LOWER CRUSTAL EARTHQUAKES IN MAGMATIC AND AMAGMATIC RIFT SECTORS, EAST AFRICA

Unlike extensional basins in collapsing orogens, border fault systems in the active cratonic rifts of East Africa are characterized by lower crustal seismicity, both in magmatic sectors (e.g., Eastern rift, Main Ethiopian rift) and weakly magmatic sectors (e.g., Western rift). Do fluids released from magma degassing and mantle metasomatism cause overpressure and induces seismicity, does the presence of fluids near border faults cause fault zone weakening and microseismicity, or, is the lower crust unusually strong beneath East Africa? We examine the time-space patterns of seismicity using data from several new seismic data sets in East Africa, and place the work in the context of earlier studies to evaluate these models. Specifically, multi-disciplinary studies in the Magadi-Natron rift sectors reveal volumetrically large magmatic CO₂ degassing along border faults with seismicity along projections of surface dips to the lower crust. The magmatic CO₂ degassing and high Vp/Vs ratios and reflectivity of the lower crust implies that the border fault serves a conduit between the lower crustal underplating and the atmospheric. Crustal xenoliths in the Eastern rift sector indicate a granulitic lower crust, which is relatively weak in the presence of fluids, arguing against a strong lower crust. Within magmatic sectors, seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Within some weakly magmatic sectors, lower crustal earthquakes also occur along projections of border faults to the lower crust, and they are prevalent in areas with high Vp/Vs in the lower crust. Other amagmatic sectors, such as the northern and central Malawi rift, lack lower crustal earthquakes. Our comparative studies suggest that 1) border faults have dips > 45 to depths of 25-35 km; 2) magma degassing in the lower crust and upper mantle increases pore pressures and promotes brittle failure; and 3) lower crustal seismicity occurs around the margins of magma bodies, where present.