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.
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 magma
Magmatism in the northern MER is currently tectonically controlled in contrast to caldera‐wide deformation in the Central MER
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 2022
InSAR models and seismicity revealed co‐seismic deformation along conjugate faults with no involvement of magma motions
Our observations show that extension can be achieved through purely tectonic processes in magma‐rich continental rifts
•Low-frequency earthquakes detected beneath Tullu Moye, Ethiopia.•Low-rupture velocities caused by high pore-fluid pressures cause the low-frequencies.•Earthquake swarms last less than 30 mins ...indicate fluids are released episodically.•Fluids are sourced from a shallow (4–5 km) magma storage region beneath Tullu Moye.
The active magmatic processes beneath volcanoes in continental rifts is poorly understood. For example, until recently in the East African rift (EAR), the majority of the young volcanoes were thought to be inactive. More recent studies have shown that numerous volcanoes in the EAR are seismically active and deforming rapidly. However, an unambiguous sign of actively degassing magma hosted in shallow magma bodies has eluded most investigators. Here we present detailed analysis of the first low-frequency (LF) earthquake swarms to be observed in the Main Ethiopian Rift. The earthquakes locate to beneath Tullu Moye volcano and are directly related to the presence of a shallow magma body with a high fluid content. Using spectral modelling we show that the LF earthquakes appear to have low stress-drops (1–50 kPa) which we interpret in terms of low rupture velocities and high pore-fluid pressure. Careful relocation of the LF earthquakes place them approximately 4 km below the surface within one of two possible clusters. However, analysis of the correlation between earthquake waveforms show that each swarm contains a range of earthquake families and as such a diversity of earthquake source mechanisms. To explain these observations, we propose the seismicity is induced by H2O/CO2 fluid pulses from the shallow magma body into a highly fractured region. Fluid pulses cause high pore fluid pressures, which also cause the low rupture velocities.
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.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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.
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 InSAR
We observed a seismic sequence occurring in the lower crust along both west‐dipping and east‐dipping faults
Deep seismicity could be caused by fluid migration in the lower crust
In magma-rich continental rifts extension by magma intrusion is thought to accommodate much of the extension. We aim to constrain major melt reservoirs in the crust during magma-rich rifting by ...applying P-to-S receiver functions (RFs) using legacy teleseismic data having magnitudes Mb > 6.0 and epicentral distances ranging from 30° to 90° and collected between the years 2000 and 2013 in 17 temporary broadband stations in Ethiopia and Eritrea. The majority of the NW Plateau crust shows fast Vs of ∼4–4.7 km/s with localized slow Vs (3.2 km/s) and high Vp/Vs (1.85–2.0) in the mid-crust (∼10–25 km depth). The seismic velocity beneath the Afar crust is fairly homogeneous except beneath the current locus of strain at the magmatic segments, which have a relatively fast Vs. (∼4.5 km/s) at a shallow (∼6–14 km) depth underlain by slower Vs. (<3.2 km/s) and high Vp/Vs (2.0) at lower crustal depths (∼20–25 km). The Moho is sharp beneath most of the plateau stations and more gradational beneath Afar with estimated values of 36–44 km in the NW plateau and 26–30 km in Afar. The results point towards the presence of partial melt in localized places in the mid-crust beneath the NW plateau, and in the lower crust beneath the west of Afar, and particularly focused in the lower crust beneath the magmatic segments in Afar. The results suggest that the lower crust is an important melt reservoir for rift-related magmatic processes. The presence of melt in the NW plateau crust is more difficult to explain but is potentially linked to the broad extension of the plateau, or lateral migration of melt from the rift.
Melting of the mantle during continental breakup leads to magmatic intrusion and volcanism, yet our understanding of the location and dominant mechanisms of melt generation in rifting environments is ...impeded by a paucity of direct observations of mantle melting. It is unclear when during the rifting process the segmented nature of magma supply typical of seafloor spreading initiates. Here, we use Rayleigh-wave tomography to construct a high-resolution absolute three-dimensional shear-wave velocity model of the upper 250 km beneath the Afar triple junction, imaging the mantle response during progressive continental breakup. Our model suggests melt production is highest and melting depths deepest early during continental breakup. Elevated melt production during continental rifting is likely due to localized thinning and melt focusing when the rift is narrow. In addition, we interpret segmented zones of melt supply beneath the rift, suggesting that buoyancy-driven active upwelling of the mantle initiates early during continental rifting.
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