The Entire Crust can be Seismogenic: Evidence from Southern Malawi

The Bilila-Mtakataka Fault (BMF), at the southern end of the western branch of the East African Rift System (EARS), has been used in various scaling relation studies and arguments about the strength of the lithosphere. We present evidence for a similar, though more degraded, frontal scarp on the graben-bounding synthetic Chirobwe-Ntcheu Fault (CNF), showing that this fault is active simultaneously with the BMF. We deployed 17 geophones for ∼60 days around the southern end of Lake Malawi, across the footwall and hangingwall of the BMF. Continuous microseismicity can be seen from the surface to ∼35 km depth highlighting a plane dipping ∼42°E. Lower-crustal earthquakes have previously been found in the EARS, and based on location and focal mechanism have been hypothesized to occur on planes that line up with the surface traces of large faults. However, no previous study of the EARS has revealed a fault plane throughout the crust that shows seismicity along its full length from the surface to the base of the crust. Rather, the lack of seismicity seen at mid-lower crustal depths, has led some people to the “jelly sandwich” hypothesis. Our results show that the entire crust is seismogenic, so support the “crème brûlée” model. In our two month deployment we recorded 22 aftershocks ML ≥ 2 from the March 8th, 2018 earthquake 200 km south of our array, 7 months after the mainshock, confirming that aftershock sequences in regions of low strain have a long duration, and could be the main component of seismicity in slowly straining regions.