Stress field in the Ryukyu–Kyushu Wadati–Benioff zone by inversion of earthquake focal mechanisms

The study addresses the space distribution of the stress field in the Kyushu–Ryukyu Wadati–Benioff zone based on homogeneous data of earthquake focal mechanisms and the inverse technique by Gephart and Forsyth J. Geophys. Res. 89 (1984) 9305]. The used data set consists of 148 Harvard CMT solutions and 22 earthquake focal mechanisms listed in previous studies. The stress field parameters are determined for 0–40, 41–100 and h>100 km depth ranges. The top 100-km layer of the Wadati–Benioff zone (WBZ) is characterized by strike normal maximum compression σ₁ and steeper than the slab minimum compression σ₃, the last indicating for unbalanced slab pull force. The Tokara channel ‘divides’ the subduction into two parts of different stress regime at depth greater than 100 km. To the south of the channel the slab is under slab parallel σ₁ and slab normal σ₃ while its northern part, beneath Kyushu, is under slab parallel extension and slab normal compression. The results of recent studies on the regional velocity structure and geochemistry of the volcanic lava indicate that the most plausible reason for the observed stress field difference below 100 km in the northern and rest part of the arc is the presence of hot low viscosity upper mantle west of Kyushu.The results of this study indicate that the forces involved in the contemporary subduction dynamics in the Ryukyu–Kyushu Wadati–Benioff zone are related to the convergence between the Philippine Sea Plate and the Eurasian plate, the trench suction force, slab pull, the slab anchor force and, in the southern-central part of the arc, mantle resistance.