Abstract
AbstractDuring 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.
Highlights
During the early stages of continental rifting, extension generally focuses at the rift margin along large-offset border faults
No significant deformation has been identified in any of the coseismic interferograms in the epicentral area (Figures 5a, 5b and S2) suggesting that coseismic slip occurred at large depth and caused too small surface deformation to be measured by Interferometric Synthetic Aperture Radar (InSAR), as indicated by the deep source location obtained from seismic data
Seismic location and well-constrained focal mechanisms give constraints on the fault kinematics characterizing the marginal grabens during the time period under study
Summary
During the early stages of continental rifting, extension generally focuses at the rift margin along large-offset border faults. Recent seismic observations at mature continental rifts show both intense fault-related seismicity (e.g., Ayele et al, 2007; Illsley-Kemp et al, 2018), and magmatic intrusion (e.g., Ebinger & Belachew, 2010; Pallister et al, 2010) characterizing the incipient passive margins of the Aden, Red Sea, and Afar rifts. This posed several questions regarding the dynamics of faulting, the causes of seismicity, and the related seismic hazard along mature rift margins. Fluid migration from such emplaced melt is interpreted to influence seismicity and fault activity in the area
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