Space distribution of the contemporary stress field in the Izu-Bonin Wadati-Benioff zone by inversion of earthquake focal mechanisms

The study addresses the space distribution of the contemporary stress field in the Izu-Bonin Wadati-Benioff zone (WBZ) based on 257 earthquake Harvard centroid moment tensor (CMT) solutions and the inverse technique of Gephart and Forsyth (1984). The stress field parameters are determined along the arc for several depth ranges. The top 30 km of the WBZ are characterized by a homogeneous stress field with slab strike normal maximum compression σ1 and steeper than the slab minimum compression σ3, i.e. the best-fit stress model is related to thrust faulting at the plate interface. The solution for the depth range 31-60 km indicates a varying stress field along the arc, with different stresses in the northern and central-southern parts of the arc. In the northern part of the arc σ1 coincides with the plate convergence and σ3 dips steeply to SW; in the central and southern part σ1 coincides with the slab normal and σ3 dips steeply to the west being slab parallel. The stress inversion result for the depth range 61-200 km shows a heterogeneous stress field with broad possible variations of both σ1 and σ3. The space distribution of earthquakes shows the existence of a well-pronounced seismic gap at depth from ∼150 to 300 km in the northern part and from ∼200 to 400 km in the central-southern part of Izu-Bonin. Below the seismic gap, the dip of the deep portion of the slab changes from ∼40° in the northern part to ∼30° for the central part, and 80° for the southern part. The best-fit stress models for the deep portion of the slab indicate a dominant compressive stress regime, the principal stresses in the central and southern segments of the slab are of similar orientations and differ from those in the northern one. Another specific feature is that the orientation of σ1 correlates with the local dip of the slab in the northern part, while no similar correlation is found for the central and southern parts. A possible reason for the above peculiarities in the stresses could be that the deep-seated slab segments are under superposition of forces, which are related to a slab-slab detachments collision and the resistance of the upper mantle due to viscosity increase below 300 km in the northern part, and the 650 km discontinuity acting as a strong resister to plate penetration in the central-southern parts. The results of this study indicate that the main forces involved in the contemporary subduction dynamics in the Izu-Bonin WBZ are related to the convergence between the Pacific Plate and the Philippine Sea Plate, the slab pull, forces related to a possible slab-slab detachment collision, the mantle resistance at depth below 300 km and the resistance of the 650 km discontinuity.