Variations in the seismogenic thickness of East Africa

The well‐established variation in the depth of earthquakes along both branches of the East African Rift System offers an opportunity to probe the controls on the depth‐extent of seismogenesis, and the length‐scales over which this may vary. We present an updated compilation of well‐determined earthquake depths from teleseismic and regional seismic data for the East African Rift System, combined with a summary of the depth distribution of smaller‐magnitude microseismicity from 13 local network deployments. Moderate‐to‐large magnitude (Mw > 5) earthquakes, unrelated to the movement of magmatic fluids, beneath Afar, the Main Ethiopian Rift, and the northernmost sections of the Eastern Branch, are confined to the upper crust. Seismicity along the Western Branch, and the southern‐most sections of the Eastern Branch extends deeper, into the lower crust, in places to depths close to the local Moho. Along the Eastern Branch, in northern Tanzania, the transition between these two regimes occurs over a distance of ≤ 40 km, requiring a change to a higher‐temperature cutoff for the deeper earthquakes; an effect that must be compositional in origin. This compositional variation in the lower crust is most likely related to the degree of hydration. Earthquakes deep within the lower crust are therefore likely to be a proxy for an anhydrous crustal composition.

Plain Language Summary

The maximum depth of earthquakes provides a crucial constraint on how deep the lithosphere can sustain sufficient stresses to produce brittle failure, rather than deforming through slow ductile processes. In East Africa, the maximum depth of earthquakes varies. In the northern sections of the East Africa Rift System, earthquakes are confined to the upper crust shallower than ∼15 km, but in the southern and western sections of the rift, we observe earthquakes occurring much deeper, into the lower crust and potentially the uppermost mantle. This variation, and the short distance over which it takes place in Northern Tanzania, indicates that it is controlled by the composition of the lower crust, which is likely to be anhydrous where the deeper earthquakes occur.