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Continental rifting is currently active in East Africa, where breakup of the African continent is generally occurring in relatively focused rift zones within two uplifted plateaus, with magma intrusions the primary mechanism for strain accommodation throughout the crust and mantle lithosphere. Linking the two narrow rift valleys is the low-lying, and as-yet poorly studied Turkana Depression - an unusually broad 300km-wide region of diffuse faulting, seismicity and magmatism. How the East African Rift has developed here remains elusive and is complicated by the fact the Depression was variably stretched by several superposed episodes of failed rifting since the Mesozoic. Utilising data from the NSF-NERC-funded TRAILS seismic network, we produce the first detailed crustal and uppermost-mantle shear-wave velocity model below the Turkana Depression, illuminating Moho and lithosphere-asthenosphere boundary topography that ultimately shed light on rift development in a multiply-rifted region. We find Turkana’s lithosphere is relatively melt-poor, unlike the Ethiopian rift and Plateau further north, which have undergone extensive lithospheric modification by voluminous Cenozoic flood-basalt magmatism and magma-assisted rifting. The lower crust below rift zones in Turkana is not associated with markedly slow (melt) or fast (cooled gabbroic intrusions) wavespeeds suggesting magmatic extension has not dominated rift development in Turkana. Throughout the Depression, the thinnest crust resides within failed Mesozoic rift zones which the present-day East African Rift appears to circumnavigate, not exploit. Fast uppermost mantle wavespeeds below the thinnest crustal regions indicate post-Mesozoic rifting, re-equilibrated and possibly melt-depleted mantle lithosphere, which now renders the plate stronger and more refractory than regions not previously rifted. Refractory Proterozoic lithosphere also present in southern Ethiopia may have influenced strain localisation and the broad, complex rift zone between Ethiopia and Kenya.

We used teleseismic receiver function analysis to image the crustal structure beneath 24 broadband seismic stations densely deployed along two profiles traversing different structural units across the western Afar margin. Our high-resolution receiver function results image pronounced spatial variations in the crustal structure along the profiles and provide improved insights to understand how strain is partitioned in the crust during rifting. Beneath the western plateau next to northern Afar, the crust is likely felsic-to-intermediate in composition (average Vp/Vs 1.74), with a step like thinning of the crust from an average of 38 km beneath the western plateau to an average of 22 km beneath the marginal graben. Consistently thicker crust is observed beneath the southern profile (central Afar), showing four distinct regions of uniform crustal thickness: 1) an average crustal thickness of 42 km beneath the western plateau; 2) 34 km beneath the foothills area; 3) 28 km beneath the marginal graben and the wide extensional basin and 4) 21 km beneath the central rift axis. We use crustal thickness results to estimate a stretching factor β of 2.2 and 2.7 for central Afar and northern Afar respectively. Our estimated values are lower than β > 3.0 predicted from plate reconstructions, and we interpret that the variations are best explained by 2–5 km magmatic addition into the crust. The crustal composition beneath the southern profile is more complex with elevated Vp/Vs ratios ranging between 1.79 and 1.85 beneath the western plateau and marginal graben. This is consistent with a greater mafic component and best explained by crust altered by intrusions due to significant pre and syn-rift magmatic activity. Abnormally high Vp/Vs ratios of more than 1.90 are observed beneath the axial rift zone of central Afar, which most likely suggests the localization of partial melt within the crust.

Post-sunset ionospheric irregularities are common features of the equatorial ionosphere that affect radio communication and navigation systems; their triggering physical mechanism is not yet fully understood. Atmospheric gravity wave is considered as a seeding mechanism for the occurrence of ionospheric irregularities ( Abdu et al., 2009 ). To understand the effects of atmospheric waves, characteristics of wavelike oscillation from ionospheric total electron content (TEC) fluctuation that can be obtained from superposition of different oscillation modes have been investigated. Decomposing fluctuating TEC into different oscillation modes and investigating oscillation characteristics of each component is also important to get insight about the characteristics of individual atmospheric waves that may cause TEC fluctuation. In this paper we have investigated characteristics of components of fluctuating TEC obtained from SCINDA GPS receiver installed at Bahir Dar, (geographic coordinate, 11.5°N, 37.6° E, and dip latitude of 2.5°N) Ethiopia during April 2012. First Empirical Mode Decomposition (EMD) has been applied to decompose TEC fluctuation into different oscillation modes that are known as Intrinsic Mode Function (IMF). Hilbert-Huang Transform (HHT) and Continuous Wavelet Transform (CWT) have been applied to investigate the characteristics of wave-like oscillations. Applying EMD on fluctuating vTEC corresponding to a GPS satellite, five components are found. Results from HHT and CWT have shown excellent agreement. In addition, it is found out that the median periods of oscillation of those five components are 9, 17, 47, 78, and 118 min. Of these periods, 17 and 47 min respectively are oscillation periods of components of TEC fluctuation with occurrence frequency of 92% and 91% that may be interpreted as the manifestation of two frequently occurring components of atmospheric gravity waves that are likely generated by the motion of solar terminator.

Multiple geoscientific studies along the Main Ethiopian and Eastern rifts have revealed that extension via magma intrusion now prevails over plate stretching as the primary mechanism for strain accommodation throughout the crust and mantle lithosphere. However, problematic in this picture is where the Main Ethiopian and Eastern rifts meet, across the low-lying, broadly-rifted, and as-yet poorly-studied Turkana Depression which separates the elevated Ethiopian and East African plateaus. We have so far revealed through body-wave tomography (Kounoudis et al., 2021), that the Depression does not lack mantle dynamic support in comparison to the plateaus, suggesting a significantly thinned crust, resulting from superposed Mesozoic and Cenozoic rifting, most likely explains its low elevations. Slow uppermost-mantle wavespeeds imply the presence of either melt-intruded mantle lithosphere or ponded asthenospheric material below lithospheric thin-spots induced by the region’s multiple rifting phases. To better illuminate the Depression’s lithosphere-asthenosphere system, we conduct a surface-wave analysis to image crust and uppermost-mantle structure using data from the NSF-NERC funded Turkana Rift Arrays Investigating Lithospheric Structure (TRAILS) project broadband seismic network. In particular, we investigate the presence of melt, whether the lithosphere is melt-rich, melt-poor, and/or if ponded zones of asthenosphere exist below variably thinned lithosphere. Group velocity dispersion curves, measured using data from local and regional earthquakes, yield the first high resolution fundamental mode Rayleigh-wave group velocity maps for periods between 4 and 40s for the Turkana Depression. In collaboration with the ongoing TRAILS GPS project, we explore how these results relate to present-day versus past episodes of extension. Kounoudis, R., Bastow, I.D., Ebinger, C.J., Ogden, C.S., Ayele, A., Bendick, R., Mariita, N., Kiangi, G., Wigham, G., Musila, M. & Kibret, B. (2021). Body-wave tomographic imaging of the Turkana Depression: Implications for rift development and plume-lithosphere interactions. G3, 22, doi:10.1029/2021GC009782.

Corbetti is currently one of the fastest uplifting volcanoes globally, with strong evidence from geodetic and gravity data for a subsurface inflating magma body. A dense network of 18 stations has been deployed around Corbetti and Hawassa calderas between February 2016 and October 2017, to place seismic constraints on the magmatic, hydrothermal and fault slip processes occurring around this deforming volcano. We locate 122 events of magnitudes between 0.4 and 4.2 were located using a new local velocity model. The seismicity is focused in two areas: directly beneath Corbetti caldera and beneath the east shore of Lake Hawassa. The shallower 0-5km depth below sea level (b.s.l.) earthquakes beneath Corbetti are mainly focused in NW-elongated clusters at Urji and Chabbi volcanic centres. This distribution is interpreted to be mainly controlled by a northward propagation of hydrothermal fluids from a cross-rift pre-existing fault. Source mechanisms are predominantly strike-slip and different to the normal faulting away from the volcano, suggesting a local rotation of the stress-field. These observations, along with a low Vp/Vs ratio, are consistent with the inflation of a gas-rich sill, likely of silicic composition, beneath Urji. In contrast, the seismicity beneath the east shore of Lake Hawassa extends to greater depth (16 km b.s.l.). These earthquakes are focused on 8-10 km long segmented faults, which are active in seismic swarms. One of these swarms, in August 2016, is focused between 5 and 16 km depth b.s.l. along a steep normal fault beneath the city of Hawassa, highlighting the tectonic hazard for the local population.

The Turkana Depression, a topographically-subdued, broadly-rifted zone between the elevated East African and Ethiopian plateaus, disrupts the N–S, fault-bounded rift basin morphology that characterizes most of the East African Rift. The unusual breadth of the Turkana Depression leaves unanswered questions about the initiation and evolution of rifting between the Main Ethiopian and Eastern rifts. Hypotheses explaining the unusually broad, low-lying area include superposed Mesozoic and Cenozoic rifting and a lack of mantle lithospheric thinning and dynamic support. To address these issues, we have carried out the first body-wave tomographic study of the Depression’s upper mantle. Seismically-derived temperatures at 100 km depth exceed petrological estimates, suggesting the presence of mantle melt, although not as voluminous as the Main Ethiopian Rift, contributes to velocity anomalies. A NW–SE-trending high wavespeed band in southern Ethiopia at urn:x-wiley:15252027:media:ggge22580:ggge22580-math-0001200 km depth is interpreted as refractory Proterozoic lithosphere which has likely influenced the localization of both Mesozoic and Cenozoic rifting. At urn:x-wiley:15252027:media:ggge22580:ggge22580-math-0002100 km depth below the central Depression, a single localized low wavespeed zone is lacking. Only in the northernmost Eastern Rift and southern Lake Turkana is there evidence for focused low wavespeeds resembling the Main Ethiopian Rift, that bifurcate below the Depression and broaden approaching southern Ethiopia further north. These low wavespeeds may be attributed to melt-intruded mantle lithosphere or ponded asthenospheric material below lithospheric thin-spots induced by the region's multiple rifting phases. Low wavespeeds persist to the mantle transition zone suggesting the Depression may not lack mantle dynamic support in comparison to the two plateaus.

The Main Ethiopian Rift (MER) has developed during the 18 Ma-Recent separation of the Nubian and Somalian plates. Extension in its central and northern sectors is associated with seismic activity and active magma intrusion, primarily within the rift, where shallow (urn:x-wiley:15252027:media:ggge22586:ggge22586-math-00015 km) seismicity along magmatic centers is commonly caused by fluid flow through open fractures in hydrothermal systems. However, the extent to which similar magmatic rifting persists into the southern MER is unknown. Using data from a temporary network of five seismograph stations, we analyze patterns of seismicity and crustal structure in the Abaya region of the southern MER. Magnitudes range from 0.9 to 4.0; earthquake depths are 0–30 km. urn:x-wiley:15252027:media:ggge22586:ggge22586-math-0002 ratios of urn:x-wiley:15252027:media:ggge22586:ggge22586-math-00031.69, estimated from Wadati diagram analysis, corroborate bulk-crustal urn:x-wiley:15252027:media:ggge22586:ggge22586-math-0004 ratios determined via teleseismic P-to-S receiver function H-urn:x-wiley:15252027:media:ggge22586:ggge22586-math-0005 stacking and reveal a relative lack of mafic intrusion compared to the MER rift sectors to the north. There is a clear association of seismicity with the western border fault system of the MER everywhere in our study area, but earthquake depths are shallow near Duguna volcano, implying a shallowed geothermal gradient associated with rift valley silicic magmatism. This part of the MER is thus interpreted best as a young magmatic system that locally impacts the geothermal gradient but that has not yet significantly modified continental crustal composition via rift-axial magmatic rifting.

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.

An ambassador for African geosciences and for Ethiopia, this celebrated geophysicist will be remembered for his contributions in research as well as his steadfast support of colleagues.

Transform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid‐ocean ridges in ocean basins worldwide. The current consensus is that oceanic transform faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of transform fault formation. Here we integrate evidence from surface faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto‐transform fault is initiating during the final stages of continental breakup. This is the first direct observation of proto‐transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late‐stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading., Geochemistry, Geophysics, Geosystems, 19 (12), ISSN:1525-2027

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 hypocentre 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 two‐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, all suggest 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.

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.

During the evolution of continental rift systems, extension focuses along on-axis magmatic segments while extensional structures along the rift margins seem to progressively become inactive. However, how strain is partitioned between rift axes and rift margins is still poorly understood. The Afar Rift is the locus of extension between Nubia, Arabia and Somalia and is believed to record the latest stages of rifting and incipient continental break-up. The Afar rift axis is bounded at its western margin by a seismically active system of normal faults separating the Afar depression from the Ethiopian Plateau through a series of large bounding faults and marginal grabens. Although most of the extension in Afar is currently accommodated on-axis, several earthquakes with Mw > 5.0 occurred in the past decades on the Western Afar Margin (WAM). Here we analysed the most recent Mw 5.2 earthquake on the WAM on 24 March 2018 and the following seismic sequence using data recorded by a temporary seismic network, set up between 2017 and 2018. We located 800 events from the 20 March to the 30 April 2018 using twenty-three local seismic stations and a new velocity model for the WAM based on a new receiver function study. Preliminary results show that seismicity during the 2018 event focused at mid-to-low crustal depths (from ~15 km to ~35 km) along west-dipping fault planes. Shallower upper crustal earthquakes also occurred on west-dipping fault planes.The hypocentral location of the mainshock has also been investigated using InSAR. We processed four independent interferograms using Sentinel-1 data acquired from a descending track. None of them shows any significant surface deformation, confirming the large depth of the hypocenters. Furthermore, we tested possible ranges of depth by producing a series of forward models assuming fault located at progressively increasing depths and corresponding to a Mw 5.2 earthquake. Our models show that surface deformations are < 1 cm at depths greater than 15 km, in agreement with our hypocentral depth of 18 km for the main shock estimated from seismic data. Our seismicity observations of slip along west-dipping faults show that deformation across the WAM is currently accommodated by antithetic faulting, as suggested by structural geology studies. Lower crustal earthquakes might occur in a strong lower crust due to the presence of mafic lower crust and/or be induced by migrating fluids such as magma or CO2.

Lateral heterogeneities in crust and mantle structure influence the distribution of strain and magmatism in continental rift zones. Sutures between Archaean cratons and younger orogenic belts represent some of Earth’s largest lateral heterogeneities: > 170 km-thick, buoyant and relatively dry lithosphere juxtaposed to ~120 km-thick, more volatile-rich mantle lithosphere. The seismically and volcanically active Turkana Depression between the Ethiopian and East African plateau magma-rich Eastern rift formed near the eastern edge of the Archaean Tanzania craton. This area was affected by rifting in Mesozoic and Paleogene time, and may have been a thin zone when magmatism started at ~40 Ma. Several hypotheses had been proposed to explain the unusual ~300 km-breadth of the Turkana Depression. We use new data from the Turkana Rift Arrays to Investigate Lithospheric Structure (TRAILS) is to evaluate spatial variations in the location of strain, and in the direction and magnitude of seismic anisotropy, which is strongly influenced by mantle flow patterns along lithosphere-asthenosphere topography, fluid-filled cracks (e.g., dikes), and pre-existing mantle lithosphere strain fabrics. Complementary data sets provide a strong contextual framework. Our results and those of regional studies show that strain is currently localized to ~100 km-wide section of the Depression, and the western sectors are inactive. We suggest that the original location of strain and magmatism was near the eastern edge of the Tanzania craton above a steep lithosphere-asthenosphere gradient, and that rifting has migrated eastward to form a more contiguous zone between the Main Ethiopian and Eastern rift zones.

In continental rifts structural heterogeneities, such as pre-existing faults and foliations, are thought to influence shallow crustal processes, particularly the formation of rift faults, magma reservoirs and surface volcanism. We focus on the Corbetti caldera, in the southern central Main Ethiopian Rift. We measure the surface deformation between 22nd June 2007 and 25th March 2009 using ALOS and ENVISAT SAR interferograms and observe a semi-circular pattern of deformation bounded by a sharp linear feature cross-cutting the caldera, coincident with the caldera long axis. The signal reverses in sign but is not seasonal: from June to December 2007 the region south of this structure moves upwards 3 cm relative to the north, while from December 2007 until November 2008 it subsides by 2 cm. Comparison of data taken from two different satellite look directions show that the displacement is primarily vertical. We discuss potential mechanisms and conclude that this deformation is associated with pressure changes within a shallow (

The earthquakes associated with continental deformation are spatially and temporally variable and are fundamental in understanding fault activity and seismogenic hazards. We conduct KK ‐means cluster analysis on seismicity in the African–Arabian rift systems to create the first computationally objective analysis of the pattern of earthquakes. We use seismic moment to compute spatial variations in maximum credible earthquake (Mcred) and likely time to the next major release of seismic energy. Our best‐fit model has 32 clusters of ∼100–400 km∼100–400 km in length, with cluster size decreasing northward along the rift and cluster boundaries correlating with major structural segmentation of the rift. Mcred varies between MwMw 5.2 and 7.4 across the whole dataset, with the highest values estimated in portions of the rift where the majority of extension is accommodated by seismogenic failure.

Space-time distribution of earthquakes in the East African Rift System (EARS) has been investigated for the years 2013–2016. The spatial earthquakes distribution in the time window considered in this study has revealed that the Main Ethiopian Rift (MER) is structurally linked to the eastern branch of the EARS. A relatively high level of seismicity has been obtained in this study compared to the ISC (International Seismological Centre) catalog for the same time period which implies the region is seismically more active than we thought. In addition to the rift floor, the rift margins and the surrounding plateaus are found to be seismically active. The western and eastern branches of the EARS are found to be more active than the other rift segments for the time period considered. The frequency-magnitude distribution (FMD) of events during this study period provided an average b- and a -values of ~1.01 and ~6.5, respectively. The b-value result is implying relatively high stress with tectonic origin earthquakes occurrence dominating the region. On the other hand, threshold magnitude completeness of Mc 3.0 is obtained which is the minimum value that has never been observed previously for such large area. The seismic energy map of the region has been systematically investigated and peaks energy-release is observed at Afar depression, eastern and western branches. The energy mapping shows that the MER is structurally connected to the eastern branch of the EARS which is consistent with seismicity distribution. The overall depth of occurrence of events in the eastern and the western branches of the EARS are relatively deeper than the one found for Afar and MER. This implies that the seismogenic thickness increases in EARS when we go from north to south which is in agreement with rift age, magmatism and influence of the African Super plume on the crust and upper mantle deformation in EARS.

Seismic hazard calculation is carried out for the Horn of Africa region (0°–20° N and 30°–50°E) based on the probabilistic seismic hazard analysis (PSHA) method. The earthquakes catalogue data obtained from different sources were compiled, homogenized to M w magnitude scale and declustered to remove the dependent events as required by Poisson earthquake source model. The seismotectonic map of the study area that avails from recent studies is used for area sources zonation. For assessing the seismic hazard, the study area was divided into small grids of size 0.5° × 0.5°, and the hazard parameters were calculated at the center of each of these grid cells by considering contributions from all seismic sources. Peak Ground Acceleration (PGA) corresponding to 10% and 2% probability of exceedance in 50 years were calculated for all the grid points using generic rock site with Vs = 760 m/s. Obtained values vary from 0.0 to 0.18 g and 0.0–0.35 g for 475 and 2475 return periods, respectively. The corresponding contour maps showing the spatial variation of PGA values for the two return periods are presented here. Uniform hazard response spectrum (UHRS) for 10% and 2% probability of exceedance in 50 years and hazard curves for PGA and 0.2 s spectral acceleration (Sa) all at rock site are developed for the city of Addis Ababa. The hazard map of this study corresponding to the 475 return periods has already been used to update and produce the 3rd generation building code of Ethiopia.

The silicic volcanic centres of the Main Ethiopian Rift (MER) play a central role in facilitating continental rifting. Many of these volcanoes host geothermal resources and are located in heavily populated regions. InSAR studies have shown several are deforming, but regional seismic networks have detected little seismicity. A local network of 12 seismometers was deployed at Aluto Volcano from 2012 to 2014, and detected 2142 earthquakes within a 24-month period. We locate the events using a 1D velocity model that exploits a regional model and information from geothermal boreholes and calculate local magnitudes, b-values and focal mechanisms. Event depths generally range from the near surface to 15 km with most of the seismicity clustering in the upper 2 km. A significant amount of seismicity follows the Artu Jawa Fault Zone, which trends in alignment with the Wonji Fault Belt, NNE–SSW and is consistent with previous studies of strain localisation in the MER. Focal mechanisms are mostly normal in style, with the mean T-axes congruent to the orientation of extension in the rift at this latitude. Some show relatively small left-lateral strike-slip components and are likely associated with the reactivation of NE-ENE structures at the southern tip of the Aluto-Gedemsa segment. Events range from − 0.40 to 2.98 in magnitude and we calculate an overall b-value of 1.40 ± 0.14. This relatively elevated value suggests fluid-induced seismicity that is particularly evident in the shallow hydrothermal reservoir and above it. Subdividing our observations according to depth identifies distinct regions beneath the volcanic edifice: a shallow zone (− 2–0 km) of high seismicity and high b-values that corresponds to the hydrothermal system and is influenced by a high fluid saturation and circulation; a relatively aseismic zone (0–2 km) with low b-values that is impermeable to ascending volatiles; a region of increased fluid-induced seismicity (2–9 km) that is driven by magmatic intrusion from below and a deeper zone (below 9 km) that is interpreted as a partially crystalline, magmatic mush. These observations indicate that both the magmatic and hydrothermal systems of Aluto volcano are seismically active and highlight the need for dedicated seismic monitoring at volcanoes in the MER.

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.

Seismicity can be used to better understand interactions between magma bodies, hydrothermal systems and their host rocks—key factors influencing volcanic unrest. Here, we use earthquake data to image, for the first time, the seismic velocity structure beneath Aluto, a deforming volcano in the Main Ethiopian Rift. Traveltime tomography is used to jointly relocate seismicity and image 3D P- and S-wave velocity structures and the ratio between them (V P/V S). At depths of 4–9 km, the seismicity maps the top of a large low velocity zone with high V P/V S, which we interpret as a more ductile and melt-bearing region. A shallow (

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 degrees E extension across the Asela-Langano margin; however, a local variation of the extension direction from axial (Wonji) faults (similar to N90 degrees E) to the boundary faults (similar to N115 degrees E) can be observed, pointing to a stress reorientation at rift margins. New(14)C 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.

The reliable source parameters, the current deformation associated to the earthquakes and the current deformation in relationship with dynamic change of Victoria plate rotation are generally poorly understood owing to the lack of the proper techniques used in the Circum-Tanzania Craton. Therefore, the reliable estimated source parameters and the detail interpretation are very crucial. Here, moment tensor inversion is made from broadband seismic data for five earthquakes that occurred in the region in the years 2014 and 2016 with magnitude ranging from Mw 4.1 to 5.7. The 2014 sequence comprises four earthquakes where two of them occurred north of Kondoa with purely normal faulting mechanisms following the N–S oriented rift structure. The other two earthquakes from the 2014 group occurred north of Dodoma and are found to be normal faulting with significant strike-slip component with dextral sense of motion trending in the NW-SE direction which seems to agree with fault structure and aftershock distribution. On the other hand, focal mechanism of the 2016 earthquake in Kagera region with magnitude Mw 5.7 shows dominantly normal faulting trending WNW-ESE with strike-slip component and dextral sense of motion. The interpreted fault orientation for the Kagera earthquake seems to agree with macroseismic information and aftershock distribution which were recorded by seismic stations deployed immediately after the main shock (Mulibo, 2019). We therefore find that the observed normal, strike-slip and oblique slip deformation seems to be influenced by the extensional stress regime of the East African Rift System and the anti-clockwise rotation of the Victorian plate. On the other hand, it is not uncommon to observe deactivation of the NW-SE oriented weak zones in the East African Rift System which seems to agree with our fault mechanism results. In addition, the waveform inverted depths of the studied earthquakes vary from 12 to 26 km in the Circum-Tanzania Craton is implying relatively larger effective elastic thickness compared to the northern part of the East African Rift System in the Afar region.

The Bora ‐ Tullu‐Moye 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 Feb 2016 and Oct 2017, locating more than a 1000 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 Tullu‐Moye (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 pre‐existing 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 (LF) seismicity is present at a depth of 5 km beneath the TM edifice triggered by high pore fluid pressures.

Moho depths beneath four stations of the Ethiopian Seismic Station Network (ESSN) are estimated from P-wave receiver functions (RF). We used high quality seismic data recorded at ANKE (Ankober), DILA (Dilla), HARA (Harar) and SEME (Semera) stations for earthquakes located at epicentral distances ranging from 30 to 100° with magnitude mb ≥ 5.5. We applied a frequency domain deconvolution technique to remove source and propagation path effects from the earthquakes waveforms to make the RFs dependent only on the structure beneath the seismic stations. The minimum number of teleseismic earthquakes used is 14 for HARA while the maximum is 39 for SEME station. A linearized-iterative inversion is applied on the generated radial component of the receiver functions to obtain P-wave velocity models beneath the stations. We achieved a reasonably good fit between the observed and synthetic RFs, which demonstrated the high quality of the inversion process. From the obtained models we estimated Moho depths of 26 ± 2 km for SEME, 36 ± 2 km for DILA, 38 ± 2 km for HARA and 42 ± 1.7 km for ANKE. The lowest Moho depth is observed at Semera station which implies a thinned crust while the highest crustal thickness is observed at Ankober, which lies along the North western plateau margin. Our results agree with previous observations which strengthen the hypothesis that Moho depths estimated for stations that lie within the rift and rift margins are lower than those located in the plateaus. Our RFs inversions show a low velocity gradient at about 16 km depth at Semera station, interpreted as evidence for lower crustal storage of partial melt.

Observations of ground deformation in East Africa have been fundamental for unveiling the tectonics of continental rifting, assessing the seismic and volcanic hazard to development, and identifying geothermal resources. Here we investigate the active natural and anthropogenic processes in the Tendaho Graben, Afar using Interferometric Synthetic Aperture Radar (InSAR) collected by the Envisat satellite in 2004–2010. We used the Poly-Interferometric Rate And time series Estimation (π-RATE) method to calculate displacement in satellite line-of-sight, and a least-square inversion to decompose the line-of-sight displacement into vertical and rift perpendicular components. We observe two zones of deformation: a 20 km wide circular region of subsidence located 10 km northeast of the town of Semera with a maximum displacement rate of ∼5 cm/yr; and elongated zone (50 km) of subsidence in the area of the geothermal prospect, maximum rate of ∼4 cm/yr. The temporal characteristics of subsidence varies between these zones, with an increase in subsidence rate observed in the circular region in August 2008. We used a Bayesian inversion to find the best fitting source models and compared this to locations of seismicity and other geophysical observations. The pattern of deformation is consistent with a combination of magmatic and geothermal processes, but there does not appear to be a direct link to a sequence of dyke intrusions during 2005–2010 at Manda Hararo graben ∼60 km away, but dynamic stress changes or deep crustal flow could account for the observations.

Seasonal variations in the seismicity of volcanic and geothermal reservoirs are usually attributed to the hydrological cycle. Here, we focus on the Aluto-Langano geothermal system, Ethiopia, where the climate is monsoonal and there is abundant shallow seismicity. We deployed temporary networks of seismometers and GPS receivers to understand the drivers of unrest. First, we show that a statistically significant peak in seismicity occurred 2–3 months after the main rainy season, with a second, smaller peak of variable timing. Seasonal seismicity is commonly attributed to variations in either surface loading or reservoir pore pressure. As loading will cause subsidence and overpressure will cause uplift, comparing seismicity rates with continuous GPS, enables us to distinguish between mechanisms. At Aluto, the major peak in seismicity is coincident with the high stand of nearby lakes and maximum subsidence, indicating that it is driven by surface loading. The magnitude of loading is insufficient to trigger widespread crustal seismicity but the geothermal reservoir at Aluto is likely sensitive to small perturbations in the stress field. Thus we demonstrate that monsoonal loading can produce seismicity in geothermal reservoirs, and the likelihood of both triggered and induced seismicity varies seasonally.

A major rifting episode began in the Afar region of northern Ethiopia in September 2005. Over a 10-day period, c. 2.5 km3 of magma were intruded into the upper crust along a 60 km-long dyke separating the Arabian and Nubian plates. There was an intense seismic swarm and a small rhyolitic eruption; extension of up to 10 m occurred across the rift segment. Over the next five years, a further 13 dyke intrusions caused continued extension, eruptions and seismicity. The activity in Afar led to a renewed international focus on the role of magmatism in rifting, with major collaborative projects involving researchers from Ethiopia, the UK, the USA, France, Italy and New Zealand working in Afar and Ethiopia to study the ongoing activity and to place it in a broader context. This book brings together articles that explore the role of magmatism in rifting, from the initiation of continental break-up through to full seafloor spreading. We also explore the hazards related to rifting and the associated volcanism. This renewed focus on magmatism and its role in rifting has implications for our understanding of how continents break-up and the associated distribution of resources in rift basins and continental margins.

During the breakup of continents in magmatic settings, the extension of the rift valley is commonly assumed to initially occur by border faulting and progressively migrate in space and time toward the spreading axis. Magmatic processes near the rift flanks are commonly ignored. We present phase velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only the upper crust beneath the axial volcanic systems but also both upper and lower crust beneath the rift flanks where ongoing volcanism and hydrothermal activity occur at the surface. Magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process and may persist through to early seafloor spreading.

The East African Rift System (EARS) has natural hazards – earthquakes, volcanic eruptions, and landslides along the faulted margins, and in response to ground shaking. Strong damaging earthquakes have been occurring in the region along the EARS throughout historical time, example being the 7.4 (Ms) of December 1910. The most recent damaging earthquake is the Karonga earthquake in Malawi, which occurred on 19th December, 2009 with a magnitude of 6.2 (Ms). The earthquake claimed four lives and destroyed over 5000 houses. In its effort to improve seismic hazard assessment in the region, Eastern and Southern Africa Seismological Working Group (ESARSWG) under the sponsorship of the International Program on Physical Sciences (IPPS) carried out a study on active fault mapping in the region. The fieldwork employed geological and geophysical techniques. The geophysical techniques employed are ground magnetic, seismic refraction and resistivity surveys but are reported elsewhere. This article gives findings from geological techniques. The geological techniques aimed primarily at mapping of active faults in the area in order to delineate presence or absence of fault segments. Results show that the Karonga fault (the Karonga fault here referred to as the fault that ruptured to the surface following the 6th–19th December 2009 earthquake events in the Karonga area) is about 9 km long and dominated by dip slip faulting with dextral and insignificant sinistral components and it is made up of 3–4 segments of length 2–3 km. The segments are characterized by both left and right steps. Although field mapping show only 9 km of surface rupture, maximum vertical offset of about 43 cm imply that the surface rupture was in little excess of 14 km that corresponds with Mw = 6.4. We recommend the use or integration of multidisciplinary techniques in order to better understand the fault history, mechanism and other behavior of the fault/s for better urban planning in the area.

The Danakil region of northern Afar is an area of ongoing seismic and volcanic activity caused by the final stages of continental breakup. To improve the quantification of seismicity, we developed a calibrated local earthquake magnitude scale. The accurate calculation of earthquake magnitudes allows the estimation of b ‐values and maximum magnitudes, both of which are essential for seismic‐hazard analysis. Earthquake data collected between February 2011 and February 2013 on 11 three‐component broadband seismometers were analyzed. A total of 4275 earthquakes were recorded over hypocentral distances ranging from 0 to 400 km. A total of 32,904 zero‐to‐peak amplitude measurements ( A ) were measured on the seismometer’s horizontal components and were incorporated into a direct linear inversion that solved for all individual local earthquake magnitudes ( M L), 22 station correction factors ( C ), and 2 distance‐dependent factors ( n , K ) in the equation M L=log( A )−log( A )+ C . The resultant distance correction term is given by −log( A )=1.274336log( r /17)−0.000273( r −17)+2. This distance correction term suggests that attenuation in the upper and mid‐crust of northern Afar is relatively high, consistent with the presence of magmatic intrusions and partial melt. In contrast, attenuation in the lower crust and uppermost mantle is anomalously low, interpreted to be caused by a high melt fraction causing attenuation to occur outside the seismic frequency band. The calculated station corrections serve to reduce the M L residuals significantly but do not show a correlation with regional geology. The cumulative seismicity rate produces a b ‐value of 0.9±0.06, which is higher than most regions of continental rifting yet lower than values recorded at midocean ridges, further supporting the hypothesis that northern Afar is transitioning to seafloor spreading. [Electronic Supplement:][1] List of all local earthquakes used in the study with calculated local magnitudes and associated magnitude error. [1]: http://www.bssaonline.org/lookup/suppl/doi:10.1785/0120150253/-/DC1

Earthquakes of low to intermediate magnitudes are a commonly observed feature of continental rifting and particularly in regions of Quaternary to Recent volcanism such as in the Main Ethiopian Rift (MER). Although the seismic hazard is estimated to be less in the Hawassa region of the MER than further north and south, a significant earthquake occurred on the 24th January 2016 in the Hawassa caldera basin and close to the Corbetti volcanic complex. The event was felt up to 100 km away and caused structural damage and public anxiety in the city of Hawassa itself. In this paper we first refine the earthquake's location using data from global network and Ethiopian network stations. The resulting location is at 7.0404°N, 38.3478°E and at 4.55 km depth, which suggests that the event occurred on structures associated with the caldera collapse of the Hawassa caldera in the early Pleistocene and not through volcano-tectonic processes at Corbetti. We calculate local and moment magnitudes, which are magnitude scales more appropriate at regional hypocentral distances than (mb) at four stations. This is done using a local scale (attenuation term) previously determined for the MER and spectral analysis for ML and MW respectively and gives magnitude estimates of 4.68 and 4.29. The event indicates predominantly normal slip on a N-S striking fault structure, which suggests that slip continues to occur on Wonji faults that have exploited weaknesses inherited from the preceding caldera collapse. These results and two previous earthquakes in the Hawassa caldera of M > 5 highlight that earthquakes continue to pose a risk to structures within the caldera basin. With this in mind, it is suggested that enhanced monitoring and public outreach should be considered.

Thermal models predict that spreading velocity at mid-ocean rifts should influence the geometry of the underlying magma chamber. InSAR data from the slow-spreading Ethiopian Rift identify a shallow, elongated magma chamber—a feature usually associated with fast-spreading rifts—beneath the Erta Ale segment, implying that spreading velocity may not be so important after all.

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 × 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.

The Afar area is one of the biggest continental hotspots active since about 30 Ma. It may be the surface expression of a mantle “plume” related to the African Superswell. Central Africa is also characterized by extensive intraplate volcanism. Around the same time (30 Ma), volcanic activity re-started in several regions of the African plate and hotspots such as Darfur, Tibesti, Hoggar and Mount Cameroon, characterized by a significant though modest volcanic production. The interactions of mantle upwelling with asthenosphere, lithosphere and crust remain unclear and seismic anisotropy might help in investigating these complex interactions. We used data from the global seismological permanent FDSN networks (GEOSCOPE, IRIS, MedNet, GEO- FON, etc.), from the temporary PASSCAL experiments in Tanzania and Saudi Arabia and a French deployment of 5 portable broadband stations surrounding the Afar Hotspot. A classical two-step tomographic inversion from surface waves performed in the Horn of Africa with selected Rayleigh wave and Love wave seismograms leads to a 3D-model of both S V velocities and azimuthal anisotropy, as well as radial S H / S V anisotropy, with a lateral resolution of 500 km. The region is characterized by low shear-wave velocities beneath the Afar Hotspot, the Red Sea, the Gulf of Aden and East of the Tanzania Craton to 400 km depth. High velocities are present in the Eastern Arabia and the Tanzania Craton. The results of this study enable us to rule out a possible feeding of the Central Africa hotspots from the “Afar plume” above 150–200 km. The azimuthal anisotropy displays a complex pattern near the Afar Hotspot. Radial anisotropy, although poorly resolved laterally, exhibits S H slower than S V waves down to about 150 km depth, and a reverse pattern below. Both azimuthal and radial anisotropies show a stratification of anisotropy at depth, corresponding to different physical processes. These results suggest that the Afar hotspot has a different and deeper origin than the other African hotspots (Darfur, Tibesti, Hoggar). These latter hotspots can be traced down to 200 km from S-wave velocity but have no visible effect on radial and azimuthal anisotropy.

A local magnitude ( M L) scale and seismicity catalog for the Ethiopian Plateau have been developed using data collected by the 2000–2002 Ethiopia Broadband Seismic Experiment. Locations for 253 local and regional events have been obtained using P -wave arrival times recorded on four or more stations. For constructing the M L scale, waveforms were corrected for instrument response and convolved with the nominal Wood–Anderson torsion seismograph response. Maximum S -wave amplitudes were then picked on the horizontal components of ground motion (3218 total observations) and used in an inversion for event magnitudes, two model parameters, and 54 horizontal component station corrections. The distance correction obtained from the inversion is -log A =0.726log( r /100)+0.000558( r -100)+3.0, where r is the hypocentral distance in kilometers. Seven of the 253 events can be considered ground truth (GT) events, with epicentral locations accurate to within 5 km according to the GT5 local criteria of Bondar et al. (2004). In contrast to previously reported ground-motion attenuation for the Main Ethiopian Rift, we find relatively low ground-motion attenuation for the Ethiopian Plateau, reflecting variations in crustal structure between the Ethiopian Plateau and Main Ethiopian Rift.