Deep crustal earthquakes and active faults along the Rukwa trough, eastern Africa

SUMMARY Since June 1992, a seismic network of five digital stations has been operating in southwestern Tanzania, with the aim of investigating the seismic activity and its partitioning with regard to depth in the area between Lake Rukwa and Lake Malawi. During the period 1992 June 15 to 1994 December 31, a reliable location was obtained for 199 local earthquakes: 13 were recorded by all five seismic stations, 71 by four stations and 115 by three seismic stations, with both P ‐and S ‐wave arrival‐time measurements. Many earthquakes arc located in the south‐eastern part of the Rukwa basin, along its eastern border in a 20 km narrow zone centred on the Lupa fault, but the more active area is the Songwe valley, characterized by a narrowing of the basin in the proximity of the important uplift of the Mbeya range. Seismic activity is also present inside intrabasinal faulted blocks. The earthquake depth distribution presents an upper seismicity cut‐offal a depth of 10 to 12 km. This upper stability zone is followed by the peak of seismic activity in the depth range 12 to 14 km, where 20 per cent of the earthquakes occurred. At greater depths, the level of activity continuously decreases up to a depth of 34 km. A crustal thickness of 42 +4 ‐5 km is estimated using arrivals identified as the PmP waves reflected at the base of the crust, indicating that the deeper earthquakes are really occurring in the lower crust and not in the upper mantle. This analysis confirms at a local scale what is suggested by geophysical and geological observations at a global scale: in the western branch of the East African Rift System, border faults penetrate to lower crustal levels in a lithosphere that is cold and mechanically strong.