Global compilation of double seismic zones and their dependence on the intraslab stress field

Double seismic zones (DSZs), parallel planes of intermediate-depth earthquakes inside oceanic slabs, have been observed in a number of subduction zones and may well be a ubiquitous feature of downgoing oceanic plates. Early focal mechanism observations from Japan and Alaska have shown downdip compressive events in the upper and downdip extensive events in the lower plane of the DSZ, which was interpreted as a signature of plate unbending at these depths. Such a pattern of compressive over extensive events has become a hallmark of DSZ seismicity, and some models of DSZ seismogenesis explicitely rely on an unbending-dominated intraslab stress field as a mechanism for deep slab hydration.

In this 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, as in NE Japan, is only observed at about half of the DSZ locations around the globe. The occurrence of extensive mechanisms across both planes accounts for most other regions, whereas a “bending signature” of extensive over compressive events is not widely observed at all. To obtain an independent estimate of the (un)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 prevalence of slab unbending at intermediate depths, and the occurrence of DSZ seismicity does not appear to be limited to regions of slab unbending. Taking high-resolution focal mechanism information from the Northern Chile subduction zone as an example, we conclude that the intraslab stress field in subduction zones is primarily a superposition of (un)bending stresses and downdip extensive in-plane stresses. Depending on the sign (bending or unbending) and the relative contributions of these two principal stresses, an unbending signature as in NE Japan or a purely extensive pattern of focal mechanisms as in Northern Chile can emerge. We also consider possible additional contributing stresses that may further modify the intraslab stress field, such as friction along the plate interface and volume loss due to metamorphic phase changes.