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

The study addresses the depth distribution of stresses in the Kamchatka Wad ati-Benioff zone, based on detailed geometry of the deep seismically active structures, homogeneous data of earthquake focal mechanisms and the invers e technique by Gephart and Forsyth (Gephart, J.. Forsyth, D., 1984. An impr oved method for determining the regional stress tensor using earthquake foc al mechanism data: application to the San Fernando earthquake sequence. J. Geophys. Res. 89, 9305-9320). The used data set includes 205 CMT Harvard so lutions for earthquakes which belong to the Wadati-Benioff zone and 17 for a deep-seated subduction segment. The inverse technique used allows for det ermining the best fit principal stress directions sigma (1), sigma (2), sig ma (3) and the ratio R=sigma (2)-sigma (1)/sigma (3)--sigma (1) in several depth intervals in the Wadati-Benioff zone and for the deep-seated slab seg ment considered as a single body. The depth ranges 0-40 and 41-60 km are ch aracterized by maximum compressive stress at which is essentially horizonta l and trends SE, indicating the NW-SE convergence between the Pacific and E urasian lithospheric plates. The minimum compression sigma (3) in the above depth ranges is down-dipping due to the slab pull. The stress tensors obta ined 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 d ips at approximately 40 degrees to SE; the in-slab part is subjected to dow n-dipping to slab-parallel extension and shallow dipping to SE compression. This stress distribution suggests unbending of the subducting plate at dep ths greater than 61 km. The stresses in the depth range 81-90 km are of sim ilar 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 compres sion which is parallel to the dip direction of this volume and by nearly ho rizontal extension of SE trend. These results show that the main geodynamic forces which drive the presently active processes at different depths in t he Kamchatka and Northern Kurille region are the NW-SE convergence between the Eurasian and pacific plates (ridge push) causing the observed almost ho rizontal 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-paralle l extension in the in-slub part. (C) 2001 Elsevier Science Ltd. All rights reserved.