LOWER-CRUSTAL RIFTING IN THE RUKWA GRABEN, EAST-AFRICA

An M-W 5.9 earthquake occurred in the Lake Rukwa rift, Tanzania, on 19 94 August 18, and was well recorded by 20 broad-band seismic stations at distances of 160 to 800 km and 21 broad-band stations at teleseismi c distances. The regional and teleseismic waveforms have been used to investigate the source characteristics of the main shock, and also to locate aftershocks that occurred within three weeks of the main shock. Teleseismic body-wave modelling yields the following source parameter s for the main shock: source depth of 25+/-2 km, a normal fault orient ation, with a horizontal tension axis striking NE-SW and an almost ver tical pressure axis (Nodal Plane I: strike 126 degrees-142 degrees, di p 63 degrees-66 degrees and rake 280 degrees-290 degrees. Nodal PIane II: strike 273 degrees-289 degrees, dip 28 degrees-31 degrees, and rak e 235 degrees-245 degrees), a scalar moment of 4.1 x 10(17) N m, and a 2s impulsive source time function. Four of the largest aftershocks al so nucleated at depths of 25 km, as deduced from regional sPmp-Pmp tim es. The nodal planes are broadly consistent with the orientation of bo th the Lupa and Ufipa faults, which bound the Rukwa rift to the northe ast and southwest, respectively, The rupture radius of the main shock, assuming a circular fault, is estimated to be 4 km with a correspondi ng stress drop of 6.5 MPa. Published estimates of crustal thickness be neath the Rukwa rift indicate that the foci of the main shock and afte rshocks lie at least 10 km above the Moho. The presence of lower-crust al seismicity beneath the Rukwa rift suggests that the pre-rift therma l structure of the rifted crust has not been strongly modified by the rifting, at least to depths of 25 km.