The Cauca region is the only documented site in the world where extensive intermediate depth seismicity occurs over multiple decades above a subducting slab. Here, the subducting Nazca oceanic plate descends beneath a mosaic of terranes derived from the Caribbean plate and accreted to continental South America from the Cretaceous to the present. Through relative relocation of >6,000 earthquakes from 2010 to 2019 we show that seismic activity within the Nazca slab is concentrated immediately inboard of the most recently accreted terrane (the Panamá-Chocó Block) and that supraslab seismicity is occurring within the subducted continuation of this terrane. The deepest extent of this seismicity occurs only within the Colombian forearc and a gap in the active volcanic arc, indicating that the continuation of this terrane at depth has perturbed the thermal structure of the subduction zone. This perturbation is likely what permits brittle failure to occur above the slab. Within the context of the long-term evolution of the Colombian subduction zone, this seismicity must represent either a transient phenomenon as the continuation of the Panamá-Chocó Block warms and becomes incorporated into the convecting mantle wedge or a site where fluids released by the subducting Nazca slab have been focused, promoting hydrofracture. While additional tests are necessary to distinguish between these possibilities, seismicity within the Nazca slab is most intense directly beneath the locations where supraslab seismicity is concentrated, consistent with hydrofracture due to fluids escaping the slab. Similar transient processes may have affected terrane accretion in the geologic past.