Fig. 2: SZI forcing endmembers and SZI types.

The two endmembers characterising SZI indicate the dominant forcing to be either—but never exclusively—a vertical (i.e., some combination of plate buoyancy force, the force from any surface load, and vertical mantle-flow force), or b horizontal (i.e., some combination of tectonic force and horizontal mantle-flow force). All known SZI events can further be grouped into one of the three types, c Newly destructive (a subduction fault establishing from an intact-plate portion or some sort of non-subduction-related plate weakness), d Episodic subduction (a subduction fault establishing at the same location following a previous, yet terminated subduction zone with the same polarity), and e Polarity reversal (formation of a new subduction fault with opposite polarity to the fault of the pre-existing, terminating subduction zone).