Fine details of the Wadati‐Benioff zone under Indonesia and its geodynamic implications

Abstract

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 <30 km, at depths >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.