| Abstrakt: |
Double seismic zones (DSZs), parallel planes of intermediate‐depth earthquakes inside oceanic slabs, have been observed in a number of subduction zones and may be a ubiquitous feature of downgoing oceanic plates. Focal mechanism observations from DSZ earthquakes sample the intraslab stress field at two distinct depth levels within the downgoing lithosphere. A pattern of downdip compressive over downdip extensive events was early on interpreted to indicate an unbending‐dominated intraslab stress field. In the present study, we show that the intraslab stress field in the depth range of DSZs is much more variable than previously thought. Compiling DSZ locations and mechanisms from literature, we observe that the "classical" pattern of compressive over extensive events is only observed at about half of the DSZ locations around the globe. The occurrence of extensional mechanisms across both planes accounts for most other regions. To obtain an independent estimate of the bending state of slabs at intermediate depths, we compute (un)bending estimates from slab geometries taken from the slab2 compilation of slab surface depths. We find no clear global prevalence of slab unbending at intermediate depths, and the occurrence of DSZ seismicity does not appear to be limited to regions of slab (un)bending. Focal mechanism observations are frequently inconsistent with (un)bending estimates from slab geometries, which may imply that bending stresses are not always prevalent, and that other stress types such as in‐plane tension due to slab pull or shallow compression due to friction along the plate interface may also play an important role. Plain Language Summary: In subduction zones, a plate of oceanic lithosphere descends into the mantle. This means it gets bent from a horizontal orientation offshore the subduction zone to an inclined orientation. Analogous to the bending of a solid beam, this bending of the oceanic lithosphere creates extension in the upper part and compression in the lower part of the oceanic plate. The orientation of these stresses can be retrieved from earthquake focal mechanisms for events that occur in the outer rise region, that is, offshore the actual subduction zone. At deeper depths, downgoing slabs are thought to straighten, which means they decrease their curvature and "unbend." This has the opposite signature in earthquake focal mechanisms as the bending. We compiled focal mechanism information from in‐slab earthquakes from global subduction zones, in order to check if such an "unbending" signature is present everywhere at depths of 50–300 km. We find that only about half of the investigated regions show such a signature, while the other ones are extensive everywhere. We then compare these findings with global slab shapes and try to constrain what different processes (e.g., stretching of the entire slab due to its weight, bending forces) influence the stress field inside downgoing plates. Key Points: Double seismic zone earthquake mechanisms globally show downdip extensive lower planes, upper planes can be downdip compressive or extensiveSlab bending and unbending estimates derived from slab2 grids show no ubiquitous presence of plate unbending at intermediate depthsIntraslab stress fields are influenced by in‐plane tension, plate bending and megathrust friction; what dominates where is hard to predict [ABSTRACT FROM AUTHOR] |
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