During the August 1989 sequence of earthquakes beneath the Dobi graben in Central Afar, principal events (including 10 shocks with ML≥5.5) propagated -50 km northwestwards in -50 h. The sequence is analyzed in terms of triggering by propagation of a critical pore pressure wave in a compressible fluid through a fractured medium. The macroscopic fluid diffusion tensor D in the crust beneath the graben is estimated. Its principal eigenvector strikes ~N155 ¿E nearly parallel to the graben axis. With a value D¿xx~2 km2/min, it is about 10 times larger than the other two, consistent with fissure anisotropy in the graben. The order of magnitude of the permeability (~10-8 m2) is compatible with characteristic fissure widths ranging from 1 mm to 3 cm. Estimating such parameters in other extensional regions might help to understand better the temporal evolution of seismic swarms.¿ 1997 American Geophysical Union