Stress Accumulation and Dissipation in Subducting Lithosphere and the Origin of Double and Triple Seismic Zones

A striking feature of plate tectonics is the seismicity caused by subduction of oceanic lithosphere known as the Wadati-Benioff Zone (WBZ). In many subduction zones the seismicity at intermediate depths (70-300km) splits up into two or three distinct layers forming double (DWBZ) or triple (TWBZ) Wadati-Benioff Zones. A 1D "constant force" lithosphere visco-elastic model has been used to investigate the accumulation and dissipation of stress at intermediate depths in subducting lithosphere slab due to the contributions of down-dip slab pull, phase transformation from basalt to eclogite, bending and unbending of the slab, and thermal expansion due to heating. This model has been used to explore processes contributing to double and triple seismic zones within the Wadati-Benioff Zone. Beneath NE Japan, based on focal mechanisms, a triple Wadati-Benioff Zone (TWBZ) has been proposed, partitioning seismicity into an upper layer with down-dip extension, a middle layer with down-dip compression, and a lower layer with down-dip extension. In order to predict the NE Japan TWBZ seismicity pattern, the 1D "constant force" lithosphere visco-elastic model requires the contribution of the basalt to eclogite phase transition in all cases, together with either contributions of slab unbending (c.f. Wang 2002) and thermal stresses, or slab unbending or thermal stresses alone. Down-dip slab pull is not necessary in all models to generate the NE Japan TWBC pattern, depending on the magnitude of unbending and/or thermal stresses. In contrast beneath N Chile, based on relative relocations, a separation of the Wadati-Benioff Zone into two zones has been observed with down-dip extension in both layers. All successful models of this N Chile DWBZ seismicity pattern require the contribution of the basalt to eclogite phase transition and slab pull, together with only small contributions from bending stresses and little or no thermal stresses. The contribution of slab pull, bending/unbending stresses and thermal stresses to stress accumulation in the TWBZ of NE Japan and the DWBZ of N Chile appear to be substantially different, and there is ambiguity regarding the contributions of thermal and slab bending/unbending processes. The relative contributions of thermal, bending/unbending mechanisms to the accumulation of stress within the subducting slab is critical to developing a unified model of Double and Triple Seismic Zones. Additional constraints on the contribution of bending/unbending stresses are required to resolve this ambiguity. Rates of stress accumulation from slab bending/unbending may be derived from observations of Wadati-Benioff Zone curvature.