The Entire Crust can be Seismogenic: Evidence from Southern Malawi

• Published in: Tectonics (Washington, D.C.) D.C.), 2021-06, Vol.40 (6), p.n/a
• Main Authors: Stevens, V. L., Sloan, R. A., Chindandali, P. R., Wedmore, L. N. J., Salomon, G. W., Muir, R. A.
• Format: Article
• Language: English
• Subjects: Blasting; Cliffs; Cracks; Crustal Strength; Deployment; Depth; Ears; Earthquakes; East African Rift; Fault lines; Faults; Graben; Instruments; Lakes; Lithosphere; Regions; Scaling; Seismic activity; Seismicity; Seismology; Seismometers
• ISSN: 0278-7407; 1944-9194
• DOI: 10.1029/2020TC006654

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. Plain Language Summary We set out instruments around the southern end of Lake Malawi to detect earthquakes. There is a large fault in the area, the Bilila‐Mtakataka Fault, where a large earthquake has occurred previously. Evidence for this comes from a ∼110 km‐long cliff, averaging 14 m high, showing movement in a past earthquake, which probably happened thousands of years ago. We found a ∼80 km‐long cliff at the base of another large nearby fault, called the Chirobwe‐Ntcheu Fault. This means that large earthquakes could also occur on this fault. There was no previous evidence for what these faults looked like beneath the surface, or whether many small earthquakes were happening on it. We discovered that these faults are probably a continuous straight line underground, highlighted by small earthquakes that occur from the surface to almost 35 km depth. This shows that a very large earthquakes could occur, reaching from this large dep