Depth distribution of stresses in the Kamchatka Wadati-Benioff zone inferred by inversion of earthquake focal mechanisms

Abstract

The study addresses the depth distribution of stresses in the Kamchatka Wadati-Benioff zone, based on detailed geometry of the deep seismically active structures, homogeneous data of earthquake focal mechanisms and the inverse technique by Gephart and Forsyth (Gephart, J., Forsyth, D., 1984. An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence. J. Geophys. Res. 89, 9305–9320). The used data set includes 205 CMT Harvard solutions for earthquakes which belong to the Wadati–Benioff zone and 17 for a deep-seated subduction segment. The inverse technique used allows for determining the best fit principal stress directions σ 1, σ 2, σ 3 and the ratio R= σ 2− σ 1/ σ 3− σ 1 in several depth intervals in the Wadati–Benioff zone and for the deep-seated slab segment considered as a single body. The depth ranges 0–40 and 41–60 km are characterized by maximum compressive stress σ 1 which is essentially horizontal and trends SE, indicating the NW–SE convergence between the Pacific and Eurasian lithospheric plates. The minimum compression σ 3 in the above depth ranges is down-dipping due to the slab pull. The stress tensors obtained for the depth layer 61–80 km and K2, the lower part of the subduction zone, indicate a heterogeneous stress field. Further analysis of the above sub-volumes showed that the stress heterogeneity is attributed to different stress regimes in the frontal and in-slab parts of the Wadati–Benioff zone: the frontal part is under slab-parallel compression and extension which dips at approximately 40° to SE; the in-slab part is subjected to down-dipping to slab-parallel extension and shallow dipping to SE compression. This stress distribution suggests unbending of the subducting plate at depths greater than 61 km. The stresses in the depth range 81–90 km are of similar orientations to those in the in-slab parts of 61–80 and 91–230 km but of different dips. The deep-seated slab segment is characterized by compression which is parallel to the dip direction of this volume and by nearly horizontal extension of SE trend. These results show that the main geodynamic forces which drive the presently active processes at different depths in the Kamchatka and Northern Kurille region are the NW-SE convergence between the Eurasian and Pacific plates (ridge push) causing the observed almost horizontal compression of SE trend at 0–60 km depth, the slab pull, unbending forces at depths 61–230 km causing the observed slab-parallel compression in the frontal part of the Wadati–Benioff zone and down-dip to slab-parallel extension in the in-slab part.