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

Abstract

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.

Spatiotemporal Variations of Intermediate‐Depth Earthquakes Before and After 2011 Tohoku Earthquake Revealed by a Template Matching Catalog

“…It is well known that the background stress state in our study region is downdip compression in the upper plane and downdip extension in the lower plane (Fujita & Kanamori, 1981; Hasegawa et al., 1978; Sippl et al., 2022). The opposing stress states in the upper plane and the lower plane are generally attributed to slab unbending at the intermediate‐depth range (Engdahl & Scholz, 1977).…”

“…It is well known that the background stress state in our study region is downdip compression in the upper plane and downdip extension in the lower plane (Fujita & Kanamori, 1981;Hasegawa et al, 1978;Sippl et al, 2022).…”

A Comprehensive Stress Drop Map From Trench to Depth in the Northern Chilean Subduction Zone

“…In northern Chile, the plate undergoes strong geometric changes as it bends at shallow depths (<60 km), stops bending below (60-100 km), followed by bending that ends with a strong downward kink (Sippl et al, 2019(Sippl et al, , 2023. The strongest geometric deformation is observed between 100 and 140 km depth (Sippl et al, 2022), where the plate is bent to a significantly steeper subduction angle. As the depth of this bending process varies laterally, an increasing number of high stress drop events is gathered to calculate their median, which could be the cause of the continuous increase in the median stress drop that would otherwise be more abrupt.…”

Plate bending earthquakes and the strength distribution of the lithosphere

“…Without this transformation, the relationship between slab geometry,and the components of the strain rate and stress tensors, is ambiguous, because the angle of slab WRT to the basis vectors changes along the slab. This can a result, for instance, in the misinterpretation of a change in slab stress state at intermediate depths ('deep bending') (e.g.,Sippl et al, 2022).…”