Global Constraints on Intermediate‐Depth Intraslab Stresses From Slab Geometries and Mechanisms of Double Seismic Zone Earthquakes

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2022-09, Vol.23 (9), p.n/a
• Main Authors: Sippl, C., Dielforder, A., John, T., Schmalholz, S. M.
• Format: Article
• Language: English
• Subjects: Bending stresses; Compression; Deformation; Depth; double seismic zone; Earthquakes; intraslab stress field; in‐plane stresses; Lithosphere; Offshore; Seismic activity; Seismic zones; Seismicity; Slabs; Subduction; subduction zone; Subduction zones
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010498

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