Lower Crustal Earthquakes in the March 2018 Sequence Along the Western Margin of Afar

Abstract

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.

Novel Bathymetry of Lake Afdera Reveals Fault Structures and Volcano-Tectonic Features of an Incipient Transform Zone (Afar, Ethiopia)

“…Onshore faults have been mapped using satellite imagery (ESRI) and DEM (SRTM, 1 Arc-Second Global Elevation data) in a zone extending from Erta Ale in the north to Manda Harraro in the south. These faults are basically the same as already identified by Varet et al (1975), Illsley-Kemp et al (2018) and La Rosa et al (2019Rosa et al ( , 2021. The lineaments of an area of 60 × 60 km around the lake were analyzed and data plotted in a rose diagram.…”

“…As demonstrated by Illsley-Kemp et al (2018) and La Rosa et al (2019, 2021, the Erta Ale-Tat'Ali Transfer Zone formed the L4 (NW-SE) orientation but also the L2 (N-S), which is shared with the Alayta-Afdera transfer system. The L2 orientation is hence common for the two transfer systems.…”

“…A comparison between the fault pattern of the Alayta-Afdera Transfer Zone and analogue modeling of oblique rifting (McClay et al, 2002, Figure 5D) shows striking similarities, thus providing a conceptual model for the fracture pattern development. The Alayta-Afdera Transfer Zone would thus constitute a broad area of diffuse deformation and important magmatism forming a sinistral linkage zone between the Rosa et al (2019Rosa et al ( , 2021. It shows that this transfer zone formed L2 and L4 orientations.…”

“…Lake Afdera is situated in an area influenced by two transfer zones linking different rift segments. The Alayta-Afdera Transfer Zone links Erta Ale and Tat'Ali rift segments in the NE and the Dabbahu Manda-Hararo in the SW (Tazieff et al, 1972;Barberi et al, 1974;Barberi and Varet, 1977;Stab et al, 2016;Gardo and Varet, 2018;Le Gall et al, 2018;Varet, 2018;Ayalew et al, 2019;Zwaan et al, 2020; see also discussion in Structural Lineaments) and the Erta Ale-Tat'Ali Transfer Zone links Erta Ale and the Tat'Ali rift segments (Illsley-Kemp et al, 2018;La Rosa et al, 2019;2021). The lake is bounded by two volcanic ranges, the Erta Ale range in the NW and the Tat'Ali range in the E, of which the Borwali volcano directly overlooks the lake.…”

“…The second transfer zone is the smaller dextral Erta Ale-Tat'Ali Transfer Zone, which links the Erta Ale and Tat'Ali rift segments and that has been investigated based on seismicity, InSAR, structural geology and described in great details by Illsley-Kemp et al (2018) and La Rosa et al (2019Rosa et al ( , 2021 see Figure 5E.…”