We examine a new data set of reliably relocated earthquakes deeper than 400 km under Indonesia, developed by [Sch¿ffel and Das, 1999>. The accuracy of this data allows us to demonstrate that the slab thickens, shortens and weakens before penetrating below 670 km by shearing along conjugate fault planes on the upper and lower portions of the seismic zone. We also show that the earthquakes in the lower portion of the Wadati-Benioff zone width are generally fewer and smaller than those in the central portions, with only 8% of the total moment released in the period 1976--1996 being due to earthquakes on the lower side. ¿ 2000 American Geophysical Union