How the state of stress varies in the Wadati-Benioff zone: indications from focal mechanisms in the Wadati-Benioff zone beneath Sumatra and Java

This study concentrates on the detailed determination of the state of stres s in the Wadati-Benioff zone in the lithosphere subducting under Sumatra an d Java. The orientations of P- and T-axes of earthquake focal mechanisms (H CMT) are used to define a system of domains in which a uniform state of str ess is assumed. The method of Gephart & Forsyth is then used to determine t he state of stress in each domain. We succeeded in delimiting eight domains: three (SI-SIII) in the Sumatra re gion and five (JI-JV) in the Java region. Domains with similar states of st ress occur in both regions in similar positions. The maximum compression al is perpendicular to the trench in domains SI, SII and JIT, that is, within the depth range 0-165 km. The orientation of al is almost parallel to the trench in domains SIII and JIII (depth range 25-225 km). The boundary betwe en domains SII and SIII, and JII and JIII is not distinct (covering the dep th range 40-165 km). It seems that the focal mechanisms belonging to domain s SII and SIII, and similarly to domains JII and JIII, occur in different s tress layers and that we observe an overlap of earthquakes with different f ocal mechanisms from two different stress-state layers, parallel to the Wad ati-Benioff zone, In domains JIV and JV, the states of stress correspond to the stress pattern observed in other subduction zones: slab-dip-parallel e xtension is observed in domain JIV (depth range 225-315 km) and slab-dip-pa rallel compression is observed in domain JV (deeper than 400 km).