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.