Fluid overpressure and fluid migration are known to be able to trigger or induce fault slip. However, relatively little is known about the role of fluids on generating earthquakes in some of the ...major continental rifts. To address this, we investigate the interaction between fluids and faults in the Main Ethiopian Rift (MER) using a large seismicity catalog that covers both the rift axis and rift margin. We performed cross‐correlation analysis on four major earthquake clusters (three within the rift and one on the rift margin) in order to significantly improve accuracy of the earthquake relative relocations and to quantify families of earthquakes in which waveforms are similar. We also analyzed variation of seismicity rate and seismic moment release through time for the four clusters. The major results are that for all four clusters the earthquake relocations are 5–15 km deep, aligned to clear N‐NNE striking, steeply (>60°) dipping planes. For the three clusters within the rift, the cross‐correlation analysis identifies earthquake families that occur in short swarms during which seismic rate and moment release increases. Together, this space and time pattern of the seismicity strongly points toward them being fluid induced, with fluid likely sourced from depth such as mantle derived CO2. In contrast, the seismicity on the rift margin lacks earthquake families, with occurrence of earthquakes more continuous in nature, which we interpret as pointing toward tectonic stress‐driven microseismic creep. Overall, our results suggest that deep sourced fluid migration within the rift is an important driver of earthquake activity.
Plain Language Summary
Fluids such as water and carbon dioxide that come from the deep Earth can move toward the surface by following fractures and faults. When this happens, these fluids make it easier for the faults to move, causing lots of small earthquakes to happen in short periods of time and in the same place. These earthquake swarms have typical characteristics such as waveforms that are incredibly similar to each other. In our study, we are interested in understanding how important the movement of fluids is for the generation of earthquakes during the breakup of continents. We investigated the presence of these characteristics for earthquakes in the Main Ethiopian Rift in East Africa. Major findings are that earthquake swarms within the rift have characteristics that indicate earthquakes are generated by fluid flow along faults. In contrast at the edges of the rift, the earthquakes are different in character, which indicates that they are caused by tectonic motion of the plates rather than fluid migration.
Key Points
Cross correlation and high‐resolution relocation are used to investigate driving mechanisms of seismicity during continental rifting
Earthquake relocation aligns seismicity to N−NNE striking, ∼60° dipping planes corresponding to rift normal faults
The cross‐correlation analysis identifies similar earthquakes that occur in swarms in the rift, pointing toward them being fluid induced
The evolution of the Caribbean plate has resulted in the formation of volcanic arcs, the Caribbean Large Igneous Province (CLIP) and micro‐plates across the plate boundary zones. The northern plate ...boundary with the North American plate has been particularly segmented with the transition from oblique subduction to oblique collision moving from east to west. However, there are few constraints on the seismic structure of the upper mantle across the plate boundary. Here we use S‐to‐P receiver functions to map seismic velocity discontinuities across the plate boundary, placing constraints on crustal and lithospheric thicknesses, as well as the structures associated with subduction and collision. We image a velocity increase with depth, consistently seen at 28–34 ± 4 km along the plate boundary, which corresponds to the Moho. A second strong velocity increase with depth is observed at depths of 64–66 ± 5 km, which is related to the presence of subducting slabs and anisotropic effects. We image a velocity decrease with depth at 95–135 ± 7 km, which reflects a lithosphere‐asthenosphere boundary that varies in depth across the plate boundary. The deepest negative discontinuity spatially maps to the CLIP. We suggest that a deep melting depth at 135 km, associated with an elevated potential mantle temperature of 1585 ± 20°C during CLIP formation, caused a depleted and dehydrated root to the base of melting, thus thickening the lithosphere.
Key Points
We image a crustal thickness of 28–34 km on islands across the North American‐Caribbean plate boundary
We image a thicker lithosphere beneath the Caribbean Large Igneous Province (CLIP) compared to the remainder of the plate boundary
Elevated mantle temperatures and deeper melting during formation of the CLIP resulted in a thicker depleted and dehydrated lithospheric root
Central Afar is shaped by the interaction between the Red Sea (RS) and Gulf of Aden (GoA) rifts. While there have been several studies conducted in the region, we know surprisingly little about the ...mechanism of connection between these two rift branches. Here we use high‐resolution 3D lithospheric scale geodynamic modeling to capture the evolution of linkage between the RS and GoA rifts in central Afar. Our results demonstrate that the two rifts initially overlap and interact across a broad zone of faulting and vertical axis block rotation. However, through time, rift overlap is abandoned in favor of direct linkage which generates a series of localized en‐echelon basins. The present‐day direct linkage between the two rifts is supported by geodetic observations. Our study reconciles previously proposed models for the RS and GoA rift connection by considering spatial and temporal evolution of the rifts.
Plain Language Summary
Rifts are places where tectonic plates move away from each other. They normally start as short, isolated features, and then grow and connect together to eventually form oceans. Central Afar in East Africa is a great location to study how these rifts form and grow by connecting with other rifts. In this area, the Red Sea (RS) and Gulf of Aden (GoA) rifts interact, but it is not clear how this happened through time. Some studies suggest that the two rifts form an overlap zone where blocks within the overlap rotate, while others argue that the two rifts directly link and form a continuous rift zone. To resolve this debate, we conducted a high‐resolution computer simulation of the evolution of the RS and GoA rifts in central Afar. We compared our model results with earthquake positions and satellite data that constrain the present‐day motion of the plates. Our results demonstrate that the RS and GoA rifts first overlapped for a few millions of years, and then formed a direct linkage. Our study suggests that both conceptual models can be reconciled when we consider the temporal evolution of the two rifts through geological times.
Key Points
Analyzing the spatiotemporal evolution of rifts can address the controversial issue on the connection between propagating rifts
We present results from InSAR and geodynamic models to describe the connection between Red Sea (RS) and Gulf of Aden (GoA) rifts in central Afar
The connection between the two rifts evolves from rift overlap to direct linkage elucidating the observed deformation in the region
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.
Continental rift systems form by propagation of isolated rift segments that interact, and eventually evolve into continuous zones of deformation. This process impacts many aspects of rifting ...including rift morphology at breakup, and eventual ocean-ridge segmentation. Yet, rift segment growth and interaction remain enigmatic. Here we present geological data from the poorly documented Ririba rift (South Ethiopia) that reveals how two major sectors of the East African rift, the Kenyan and Ethiopian rifts, interact. We show that the Ririba rift formed from the southward propagation of the Ethiopian rift during the Pliocene but this propagation was short-lived and aborted close to the Pliocene-Pleistocene boundary. Seismicity data support the abandonment of laterally offset, overlapping tips of the Ethiopian and Kenyan rifts. Integration with new numerical models indicates that rift abandonment resulted from progressive focusing of the tectonic and magmatic activity into an oblique, throughgoing rift zone of near pure extension directly connecting the rift sectors.
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.
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
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