Tomographic Imaging of the Three-Dimensional P-wave Velocity Structure Beneath Costa Rica: Constraints on Subduction Processes

Subduction of the Cocos plate beneath Central America and its associated processes cause a high seismic activity in Costa Rica. Two separate seismic networks (OVSICORI-UNA and RSN) exist in Costa Rica to routinely locate earthquakes associated with the tectonic activity. We merged these two independent datasets to achieve a high-quality and consistent earthquake data set for Costa Rica. From the merged dataset we selected 3790 well locatable events with a GAP less than 180 degrees and more than 6 P-wave observations to invert for three-dimensional P-wave velocity structure and hypocenter locations. Resolution of our tomographic model is good throughout most of Costa Rica down to a depth of 60-70 km. At greater depth, good resolution is limited to a region parallel to the subducting slab and the overlying mantle wedge. Except for southern Costa Rica, we identify the subducting Cocos plate as a dipping high-velocity feature consistent with relatively cold and mafic oceanic crust being subducted. In southern Costa Rica subducting of the extremely thickened and young oceanic crust of the Cocos Ridge, causes lower velocities. Based on seismicity and seismic velocities we are able to trace the subducting Cocos plate down to a depth of 150 km beneath northern Costa Rica. Seismicity ceases beneath central Costa Rica and we loose the trace of the subducting Cocos plate at a depth of 80 km. A zone of very low seismic P-wave velocities and high seismic activity is found at 60 to 80 km beneath Central Costa Rica. We interpret these low velocities and the abundance of seismicity at this depth as an indication of ongoing dehydration and subsequent release of fluids in the subducting slab. A zone of high seismic P-wave velocities, possibly representing subducted cold material, separates the mantle wedge underlying northern and central Costa Rica. The position of this zone coincides with a change in the dip of the subducting Cocos plate with a steep subduction angle to the north and a shallowed subduction angle to the south. It also coincides with the transition in the geochemical signature of arc lavas showing a more depleted mantle source in northern Costa Rica and southern Nicaragua and a more enriched mantle source in central Costa Rica. We interpret our results as more evidence that two different kinds of mantle material underlie Costa Rica separated by a zone of colder subducted material.