Testing Models for Upper Mantle Earthquakes in the Tanganyika-Rukwa Rift, Africa

Abstract

Rifts that initiate in mechanically strong, stable continental lithosphere are characterized by M > 5 earthquakes at depths >35 km near or below the crust mantle interface. Current models for deep rift zone earthquakes invoke elevated pore pressures associated with magmatism, and rapid stressing from magma intrusions. We evaluate the role of static stress changes caused by lateral density contrasts (magma intrusions) in the upper mantle and crust-mantle interface on mantle earthquakes. We use numerical models of static stress changes, and test models against seismic data from the Tanganyika rift, East Africa which has upper mantle earthquakes and locally elevated crustal Vp/Vs. The earthquake source mechanisms from these ML2.4–2.8 earthquakes show steep nodal planes and little correlation to crustal source mechanisms and E–W extension direction. Likewise, the direction of fast splitting from seismic anisotropy studies is oblique to predictions from earthquake source mechanisms. The models of density contrasts (magma intrusions) explored in this study predict differential stresses of 1–10 MPa, stresses consistent with average earthquake stress drops. The local stress field rotations around the edges of intrusive bodies can explain the variable source mechanisms and match the local rotation of shear wave splitting direction, providing a plausible mechanism for mantle earthquakes in cratonic rifts. By analogy, exhumed mantle from rift zones reveals pseudotachylites near intrusions. Metasomatic reactions in contact aureoles broaden density contrasts and reduce upper mantle strength, potentially enabling extension of initially cold, strong continental lithosphere.