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

Summary This study concentrates on the detailed determination of the state of stress in the Wadati–Benioff zone in the lithosphere subducting under Sumatra and Java. The orientations of P‐ and T‐axes of earthquake focal mechanisms (HCMT) are used to define a system of domains in which a uniform state of stress is assumed. The method of Gephart & Forsyth is then used to determine the state of stress in each domain. We succeeded in delimiting eight domains: three (SI–SIII) in the Sumatra region and five (JI–JV) in the Java region. Domains with similar states of stress occur in both regions in similar positions. The maximum compression σ 1 is perpendicular to the trench in domains SI, SII and JII, that is, within the depth range 0–165 km. The orientation of σ 1 is almost parallel to the trench in domains SIII and JIII (depth range 25–225 km). The boundary between domains SII and SIII, and JII and JIII is not distinct (covering the depth range 40–165 km). It seems that the focal mechanisms belonging to domains SII and SIII, and similarly to domains JII and JIII, occur in different stress layers and that we observe an overlap of earthquakes with different focal mechanisms from two different stress‐state layers, parallel to the Wadati–Benioff zone. In domains JIV and JV, the states of stress correspond to the stress pattern observed in other subduction zones: slab‐dip‐parallel extension is observed in domain JIV (depth range 225–315 km) and slab‐dip‐parallel compression is observed in domain JV (deeper than 400 km).