Eos, Vol. 73, No. 33, August 18, 1992 VOLUME 73, NUMBER 33 AUGUST 18, 1992 PAGES 345-360 Kenya Rift International Seismic Project, 1989-1990 Experiment PAGES 345, 349-351 G. Randy Keller, L. W. BraUe, P. M. Davis, R. P. Meyer, W. D. Mooney, and the KRISP Working Group Ever since Gregory's work at the turn of the century, the extensional structures ex- tending through central Kenya have been recognized as the classic example of a conti- nental rift zone. Because of the scale and variety of rift features present, this locality has fascinated geologists and geophysicists since its discovery. Rifting is a key process in the evolution of the continents, and the Kenya rift repre- sents a model of comparison for other rifts. Although the surface structure of the Kenya rift is fairly well known, its relationship to deeper features is not. The purpose of the Kenya Rift International Seismic Project (KRISP) is to provide essential seismic data for determining the deep structure of this feature and to use this information to better understand rift processes on a global basis. In this report, we review the results of a pre- liminary experiment conducted in 1985 and report on the initial results of the major ex- periment conducted during 1989-1990. The 1990 refraction results are also discussed by the KRISP Working Party [1991]. Together, these results provide a picture of the Kenya rift that is significantly different than previ- ously inferred, particularly in that the crustal thickness varies greatly along the rift and abrupt and generally symmetrical variations in lithospheric structure occur across the rift. The 1985 Preliminary Experiment In 1985, the KRISP Working Group con- ducted a modest preliminary study to learn G. Randy Keller, Department of Geological Sci- ences, University of Texas at El Paso, El Paso, TX 79968; L. W. Braile, Department of Earth and Atmo- spheric Sciences, Purdue University, West Lafay- ette, IN 47907; P. M. Davis, Department of Earth and Space Sciences, University of California at Los Angeles, Los Angeles, CA 90024; R. P. Meyer, De- partment of Geology and Geophysics, Week Hall, 1215 W. Dayton, University of Wisconsin-Madison, Madison, WI 53708; W. D. Mooney, United States Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025; and the KRISP Working Group (names and affiliations appear at end of article) about local conditions, seismic source prob- lems, and site suitability for long-term re- cording and to obtain enough data to deter- mine the feasibility and design of a larger experiment. This effort included both passive local earthquake and teleseismic monitoring and active seismic refraction profiling. Tele- seismic recording consisted of deployment of a long profile and a local array of seismo- graphs (Figures 1 and 2). The teleseismic data produced evidence of high lower-crustal velocities and low velocities in the upper mantle to depths of 200 km. The upper man- tle low velocities were most pronounced in the 65- to 105-km depth range, where varia- tions across the rift are typically 5-10%. There was also an indication of axial vari- 10 - 35 km percent velocity perturbation Fig. 1. Results of the ACH (Aki-Christoffersson-Husebye) inversion for four depths. Red col- ors indicate the greatest decrease in velocity. [Original color image appears in the back of this volume. ] This page may be freely copied. Eos, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION