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.

The Cauca Cluster of seismicity in western Colombia is the most extensive and persistent collection of intermediate (>60 km) depth earthquakes that cannot be easily associated with a subducting slab. The cluster stretches over an area of ~390 km by ~250 km from 2.5°N to 6°N and from 75.5°W to 77.75°W in a wedge-like zone of seismicity thickening from ~25 km in the west to ~125 km in the east. Prior tomographic results suggest that the lower edge of this feature is contained within the subducting Nazca plate’s oceanic lithosphere and corresponds to the Wadati-Benioff zone (WBZ) seen in most slabs, however this cannot explain the full thickness of the cluster. Large earthquakes within the cluster have been reported since at least the 1960s-70s and a great number of smaller events have been reported since the establishment of the Red Sismológica Nacional de Colombia (RSNC)’s regional catalog in 1993. Here, we relocate more than 6,700 events from the RSNC’s catalog beginning in 2010 and extending for ~10 years to dissect the Cauca Cluster’s structure and relationship to seismicity below 10 km depth. We find that while 40% of this seismicity can be associated with the Nazca plate’s WBZ, 35% occurs as features completely or partially within the overlying forearc mantle. These features include: 1) three focused centers of seismicity at ~90, ~100, and ~120 km depth extending ~30 km perpendicular to the WBZ; 2) a feature dipping at an angle shallower than the WBZ between 10 km and ~75 km depth; and 3) a diffuse zone of seismicity extending from the WBZ to ~10 km depth. None of these features extend beneath the active volcanic arc, and as such are limited to the stagnant corner of the mantle wedge. We find that these features are also limited to an area affected by the late Miocene accretion of the Panama-Choco terrane, the top of which we associate with the dipping feature between 10 and 75 km depth. This accretion has likely cooled the deep forearc to a point that allows for seismicity. The occurrence of these mantle wedge earthquakes in an immobile part of the subduction system suggests they are not produced directly by dehydration. They may instead be a result of fracture induced by the upward movement of fluids from the slab or of self-localizing thermal shear runaway triggered by reheating of the cooled forearc along its arc-ward edge.