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Using about 5200 handpicked P, S, pP, sP, PcP, and ScP phases from digitally recorded seismograms, together with International Seismological Centre reported phases, we obtain improved hypocentral locations for ∼1790 earthquakes, ∼983 of them having 90% confidence limits 50 km, in the period 1962 to September 1996, along the Indonesian subduction zone. We use an improved joint hypocenter determination method in which the solution is more stable than in the original one. We show that for deep earthquakes, use of the core-reflected phases is as good as use of depth phases, and since such core phases often have larger amplitudes and sharper onsets than the depth phases, their arrival times can be read very accurately. The positions of the relocated hypocenters show that (1) a portion of the Indonesian arc between ∼ 110° E and 123° E longitude, and deeper than ∼500 km, is dipping southward at an angle of ∼75°, that is, in a direction opposite to the upper part of the north dipping slab, and (2) east of about 108° E, the seismic zone is wider near 670 km than near the 500 km depth. The first suggests southward lateral flow in the mantle, relative to the plate motion vector here. From the contortion of the seismic zone along the eastern portion of the Indonesian arc, we can estimate the average lateral shear strain rate in the 300 to 670 km depth range to be of the order of 10-16S-1, over the last 10-20 Myr. Copyright 1999 by the American Geophysical Union.

We examine a new data set of reliably relocated earthquakes deeper than 400 km under Indonesia, developed by [Schoffel 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.