Variations in the Seismogenic Thickness of East Africa

摘要

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 (M-w > 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.