Abstract
AbstractWe combine numerical modeling of lithospheric extension with analysis of seismic moment release and earthquake b‐value in order to elucidate the mechanism for deep crustal seismicity and seismic swarms in the Main Ethiopian Rift (MER). We run 2‐D numerical simulations of lithospheric deformation calibrated by appropriate rheology and extensional history of the MER to simulate migration of deformation from mid‐Miocene border faults to ∼30 km wide zone of Pliocene to recent rift floor faults. While currently the highest strain rate is localized in a narrow zone within the rift axis, brittle strain has been accumulated in a wide region of the rift. The magnitude of deviatoric stress shows strong variation with depth. The uppermost crust deforms with maximum stress of 80 MPa, at 8–14 km depth stress sharply decreases to 10 MPa and then increases to a maximum of 160 MPa at ∼18 km depth. These two peaks at which the crust deforms with maximum stress of 80 MPa or above correspond to peaks in the seismic moment release. Correspondingly, the drop in stress at 8–14 km correlates to a low in seismic moment release. At this depth range, the crust is weaker and deformation is mainly accommodated in a ductile manner. We therefore see a good correlation between depths at which the crust is strong and elevated seismic deformation, while regions where the crust is weaker deform more aseismically. Overall, the bimodal depth distribution of seismic moment release is best explained by the rheology of the deforming crust.
Highlights
The depth distribution of seismicity in the East African Rift System (EARS) generally shows earthquakes only in the upper crust or alternatively a clear bimodal pattern with peaks in the upper crust and in either the lower crust or upper mantle (e.g., Craig et al, 2011; Yang & Chen, 2010)
We present a detailed numerical modeling study of lithospheric extension and deviatoric stress state in the Main Ethiopian Rift (MER)
Model results are compared with depth distribution of seismicity and seismic swarms, b‐value, and seismic moment release in order to propose a mechanism for deep crustal earthquakes in the MER
Summary
The depth distribution of seismicity in the East African Rift System (EARS) generally shows earthquakes only in the upper crust or alternatively a clear bimodal pattern with peaks in the upper crust and in either the lower crust or upper mantle (e.g., Craig et al, 2011; Yang & Chen, 2010). Geochemistry, Geophysics, Geosystems (1994) proposed that lower crustal earthquakes occur due to high pore fluid pressure from fluids migrating from the upper mantle. This mechanism is recently used to explain most of deep crustal seismicity in the Tanzanian and Kenyan rifts (e.g., Lindenfeld et al, 2012). The bimodal distribution of earthquakes suggests that the deviatoric stress in parts of the middle crust must be lower than that of the upper crust and of the deep crust/upper mantle (Yang & Chen, 2010)
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