Spatial distribution of the contemporary stress field in the Kurile Wadati-Benioff zone by inversion of earthquake focal mechanisms

Abstract

The study addresses the spatial distribution of the contemporary stress field and stress regime in the Kurile Wadati-Benioff zone (WBZ) based on homogeneous data of earthquake focal mechanism solutions (FMS) and the inverse technique by Gephart and Forsyth (1984). The data set used consists of 829 Centroid Moment Tensor solutions (time period 1977–2010) and 38 FMS listed in previous studies for intermediate-depth and deep events that occurred prior to 1977.The detailed analysis of the spatial distribution of orientation of P (compression) and T (tension) axes of the individual FMS relative to the local geometry of the subducting slab allowed the outlining of 19 WBZ subvolumes along and across the arc for which the stress field parameters and stress regime (based on the orientations of the principal stresses in a slab's reference frame and the value of R) were evaluated. The stress inversion results show that the shallow portion of the slab (5 WBZ subvolumes), is characterized by sub-horizontal and close to strike-normal maximum compression σ1 and down-dipping minimum compression σ3, the stress regime is of general tension. A two-planar stress pattern with slab-parallel or in-slab σ1 and σ3 in the upper and lower planes, respectively, is observed at intermediate depth all along the arc. An exception of this pattern is found for the slab segment beneath Iturup island where the upper plane is ‘missing’ and the orientations of principal stresses in the depth range 61–140km are similar to these for the lower plane in the slab segment located to the south. Five sub-volumes have been outlined within the Kurile slab at depth greater than 220km, each with their own characteristic focal mechanisms and stress distribution. The subvolume V1 (depth range 225–380km), stretching along the entire arc, is characterized by σ1 and σ3 of SW and ENE orientation, respectively, the stress regime being compressional.The results obtained for the deep portion of the slab indicate along-arc variations in the stress field. The southernmost deep segment of the slab (subvolume V2) is dominated by a tensional stress regime with strike-normal σ1 and close to strike-aligned trending NE σ3. The rest of the deep-seated WBZ subvolumes V3, V4 and V5, situated to the north of V2, are characterized by a compressional stress regime with slab-parallel or in-slab σ1 and slab-normal or strike-normal σ3. However, σ1 is close to slab-parallel only within V4 in the central part of the arc. In the subvolumes V3 and V5, located to the south and north of V4, σ1 rotates counterclockwise relatively to the slab-dip direction at about 50° and 60°, respectively.The results obtained indicate that the main geodynamic forces that drive the presently active processes at different depths along the Kurile Wadati-Benioff zone are the slab pull and ridge push at shallow depths, unbending of the slab at intermediate depths causing the observed two-planar stress pattern, and the mantle resistance in the central and northern deep-seated slab segments. The stress inversion results indicate also additional forces that participate in the contemporary dynamics within several WBZ subvolumes: lateral tension that modifies the σ3 orientation from slab-parallel into strike-aligned, trending NNE and NE in the lower plane of segment K1-2, in the intermediate-depth subvolume beneath Iturup island, and in the deep subvolume V2; forces modifying the orientation of σ1 within the subvolumes V1, V3 and V5 from slab-parallel into strike-aligned, trending SW, W and SW, respectively. The origin of these forces remains unsolved thus posing open questions for further investigations into the dynamics of the Kurile Wadati-Benioff zone.