Seismicity and Crustal Structure of the Southern Main Ethiopian Rift: New Evidence From Lake Abaya

C S Ogden,S Marcou,B A Kibret,I D Bastow,S Gudbrandsson,A Ayele,A Woodward,D Keir,F Ugo

doi:10.1029/2021gc009831

Abstract

AbstractThe Main Ethiopian Rift (MER) has developed during the 18 Ma‐Recent separation of the Nubian and Somalian plates. Extension in its central and northern sectors is associated with seismic activity and active magma intrusion, primarily within the rift, where shallow (5 km) seismicity along magmatic centers is commonly caused by fluid flow through open fractures in hydrothermal systems. However, the extent to which similar magmatic rifting persists into the southern MER is unknown. Using data from a temporary network of five seismograph stations, we analyze patterns of seismicity and crustal structure in the Abaya region of the southern MER. Magnitudes range from 0.9 to 4.0; earthquake depths are 0–30 km.ratios of1.69, estimated from Wadati diagram analysis, corroborate bulk‐crustalratios determined via teleseismic P‐to‐S receiver functionH‐stacking and reveal a relative lack of mafic intrusion compared to the MER rift sectors to the north. There is a clear association of seismicity with the western border fault system of the MER everywhere in our study area, but earthquake depths are shallow near Duguna volcano, implying a shallowed geothermal gradient associated with rift valley silicic magmatism. This part of the MER is thus interpreted best as a young magmatic system that locally impacts the geothermal gradient but that has not yet significantly modified continental crustal composition via rift‐axial magmatic rifting.

Highlights

The Main Ethiopian Rift (MER) captures the transition from embryonic fault-controlled continental rifting in the south to more evolved magma-rich continental rifting in the north (e.g., Hayward & Ebinger, 1996)
VP / VS ratios of 1.69, estimated from Wadati diagram analysis, corroborate bulk-crustal VP / VS ratios determined via teleseismic P-to-S conversion (Ps)-to-S receiver function H- stacking and reveal a relative lack of mafic intrusion compared to the MER rift sectors to the north
There is a clear association of seismicity with the western border fault system of the MER everywhere in our study area, but earthquake depths are shallow near Duguna volcano, implying a shallowed geothermal gradient associated with rift valley silicic magmatism

Summary

Introduction

The Main Ethiopian Rift (MER) captures the transition from embryonic fault-controlled continental rifting in the south to more evolved magma-rich continental rifting in the north (e.g., Hayward & Ebinger, 1996). Seismicity (e.g., Keir et al, 2006, 2009; Wilks et al, 2017) and magnetotelluric (MT) (e.g., Samrock et al, 2015, 2018, 2021; Whaler & Hautot, 2006) studies reveal well-developed hydrothermal systems, which are increasingly being exploited for geothermal power (e.g., Kebede, 2017). Such tectonic and hydrothermal processes are ongoing as far south as Corbetti volcano in the central MER (e.g., Greenfield et al, 2019; Lavayssière et al, 2019; Figure 1). Geophysical constraints on crustal structure and patterns of seismicity are lacking in this region, meaning the magma-poor hypothesis for the southern MER remains untested

Results

Discussion

Conclusion