Seismicity and the deformation of subducting slabs

Earthquakes associated with the subduction of tectonic plates into the Earths’ interior occur in four dominant settings: (i) major seismic failure on the interface between the two plates (the subduction “megathrust”); (ii) shallow crustal earthquakes within the overriding plate; (iii) earthquakes within the outer rise region of the downgoing plate, as it bends into the subduction zone; and (iv) earthquakes that occur within the subducting slab as it descends into the mantle. These latter two classes of earthquakes reflect the internal deformation of the downgoing plate, and are the focus for this talk.  

Both of these types of intraslab, or intraplate, earthquakes are principally driven by bending of the plate.  The bending and flexure of subducting plates creates complex stress and strain fields, which are highly variable across short distances, both vertically and laterally.  This leads to complex patterns of intraplate seismicity, which offers our principal insight into the dynamics of the subducting slab.  Beyond this, these intraslab earthquakes offer our main observational constrains on the rheology of the slab, and its rheological evolution over the its initial stages of subduction.  From the potential for fluid infiltration into the subducting plate at the outer rise, to the role that metamorphism and fluid release play in permitting intermediate depth intraslab seismicity, these earthquakes also relate to the chemical evolution of the plate during subduction.  Lastly, although less common, and usually smaller in magnitude, than earthquakes occurring on the subduction interface, intraplate subduction zone earthquakes pose a critical, and often under-appreciated, hazard for the overlying regions, particularly given their potential to trigger secondary hazards in the setting of the supra-subduction arc.

Here, I will consider earthquakes associated with the internal deformation and bending of subducting plates through the initial phases of subduction, from an intact oceanic plate entering the subduction zone to the descent of the plate into the mid-mantle, the controls on their occurrence, and their geodynamic implications.