The Main Ethiopian Rift (MER) is characterized by extensional tectonics and volcanism, associated with active hydrothermal systems, hydrothermal alteration and fumarolic deposits. The spatial distribution of these hydrothermal products and their link with faults and rock types provides important clues to what controls fluid flow in the subsurface. However, little is known about this in the East African Rift. We address this issue with a multidisciplinary approach in the Fentale-Dofan magmatic segment of the MER, an area characterized by intense volcanic and tectonic activity and a geothermal prospect. Primarily we conduct mapping of hydrothermal alteration and fumarolic deposits, and rock lithologies using a surface feature classification technique of multispectral satellite images. Then we interpret the map using a new database of faults and active hydrothermal manifestations such as hot-springs and fumaroles. We find that the surface hydrothermal alteration and deposits are mainly focused near Fentale and the Dofan Volcanic Complex (DVC). At DVC the hydrothermal products are focused on rhyolites on the western side of the volcano, in an area of intense NNE striking, rift parallel faults. At Fentale volcano the hydrothermal products are mainly associated with ignimbrite and show a circular pattern around the volcanic edifice, but also in places follow the NNE striking faults. At Fentale, the more complex association of hydrothermal products and active manifestations around the edge of the ignimbrite suggests formation contacts may also localize fluid flow in places. At both volcanoes the association between hydrothermal products with either the rhyolites and ignimbrites is likely due to them being relatively easily altered (in comparison to basalt), and also their brittle nature allows for fracturing through which localized fluid flow can occur (as opposed to the sediments). The general pattern of hydrothermal products suggests a stronger structural influence at the DVC with respect to Fentale. The presence of hydrothermal products and active hydrothermal manifestations, along with other lines of evidence such as locus of subsurface dike intrusion at the volcanic centres, suggest that discrete and localized magma reservoirs beneath Fentale and the DVC are the heat source for hydrothermal circulation. Our study also demonstrates that geology, including hydrothermal deposits, can be successfully mapped using automated remote sensing based classification.

Fluids and their migration can influence faulting in various ways, primarily by modifying the state of crustal stresses and hence affecting the nucleation, propagation, and arrest of earthquakes (Hickman et al., 1995;Sibson, 2000;Yamashita & Tsutsumi, 2018). Fluids can modify the elastic parameters of rocks, as an increase in the pore fluid pressure causes an increase of the Poisson's ratio and a decrease in P-and S-wave velocities (Christensen, 1984). Fluids can also control whether a fault zone behaves in a stick-slip or continuous creep fashion (Yamashita & Tsutsumi, 2018).The most common fluids in the crust are H 2 O and CO 2 , which can have a range of origins such as from dehydration of minerals, from magma degassing or simply as fluids trapped in pore spaces, or from percolation of meteoric water (Hickman et al., 1995). In particular, the presence of CO 2 in rifts from magma degassing has been shown to be of potential global significance (Brune et al., 2017). The CO 2 from the mantle can migrate upward as magma ascends through the lithosphere, decompresses, and starts crystallizing and exsolving volatiles, which can then reach the surface through faults and fractures (Lee et al., 2016).

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Recent observations from geology, geodesy and seismicity at magma-rich continental rifts suggest that dike intrusion is the primary mechanism achieving plate extension (Ebinger et al., 2010;Sigmundsson et al., 2015;Wright et al., 2012). In these settings most medium-to-large magnitude, normal slip earthquakes are induced by dikes, while purely tectonic normal faulting is less common (Ebinger et al., 2010). For example, in the magma-rich rifts of Ethiopia (Afar and the Main Ethiopian rift) all the geodetically measured examples of normal faulting (i.e., since the onset of InSAR measurements in the area in 1994) have been induced by dike intrusion (e.g., Nobile et al., 2012;Wright et al., 2012).An earthquake sequence with a mainshock-aftershocks pattern occurred between 26 and 28 December 2022 in northern Afar (Bada region), with several earthquakes recorded globally. The National Earthquake Information Center (NEIC) recorded an earthquake M w 5.5 on 26 December at 12:21:07 (UTC), followed by a second mb 4.6 earthquake at 16:27:13 (UTC) on the same day, and a mb 4.9 at 16:14:57 (UTC) on 28 December (Figure 1). The European Mediterranean Seismological Center (EMSC) recorded the mainshock on 26 December with mb 5.2 and the second earthquake as mb 4.6, similar to the NEIC. On 28 December the EMSC recorded two earthquakes that were not detected by NEIC, both mb 4.5, at 14:02:27 (UTC) and at 14:35:19 (UTC), respectively, and later a mb 4.9 earthquake, the same as the M w 4.9 recorded by NEIC (Figure 1). The focal mechanism of the M w 5.5 earthquake calculated by the Global Centroid Moment Tensor (gCMT) project showed mainly normal faulting on a NNW-SSE striking plane, and with a minor component of strike-slip (Figure 1).