Extensional failure at the tip of a weak slab under slab pull -- the 2023 Mw 6.4 Zacualpa, Guatemala, earthquake

The 2023 Mw 6.4 Zacualpa earthquake is the deepest recorded major (Mw > 6) earthquake to have occurred in the Cocos slab beneath Central America, at a depth of ~ 255 km. Here, we refine the source parameters of both the Zacualpa earthquake, and the only other event at comparable depths (the 1997 Mw 5.5 Santa Catarina Mita earthquake), confirming both their exceptional depth within the downgoing slab, and their consistent down-dip extensional mechanism. That the Cocos slab remains capable of hosting major intraslab earthquakes, with mechanisms consistent with down-dip extension, near, or at, the tip of the contiguous slab, suggests that the slab itself is weak, such that the minimal stresses derived from supporting the negative buoyancy of the short section of slab down-dip from this earthquake are still sufficient to lead to brittle failure of the slab.