Local seismicity near the actively deforming Corbetti volcano in the Main Ethiopian Rift

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 tectonic processes in the region. We locate 122 events of magnitudes between 0.4 and 4.2 using a new local velocity model. The seismicity is focused in two areas: directly beneath Corbetti caldera and beneath the city of Hawassa. The shallower 0–5 km depth below sea level (b.s.l.) earthquakes beneath Corbetti are mainly focused in EW- to NS-elongated clusters at Urji and Chabbi volcanic centres. This distribution is interpreted to be mainly controlled by a northward propagation of hydrothermal fluids away 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 Corbetti. In contrast, the seismicity beneath 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 earthquake hazard for the local population

Automatic detection for a comprehensive view of Mayotte seismicity

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Initial results from a hydroacoustic network to monitor submarine lava flows near Mayotte Island

In 2019, a new underwater volcano was discovered at 3500 m below sea level (b.s.l.), 50 km east of Mayotte Island in the northern part of the Mozambique Channel. In January 2021, the submarine eruption was still going on and the volcanic activity, along with the intense seismicity that accompanies this crisis, was monitored by the recently created REVOSIMA (MAyotte VOlcano and Seismic Monitoring) network. In this framework, four hydrophones were moored in the SOFAR channel in October 2020. Surrounding the volcano, they monitor sounds generated by the volcanic activity and the lava flows. The first year of hydroacoustic data evidenced many earthquakes, underwater landslides, large marine mammal calls, along with anthropogenic noise. Of particular interest are impulsive signals that we relate to steam bursts during lava flow emplacement. A preliminary analysis of these impulsive signals (ten days in a year, and only one day in full detail) reveals that lava emplacement was active when our monitoring started, but faded out during the first year of the experiment. A systematic and robust detection of these specific signals would hence contribute to monitor active submarine eruptions in the absence of seafloor deep-tow imaging or swath-bathymetry surveys of the active area

Multiscale characterisation of chimneys/pipes: Fluid escape structures within sedimentary basins

Evaluation of seismic reflection data has identified the presence of fluid escape structures cross-cutting overburden stratigraphy within sedimentary basins globally. Seismically-imaged chimneys/pipes are considered to be possible pathways for fluid flow, which may hydraulically connect deeper strata to the seabed. The properties of fluid migration pathways through the overburden must be constrained to enable secure, long-term subsurface carbon dioxide (CO2) storage. We have investigated a site of natural active fluid escape in the North Sea, the Scanner pockmark complex, to determine the physical characteristics of focused fluid conduits, and how they control fluid flow. Here we show that a multi-scale, multi-disciplinary experimental approach is required for complete characterisation of fluid escape structures. Geophysical techniques are necessary to resolve fracture geometry and subsurface structure (e.g., multi-frequency seismics) and physical parameters of sediments (e.g., controlled source electromagnetics) across a wide range of length scales (m to km). At smaller (mm to cm) scales, sediment cores were sampled directly and their physical and chemical properties assessed using laboratory-based methods. Numerical modelling approaches bridge the resolution gap, though their validity is dependent on calibration and constraint from field and laboratory experimental data. Further, time-lapse seismic and acoustic methods capable of resolving temporal changes are key for determining fluid flux. Future optimisation of experiment resource use may be facilitated by the installation of permanent seabed infrastructure, and replacement of manual data processing with automated workflows. This study can be used to inform measurement, monitoring and verification workflows that will assist policymaking, regulation, and best practice for CO2 subsurface storage operations

Imaging lithospheric discontinuities beneath the Northern East African Rift using S -to-P receiver functions

Imaging the lithosphere is key to understand mechanisms of extension as rifting progresses. Continental rifting results in a combination of mechanical stretching and thinning of the lithosphere, decompression upwelling, heating, sometimes partial melting of the asthenosphere, and potentially partial melting of the mantle lithosphere. The northern East African Rift system is an ideal locale to study these processes as it exposes the transition from tectonically active continental rifting to incipient seafloor spreading. Here we use S‐to‐P receiver functions to image the lithospheric structure beneath the northernmost East African Rift system where it forms a triple junction between the Main Ethiopian rift, the Red Sea rift, and the Gulf of Aden rift. We image the Moho at 31 ± 6 km beneath the Ethiopian plateau. The crust is 28 ± 3 km thick beneath the Main Ethiopian rift and thins to 23 ± 2 km in northern Afar. We identify a negative phase, a velocity decrease with depth, at 67 ± 3 km depth beneath the Ethiopian plateau, likely associated with the lithosphere‐asthenosphere boundary (LAB), and a lack of a LAB phase beneath the rift. Using observations and waveform modeling, we show that the LAB phase beneath the plateau is likely defined by a small amount of partial melt. The lack of a LAB phase beneath the rift suggests melt percolation through the base of the lithosphere beneath the northernmost East African Rift system

Evolution of the Volcano-Tectonic seismicity associated with Mayotte's active magmatic system

International audienceIn May 2018, a seismic crisis started East of Mayotte which was widely felt on the island and has been linked to the discovery of a new active submarine edifice. In order to reconstruct the evolution of the crisis with more details on the active structures we re-analyze the seismicity from March 2019 to February 2021 and focus on its evolution throughout the eruption. We use a catalog built by using the neural network-based method PhaseNet, which considerably increased the number of detected earthquakes and enabled a deeper analysis of the seismicity variations. Moreover, the development of a new velocity model for the region allowed a precise location of these earthquakes using only land-stations data. We show that the geometry of the seismicity has evolved in the two main clusters of activity with a general decrease in activity in the proximal cluster and no decrease in activity in the distal cluster. Spatial evolutions of different sub-clusters can be identified over time. This analysis is essential to understand the dynamics of the volcanic and magmatic processes beneath Mayotte island and will provide crucial details on the dynamics of submarine eruptions

Evidence of Fluid Induced Earthquake Swarms From High Resolution Earthquake Relocation in the Main Ethiopian Rift

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. 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 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

ANALYSIS OF EARTHQUAKE SWARMS IN THE NORTHERN MAIN ETHIOPIAN RIFT

The Main Ethiopian Rift (MER) is a continental rift with northward increase in maturity in stages of continental break-up. The MER is characterized by ~60-km-long border faults with high vertical offset that delimit the rift. The majority of rift valley extension is focused to in-rift magmatic segments that have networks of small-offset faults, volcanic centres and associated cone fields. We have studied the spatial, temporal and waveform characteristics of local seismicity from the northern sector of MER. The seismic database contains events from October 2001 to January 2003, and acquired by the Ethiopia Afar Geoscientific Experiment (EAGLE Project). The earthquakes have been relocated with NLLoc using a new 3D velocity model derived from a wide-angle controlled source experiment. The relocated catalog contains a total of 1543 events with magnitudes between 0 and 4. The seismicity is mainly concentrated in two areas: near the Ankober border fault and within the rift near Fentale and Dofen volcanoes. On the border fault, events mostly occur down to 20 km depth, with an average depth of ~ 12 km. Within the rift, the events mostly happen down to 15 km depth, with an average depth of ~ 9 km. The seismicity is divided into several clusters aligned parallel to the rift direction, and in profile sections the clusters are mostly dipping steeply. The analysis of the temporal-spatial distribution of earthquakes shows that some of the clusters are strongly concentrated in time and in space, and therefore swarm-like. b-values were calculated for the identified clusters using the Maximum Likelihood method, with results showing values of b higher than 1. We have conducted a waveform cross correlation on waveform cut 10 seconds before and 60 seconds after P waves arrivals in order to individuate similar events and group similar earthquakes into families. Most of the earthquake clusters are composed of several swarms within which earthquakes are highly correlated, but with different swarms not correlating well with each other. Finally, the cross-correlated P arrivals were used in a new relocation with the HypoDD double-differencing software. Comparison between the different families of swarms, the mapped faults and the active geothermal sites suggest that some seismic swarms could be induced by geothermal fluids

Core level photoelectron spectromicroscopy with Al Kalpha1 excitation at 500 nm spatial resolution

International audienceCore level photoelectron spectromicroscopy in laboratory conditions (XPS imaging) with standard Al Kalpha1 excitation (1486.6 eV), either in scanning or parallel imaging mode, is currently limited to a spatial resolution of ∼4 um. Using energy-filtered X-ray photoelectron emission microscopy (XPEEM) and a bright monochromated Al Kalpha source (photon flux ∼1012 photons/(s.mm2)), we demonstrate refined results regarding lateral and energy resolutions on cross-sectioned epitaxial Si/SiGe layers imaged with photoelectrons of 266.4 eV energy referred to the Fermi level of the sample (Ge 2p3/2 transition). Despite an elemental contrast of only 50%, XPS imaging performed this way has an edge lateral resolution of 480 nm and an energy resolution of 0.56 eV, the spectroscopic information being available at the decanometric scale. Since the lateral resolution is only limited by the counting statistics due to a modest illumination flux, this method paves the way to laterally resolved XPS and UPS in the 100 nm range