Abstract
Lower crustal earthquakes at plate boundaries and intraplate settings occur at depth where deformation is normally expected to occur in a ductile manner. Here we use the available earthquake catalogs and compute theoretical predictions for a range of conditions for the occurrence of lower crustal earthquakes beneath the Main Ethiopian Rift (MER) and adjacent north-western (NW) plateau. Yield strength envelops are constructed using information on geothermal gradient, strain rate, and composition constrained by geophysical observations. Our models suggest that away from the MER beneath the NW plateau the depth distribution of earthquakes in the lower crust is best explained by strong mafic lower crustal rheology and hydrostatic fluid pore pressure conditions. In the same region the effective elastic thickness is similar to seismogenic thickness showing that the lower crust has long-term strength and hence can physically support brittle deformation. On the contrary, in the central MER the seismogenic thickness is much larger than the effective elastic layer thickness implying that the lower crust has no long-term strength. Here our models show that both hydrostatic and near-lithostatic fluid pore pressures fail to explain the observed seismicity and instead a combination of near-lithostatic pore fluid pressure and transient high strain rate due to the movement of fluids provide a plausible mechanism for the occurrence of seismicity in the lower crust. Our interpretations are supported by occurrence of swarms of deep earthquakes beneath the MER, as opposed to more continuous background deep seismicity away from the rift. Using time-depth progression of earthquakes, we estimate permeability values of 5.9 × 10−15m2and 1.8 × 10−14m2at lower crustal depth. The range of permeability implies that seismicity can be induced by pore-pressure diffusion, likely from fluids sourced from the mantle that reactivate preexisting faults in the lower crust. Our thermo-rheological models explain the first order differences in lower crustal earthquakes both directly beneath and outboard of the rift valley.
Highlights
Lower crustal earthquakes have been observed at both plate boundaries and intraplate settings at a depth where deformation is normally expected to occur in a ductile manner
The results of our yield strength envelopes (YSE) calculations presented in Figure 4 allow us to model both the long-term strength of the crust as well as short term earthquake processes (Bürgmann and Dresen, 2008; Hauksson and Meier, 2018)
Several studies have evaluated the consistency between crustal rheology and depth distribution of earthquakes (Déverchere et al, 2001; Albaric et al, 2009; Dong et al, 2018; Hauksson and Meier, 2018; Muluneh et al, 2020) by making an assumption that increased strength results in more seismicity (Hauksson and Meier, 2018)
Summary
Lower crustal earthquakes have been observed at both plate boundaries and intraplate settings at a depth where deformation is normally expected to occur in a ductile manner. Other studies suggest that lower crustal earthquakes are facilitated by high pore fluid pressure (Lindenfeld et al, 2012; Lee et al, 2016; LaRosa et al, 2021), which can locally induce high enough strain rates to cause earthquakes even in areas with high heat flow and weak longterm rheology. It is usually unclear whether the high pore fluid pressure is a transient or a long-term feature
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