Lower Crustal Earthquakes in the March 2018 Sequence Along the Western Margin of Afar. (9th April 2021)

• Record Type: Journal Article
• Authors: La Rosa, Alessandro; Keir, Derek; Doubre, Cecile; Sani, Federico; Corti, Giacomo; Leroy, Sylvie; Ayele, Atalay; Pagli, Carolina
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 4(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-04-09
• Subjects: Afar -- deep seismicity -- rift margins; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2020GC009614
• Languages: English
• ISSNs: 1525-2027

Abstract

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. Plain Language Summary: The Earth's continents are thinned and broken by extensional forces along rift valleys. Rift valleys are bounded by big fractures (called border faults) that form at the inception of extension and that slip causing earthquakes. As thinning proceeds, molten rock (magma) can rise making its way through the crust. It is not well understood where and how the molten rocks migrate through the crust, and whether, e.g., the large border faults are exploited as pathways. The migration of magma, and the gasses and fluids it releases, can fracture rock causing earthquakes. In this study, we analyzed earthquakes occurring along border faults of the Afar rift of Ethiopia. We found that they occur deep in the crust where previous studies indicate the presence of magma. Our results could suggest that border faults could keep slipping and causing earthquakes as a result of the migration of magma into the deep parts of the crust. Key Points: We studied fault activity and kinematics at the Western Afar Margin using seismicity and InSAR We observed a seismic sequence occurring in the lower crust along both west‐dipping and east‐dipping faults Deep seismicity could be caused by fluid migration in the lower crust