Is Southeastern Costa Rica a Convergent Double Subduction Zone?

We believe the answer to this question is yes. Southeastern Costa Rica is a highly complicated region, where the Caribbean, Cocos, and Panama plates interact. On the Pacific side, the Cocos Plate together with the Cocos Ridge subducts underneath the Panama Microplate where there is an exposed forearc, a lack of deep (70 km) seismicity, and a ~200-km gap in the Quaternary volcanic arc, which is also being uplifted. On the Caribbean side, the North Panama Deformed Belt (NPDB) has been described as a system of reverse faults marking the northern limit of the Panama Microplate. In 1991, this region was the setting of the latest large earthquake (Mw 7.7) between the Caribbean Plate and the Panama Microplate. Despite its high seismicity and advances in seismic network coverage, until now this area lacked detailed studies of seismic velocities. Using the VELEST and SIMULPS packages, we derive the one-dimensional and three-dimensional P-wave velocity models for Southeastern Costa Rica by simultaneously inverting 35,964 travel times from 1208 earthquakes recorded between 1998 and 2020 by the National Seismological Network (RSN) of the University of Costa Rica. This new tomography shows a ~15-km thick band of low velocities dipping from the Caribbean coast towards the southwest down to depths of 50 km below the extinct magmatic arc. In this image, the NPDB coincides with a zone of low velocities down to depths of 10 km, which corresponds to the region that has been described in previous works as highly faulted and folded, resembling an accretional prism of a subduction zone. The deeper part of the low-velocity anomaly includes the hypocenter of the 1991 earthquake and seismicity down to 30 km. We explain this configuration as the subduction of the Caribbean Plate under the Panama Microplate and the 1991 earthquake as an interplate seismogenic zone event. We estimate the Caribbean subduction onset at a minimum age of 14.5 Myr. The interpretation presented here implies that southeastern Costa Rica is a convergent double subduction zone, in which the subducted slabs of the Cocos and Caribbean plates approach each other below the magmatic arc. This geometry provides a new framework for understanding the complex tectonics of the region and its evolution, as well as the origin of the major earthquakes and tsunamis in the Caribbean of Costa Rica.