Dabbahu-Manda Hararo rift segment. Strain is accommodated primarily by axial dike intrusions fed from mid-segment magma chamber(s). These findings show that episodic (approximate century interval), rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation. The scale (~65 km x 8 km) and intensity of crustal deformation (~6 m), as well as the volume of intrusive and extrusive magmatism (> 3 km³), provokes a re-evaluation of seismic and volcanic hazards in subaerial rift zones. [ABSTRACT FROM AUTHOR]

A 60-km-long dyke intruded the Dabbahu segment of the Nubia–Arabia Plate boundary (Afar, Ethiopia) in 2005 September, marking the beginning of an ongoing rifting episode. We have monitored the continuing activity using Satellite Radar Interferometry (InSAR) and with data from Global Positioning System (GPS) instruments and seismometers deployed around the rift in response to the initial intrusion. These data show that a sequence of new dyke intrusions has reintruded the central and southern section of the Dabbahu segment. The first was in 2006 June and seven new dykes were emplaced by the end of 2007. Modelling of InSAR data indicates that the dykes were between 0.5 and 2 m wide, up to ∼10 km long and confined to the upper 10 km of crust. An intrusion in 2007 August was associated with a 5-km-long basaltic fissural eruption. During the new dyke injections, InSAR and GPS data show no subsidence at either of the volcanoes at the northern end of the segment, which partly fed the 2005 September dyke. Seismicity data imply that the dykes were probably fed from a source near the Ado'Ale Silicic Complex at the centre of the segment, but the lack of significant subsidence there implies that the source is very deep, or that there was minimal deflation at shallow magma sources. The new dykes are concentrated in an area where the 2005 dyke did not produce significant opening, implying that residual tensile tectonic stresses are higher in this location and are focusing the later intrusions. The sequence of dyke intrusions observed so far is similar to those seen in Iceland during the Krafla rifting episode, which lasted 9 yr from 1975 to 1984. It is likely that, with a continued magma supply, dykes will continue to be intruded until the tectonic stress is fully relieved. As observed at Krafla, eruptions are likely to become more common before the rifting episode is concluded. [ABSTRACT FROM AUTHOR]

Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred within the ∼60-km-long Dabbahu magmatic segment of the Afar rift, a nascent seafloor spreading centre in stretched continental lithosphere. Here we present a three-dimensional deformation field for the Dabbahu rifting episode derived from satellite radar data, which shows that the entire segment ruptured, making it the largest to have occurred on land in the era of satellite geodesy. Simple elastic modelling shows that the magmatic segment opened by up to 8 m, yet seismic rupture can account for only 8 per cent of the observed deformation. Magma was injected along a dyke between depths of 2 and 9 km, corresponding to a total intrusion volume of ∼2.5 km3. Much of the magma appears to have originated from shallow chambers beneath Dabbahu and Gabho volcanoes at the northern end of the segment, where an explosive fissural eruption occurred on 26 September 2005. Although comparable in magnitude to the ten year (1975–84) Krafla events in Iceland, seismic data suggest that most of the Dabbahu dyke intrusion occurred in less than a week. Thus, magma intrusion via dyking, rather than segmented normal faulting, maintains and probably initiated the along-axis segmentation along this sector of the Nubia–Arabia plate boundary. [ABSTRACT FROM AUTHOR]

Seismic anisotropy beneath broad-band stations in the vicinity of the East African rift are compared with those on stable cratonic parts of Africa and Arabia. Such measurements offer potential constraints on rift processes, absolute plate motions (APM) and tectonic structure. New SKS shear wave splitting parameters are analysed beneath the broad-band stations of FURI and AAE (Ethiopia), BGCA (Central African Republic) and RAYN (Saudi Arabia). The number of events considered at the four stations varies from 13 to 32 and provides good azimuthal coverage. Stations on or near the rift show the polarization of the fast shear wave (φ) aligned parallel to the rift axis. The magnitude of the splitting delay (δ t) increases northward along the East African rift. Previously published measurements in Kenya show the smallest splitting value (1.0 s), whilst the Djibouti station, ATD, shows the largest splitting (1.6 s). The Ethiopian results show constancy in δ t and φ with respect to backazimuth, thus, suggesting a single anisotropic layer beneath the stations. There is no observed correlation of φ with APM direction. Less splitting (δ t) is observed beneath cratonic parts of Africa. BGCA in central Africa shows splitting parallel to the inferred direction of transpression, not the APM direction. Receiver-function analysis at FURI and AAE supports evidence from refraction experiments of thick crust ( ca 40 km) in the region of continental rifting, however, the analysis shows a deeper interface at a depth of 90 km, also. This interface may mark the base of the lithosphere in this region. One interpretation of the splitting results is that the anisotropy at the Ethiopian stations is the result of aligned melt in this upper 90 km of lithosphere. per cent volume fraction of melt aligned in thin (aspect-ratio <0.03) vertical ellipsoidal pockets generates sufficient splitting to explain the data. Higher splitting magnitudes in the north correlates with higher melt production observed in the Ethiopian part of the rift. Alternatively, anisotropy may be the result of the alignment of olivine in the asthenosphere parallel to the ridge axis, as material flows laterally to fill the gap caused by lithospheric extension. This would either suggest a northward-thickening anisotropic layer beneath the rift or enhanced olivine alignment as a result of changes in strain. Whatever the mechanism, it appears that the anisotropy is sensitive to, and provides insight into, the transition from continental to oceanic rifting in the northern Ethiopian rift. [ABSTRACT FROM AUTHOR]

Integrated inversions of short-period P, broadband P, and long-period P &s waves are done for fault mechanisms, focal depths, seismic moments, and source-time functions from the largest four earthquakes of the 1985 and 1987 earthquake sequence in south-western Ethiopia. These earthquakes had similar normal-faulting mechanisms. The general trends of the fault planes follow the Main Ethiopian Rift which is in agreement with foreshock-aftershock distribution, surface breaks and geology. Despite the morphological discontinuity of the Main Ethiopian Rift at its southern tip, the mode of deformation of the continental crust under study shows its extension southward. There are no significant strike-slip components trending NW–SE in all the mechanisms which would have been associated with the Aswa Fault Zone in southern Sudan or Anza Rift in northern Kenya. We also infer that the relatively broad fracture zone at the southern extreme of the Main Ethiopian Rift demonstrates the early stage of the break-up between the Nubia and Somalia plates in comparison with the Main Ethiopian Rift proper and the Afar Depression. The main shock of the sequence (Mw = 6.3) ruptured at a depth of 6.8 km, shallower than expected since the depth of earthquakes generally increase southward from the Afar Depression. The shallow depth of earthquake occurrence is supported by surface deformations with an overall trend in the direction of the Main Ethiopian Rift. [ABSTRACT FROM AUTHOR]