Assessing the causes of slab seismicity through dehydration, temperature and strain: A global approach

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The majority of Earth's seismicity is focused at subduction zones, either in the shallow thrust zone or as intermediate-depth (~50 to 300 km depth) or deep (to ~600 km depth) slab seismicity. Intermediate-depth seismicity is commonly thought to be caused by dehydration embrittlement, in which the dehydration of hydrous minerals in the subducted crust or mantle increases the pore fluid pressure in low-permeability rocks, allowing rocks to behave brittlely; although recent studies have called this hypothesis into question due to a lack up supporting laboratory observations. Double seismic zones are obvious in some subducting slabs (e.g., Honshu), but their presence in other slabs is less clear due to the relatively large errors in hypocentral locations relative to the spacing between the two layers of seismicity, which varies among different slabs. As the lower layer of seismicity is thought to be caused by the dehydration of serpentine in the slab mantle, having a limited view of the global prevalence of double seismic zones limits the constraints on slab mantle serpentinization and dehydration. In this project, we will relocate teleseismic earthquakes within 54 small arc sections and compare these seismicity distributions with pre-existing 2D thermal and mineralogical models, with a focus on the locations of greatest dehydration of slab minerals. The double-difference relocation technique teletomoDD will be used with nested regional-global 3D velocity models, which will significantly reduce scatter and improve the absolute locations of earthquake hypocenters in comparison to existing global catalogs. By better understanding intermediate-depth seismicity and its relation to thermal and petrological models, we can gain insight to the seismogenic processes in slabs and contribute to the understanding of six key questions pertaining to subduction zones:1) What phenomenon is responsible for the majority of intermediate-depth seismicity?2) Where are double seismic zones observable?3) In double seismic zones, what is responsible for the lower layer of seismicity?4) What controls the maximum depth of slab seismicity?5) Is the distribution of slab seismicity correlated with changes in volcanism?6) Is there a relationship between slab seismicity and changes in deep plate coupling?The majority of Earth's seismicity is focused at subduction zones, where cold, dense oceanic crust and mantle descend beneath a more buoyant plate, with the ability of producing large earthquakes and volcanism. Seismicity occurs either in the shallow thrust zone, or as intermediate-depth (~50 to 300 km depth) or deep (to ~600 km depth) slab seismicity. The mechanisms that cause shallow earthquakes are relatively well understood, whereas the cause of intermediate-depth seismicity is less certain. This is due in part to the expected ductile behavior of Earth materials at high pressures and temperatures, conditions that are difficult to reproduce in the laboratory. However, by combining numerical models of temperature and mineralogical reactions within subducting crust and mantle to observations of seismicity, we can better understand what slab conditions are most closely associated with observed seismicity. In this project, we will relocate intermediate-depth earthquakes using techniques that more accurately reproduce the paths of seismic wave propagation and reduce errors in earthquake locations from existing global seismicity catalogs. We will then comprehensively compare the distributions of slab seismicity with existing temperature and mineralogical models to identify which particular reactions contribute most to the creation of potentially large intermediate-depth earthquakes.