ይህንን ጽሁፍ በአማርኛ ለማንበብ እዚህ ይጫኑ።
Si aad u akhrido qoraalkan oo af-Soomaali ah, guji halkan.
ነዚ ጽሑፍ ብትግርኛ ንምንባብ ኣብዚ ጠውቑ።
Barreeffama kana Afaan Oromootiin dubbisuuf as tuquun.
A post Qafar afat takriyuh, akket xukkuta.
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Update: January 3, 2024, 17:48 UTC. News reports indicate that Mount Dofan began to erupt, starting at around 14:00 UTC. A magnitude 5.5 earthquake was also recorded several hours later, at 17:01 UTC. Footage of the eruption is very impressive! A news article published yesterday indicated that even prior to the eruption, there had been significant damage to homes, and many people were evacuating. To see the location of Mount Dofan relative to subsurface activity, scroll to the end of this post for an InSAR image of deformation from Dec. 24-29.
Further note: perhaps the footage shows an associated mud volcano? The news report references a “fiery plume,” so may be a different feature.
January 4, 2024, 00:52 UTC. A M5.8 earthquake is the largest in the sequence so far. USGS Did-You-Feel-It reports reach up to intensity IX (violent), but fortunately the population density around the earthquake is relatively low — and likely lower than usual, following evacuations.
Many moderate magnitude earthquakes have been recorded in Ethiopia over the last two weeks: the USGS reports 55 events above M4, with the two largest reaching M5.1. Relatively little information is available about these events. A handful of reports from Addis Ababa, the capital city, indicate shaking of intensity IV (light) during the larger earthquakes; an even sparser handful from closer to the epicenters reach intensity V (moderate). After one M4.9 earthquake on New Year’s Day, one person ~100 km east of the epicenter wrote: “All things in the home are shaking.”
Ethiopia is no stranger to earthquakes: it is bisected by the East African Rift, one of Earth’s few locations where a continent is actively breaking in two. Here, the Nubia Plate to the west is pulling away from the Somali Plate to the east — at the extremely slow rate of 5 mm/yr. While the southern part of the East African Rift running from Kenya to Mozambique is still in the stage of continental rifting, with the resulting chasms providing room for the great interior lakes of Africa, in northern Ethiopia the breakup has progressed to actual ‘oceanic’ spreading. However, the ocean is absent, because the land is too high. In this area, much like Iceland, a spreading center is actually exposed to air, and we can witness the typically hidden processes that birth new crust on Earth.
At the Earth’s surface, we can trace the East African Rift using topography, seismicity, and volcanoes. At depth, scientists have mapped the system using a technique called tomography — i.e. by making maps of the velocities of seismic waves as they travel every-which-way through the mantle, from earthquakes around the globe to seismometers around the globe. These tomographic images show that a broad zone with low seismic velocity underlies the rift. These low seismic velocities can be attributed to high temperatures or low densities. This “plume” of hot mantle material — known as the African Superplume — is rising due to its low density. Above the plume, the crust has bulged upward. This interaction of a plume with a rift is probably what has caused the spreading centers in the Afar region to rise above sea level.
As the rock within the plume rises, it experiences lower pressures, and parts of it melt. This is decompression melting, where lowering pressures causes a phase transition from solid to liquid, even if the rock is held at the same temperature.
It is this plume that is thought to be responsible for the existence of the East African Rift in the first place, weakening the lithosphere of the African continent and allowing it to break apart. The stretching of the crust along the rift zone further decompresses the mantle below, causing even more volcanism. Mapping out the details of what exactly is going on down there is an area of active study, taking into account the detailed chemistry of different eruptions dating to tens of millions of years ago.
Whatever is going on at depth, the surface exposure of the East African Rift shows a combination of broad uplift, shallower tectonic deformation (steep normal faults lining a deep, stepped valley), and a speckling of volcanoes.
The recent seismic swarm occurred around the “mouth” of the East African Rift, where it broadens from the narrower rift valleys of the south, into the wide Afar Triangle to the north.
While earthquakes occur here semi-regularly, this swarm is actually pretty unusual — when we looked at rates of seismic activity over time, this one stands out well above any other period of time in global catalogs. While larger earthquakes have been recorded, those tended to have typical aftershock sequences. Just check out the histogram below! In general, we don’t like to look at numbers of earthquakes as an indicator of activity. That’s because each larger earthquake is accompanied by many smaller ones, so the “number of quakes” usually tells you more about the quality of your network than the physical process. However, in this case, the recorded events all fall within a magnitude range that should be within detectable limits since at least 2000 CE, so it seems like a fair comparison for the last quarter-century, at least.
Where is this swarm occurring? Mapped epicenters seem to range pretty widely within a region ~170 km N-S and 50 km E-W. We are guessing that most of this scatter reflects low-quality epicenter determinations from global networks, and that the earthquakes are actually more tightly centered. This would match a smaller swarm earlier in 2024 (September-October), which was more tightly clustered.
Given that the earthquakes are occurring as a swarm, rather than a mainshock-aftershock sequence, the most likely scenario here is that they are volcanic, associated with the movement of magma in the subsurface.
So, what volcanoes are nearby? As mapped, the seismicity is closest to two volcanoes: Fentale and Dofan. The Smithsonian Institution Global Volcanism Program tells us that Fentale last erupted in 1820 CE, and before that in the 13th century. For Dofan, the Smithsonian IGVP only indicates that eruptions are unknown, but credible — although the fact that the volcano is known as “smoking mountain” certainly suggests some level of historical activity. All this being said, magmatic intrusions in spreading ridges often occur in the form of long, buried dykes that may not even reach the surface. So, it is extremely difficult to say whether any particular volcano may or may not re-awaken, or a new edifice might be formed.
There is a recent precedent for this kind of event. In 2005, a seismic swarm occurred in northern Afar, which eventually culminated in the emplacement of a huge dyke. The emplaced dyke was about 80 kilometers long and opened up about 8 meters wide, shoving the surrounding crust away toward both sides. The InSAR image below, taken from Ayele et al. (2009), shows how the surrounding crust was pushed outward away from the intruding dyke. Because these intrusions fill up many meters of space between the separating plates, these kinds of events are necessarily quite rare. It will be interesting to see where this new volcanic swarm, located much farther south, will lead!
Edit, January 2 2024 17:30 EST: As noted in the comment by Prof. Dr. Eleanora Rivalta below, an InSAR image was shared on X by Prof. Carolina Pagli showing the ground deformation from Dec. 17 to Dec. 29. An InSAR image is basically a contour image of deformation, where each contour (in this case) represents 2.8 cm of motion towards or away from the satellite passing overhead — so this image represents ~40 cm of total near-vertical motion. The deforming area stretches NNE-SSW, from Fentale north towards (but not quite reaching) Dofen, aligned with mapped fault orientations and the nodal planes of earthquakes in the swarm. Prof. Pagli credits the InSAR UNIPI Group in support to Addis Abeba University.
References:
Ayele, A., Keir, D., Ebinger, C., Wright, T.J., Stuart, G.W., Buck, W.R., Jacques, E., Ogubazghi, G. and Sholan, J., 2009. September 2005 mega‐dike emplacement in the Manda‐Harraro nascent oceanic rift (Afar depression). Geophysical Research Letters, 36(20). https://doi.org/10.1029/2009GL039605
Chang, S.J., Kendall, E., Davaille, A. and Ferreira, A.M., 2020. The evolution of mantle plumes in East Africa. Journal of Geophysical Research: Solid Earth, 125(12), p.e2020JB019929. https://doi.org/10.1029/2020JB019929
Civiero, C., Lebedev, S. and Celli, N.L., 2022. A complex mantle plume head below East Africa‐Arabia shaped by the lithosphere‐asthenosphere boundary topography. Geochemistry, Geophysics, Geosystems, 23(11), p.e2022GC010610. https://doi.org/10.1029/2022GC010610
Earle, S. (2015). Physical Geology. Victoria, B.C.: BCcampus. Retrieved from https://opentextbc.ca/geology/
Hubbard, J. and Bradley, K., 2024. M5.4 earthquake below Lake Baikal. Earthquake Insights, https://doi.org/10.62481/dacecfc7
Hubbard, J. and Bradley, K., 2024. M5.7 earthquake shakes Hawaii. Earthquake Insights, https://doi.org/10.62481/d846ea76
Hubbard, J. and Bradley, K., 2023. Volcanic earthquakes in Iceland. Earthquake Insights, https://doi.org/10.62481/417cdab2
Scoon, R.N., Scoon, R.N. and van Steenbergen, 2018. Geology of national parks of central/southern Kenya and northern Tanzania. Cham: Springer. https://doi.org/10.1007/978-3-319-73785-0
Xu, H., Li, J., Wang, L., Zhang, X. and Feng, B., 2024. Influence of mantle plume on continental rift evolution: A case study of the East African rift system. Petroleum Research. https://doi.org/10.1016/j.ptlrs.2024.02.001
See the interferogram shared by Carolina Pagli from University of Pisa in collaboration with University of Addis Ababa https://x.com/SorcerInSAR/status/1873644455263428785?t=xkCtuLRaXiAd9XjvrNMhfw&s=19
So, effectively, at 5 mms per year, the rift has widened , in the north, by 5 kilometers over the last million years: not much. Yet the rift valley is quite wide, certainly many multiples of 5. Given the complexity of the rift the activity being observed today can’t be representative of activity over its entire history. Can you direct interested, non scientist readers to papers that address the complexity of the region and attempt to explain the apparent mystery of its current state relative to the physical evidence?