Exploring the connection between intermediate-depth seismicity, slab hydration, and dehydration

The dehydration of hydrous minerals has commonly been cited as the cause of intermediate-depth seismicity in subducted crust and mantle, through the process known as dehydration embrittlement. However, recent laboratory and empirical studies have called both the mechanism and seismological observation of this phenomenon into question. In order to assess the global relationship between seismicity, the presence of hydrous and dehydrating minerals, and the thermal state of slabs, we perform double-difference earthquake relocation of earthquakes at the majority of Earth's subduction zones, which reduces the scatter and improves the accuracy of the distributions of slab seismicity. The double-difference relocations are systematically calculated for each subduction zone in a version of the algorithm tomoDD that has been modified to include absolute and differential catalog P, S, and depth phase arrival times from local and teleseismic stations, as well as a three-dimensional global velocity model. Preliminary relocations demonstrate shifts of up to 15 km due to the use of a three-dimensional global velocity model. These relocations also illuminate various types of slab structures, including a range of slab morphologies, potential double seismic zones, and evidence of fault zones within slabs. At each subduction zone, these distributions are compared to previously published two-dimensional thermal and mineralogical models that have been calculated for that particular slab. The findings of these comparisons will be used to develop a set of slab conditions that describe where intermediate-depth seismicity is possible (and observed) at subduction zones.