Abstract
We use body-waveform modelling to constrain the source parameters of earthquakes occurring globally in oceanic lithosphere beneath the subduction zone outer rise and outer trench slope. These data are then used to map the stress state in the lithosphere of the downgoing plate as it bends into the subduction zone. Our results provide new constraints on the faulting of oceanic lithosphere at the outer rise, which is important for understanding the transmission of plate-driving forces through the subduction system. In all cases, shallow normal-faulting earthquakes are observed at the top of the plate, and are separated in depth from any deeper thrust-faulting earthquakes. No temporal variation associated with large thrust-faulting earthquakes on the subduction interface is seen in the depth extent of each type of faulting at the outer rise. The transition depth from trench-normal extension to compression is found to vary in agreement with models in which deformation is driven by the combination of in-plane stresses and bending stresses, resulting principally from slab pull. Combining the seismologically derived constraints on the thickness of the elastic core of the plate with estimates of the plate curvature, we place upper bounds on the strength of the lithosphere at the outer rise, which is required to be ≲300 MPa for a constant yield stress model, or governed by an effective coefficient of friction of ≲0.3.
Highlights
Beneath the outer rise and outer trench slope at subduction zones, the incoming plate bends down into the trench in response to compressive stresses transmitted across the subduction interface and to buoyancy-driven forces acting on the descending oceanic lithosphere
This study investigates the seismicity beneath the outer rise and outer trench slope in the majority of regions around the world where significant outer-rise seismic activity has occurred within the instrumental period (Fig. 1), and examines what this seismcity can tell us about the deformation and strength of the plate
The presence of an elastic core separating regions of normal faulting from deeper thrust faulting would be likely to inhibit the penetration of fluids into the deeper parts of the plate
Summary
Beneath the outer rise and outer trench slope at subduction zones, the incoming plate bends down into the trench in response to compressive stresses transmitted across the subduction interface and to buoyancy-driven forces acting on the descending oceanic lithosphere. Cycle modulates the near-trench seismicity in the downgoing plate (Christensen & Ruff 1983, 1988; Lay et al 2009) In this conceptual model, normal-faulting earthquakes in the downgoing plate become progressively inhibited as stress builds up on the subduction interface, and are enhanced following its release, with the converse being applied to compressional earthquakes. Instead of focusing on a single region, this study instead takes a more global perspective, seeking to understand the differences and similarities in outer-rise seismicity between regions
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