Seismic Expression of Rift Basins in the East African Rift System

Seismic Expression of Rift Basins in the East African Rift System

Introduction

Extensional regions are commonly associated with low level of seismic strain release and earthquakes of relatively small magnitude. In a synoptic assessment of global rift-related seismicity and magmatism, Parsons and Thompson (1991) showed that magmatic rift zones in particular are characterized by low levels of seismicity. They explained the phenomenon of suppressed faulting by stress equalization due to a buildup of magma pressure counteracting tectonic stress. In such a scenario, rift extension is thought to be accomplished by a combination of magmatic dyke intrusion and active normal faulting, accompanied by small but frequent earthquakes.

Many seismic studies of mantle structure beneath eastern Africa rift have delineated velocity anomalies spatially coincident with the seismic expression on surface in the form of the volcanism, plateau uplift, and rift faulting. In the East Africa Rift, differences in seismicity between magmatically active versus inactive areas are well documented. The Kenya rift in the eastern branch of the rift system is one of the most volumetrically important magmatic extensional areas on Earth. Kenya rift is characterized by basaltic and rhyolitic volcanism and pronounced hydrothermal activity along the tectonically active rift axis attesting to the significant thermal deformation of physical properties of the crust. The East African rift system is widely recognized as the classical example of a continental rift system which is a part of the Afro Arabian rift that extends from the Red Sea to Mozambique in the south (Figure 1). As the rift extends from the Ethiopian segment southwards it bifurcates at about 5oN into the Eastern and Western branches. The two branches of the rift skirts around the Tanzania craton and formed within the Late Proterozoic belts adjacent to the margins of the craton. However, the Eastern Branch that comprises the Ethiopian and Kenya rifts is older and relatively more volcanically active than the western branch that comprises Albertine-Tanganyika- Rukwa-Malawi rifts.

Literature Review

Fairhead and Girdler (1971) suggested that thermal overprinting by up warped asthenosphere lowers the crustal tensile strength in this region, thus decreasing the level of seismically released strain. Thinning of the crust also reduces the potential down-dip rupture width, therefore reducing the seismogenic potential of any fault. In contrast with the magmatically active Kenya rift, higher seismicity levels with historical magnitudes as large as M 7.4 characterize the virtually less volcanic western branch of the East African Rift and the Tanzania rift zone. Amongst these are the recent M 6.8 Kalemie earthquake (2005), M 7.0 Machaze earthquake (2006), and M 6.0 Cyangugu earthquake (2008). The Kenyan rift system can best be considered in two parts, the division being at the Kenya-Tanzania border approximately north and south of 2ºS. In central and southern Kenya, the Rift Valley is about 60 km wide and about 1 km deep. The rift walls are normal faults with many offsets.

Seismic expression under the magmatic rifts normally occur at depths of 25 ...