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High-resolution seismicity data from Costa Rica and southern Nicaragua have been used to image the geometry of the Wadati-Benioff zone under southern Central America. The data include 9,514 events with computed horizontal and vertical errors smaller than 4 and 5 km respectively.

We found that under the Nicaragua-Costa Rica border the Wadati-Benioff zone contorts (from steep to shallow dip angles, northwest to southeast) but does not show evidence of a brittle tear, as postulated by others. Further to the southeast—northeast from Puerto Caldera—the Wadati-Benioff zone does, however, show a tear (the Quesada Sharp Contortion) at intermediate depths (h > 70 km). Overall the dip angle of the Wadati-Benioff zone decreases from 84° under Nicaragua to 60° under central Costa Rica. The maximum depth of intraplate Wadati-Benioff zone earthquakes also decreases from northwest (-200 km under Nicaragua) to southeast (-125 km under central Costa Rica). In southern Costa Rica, east of 83°55′W, we Find no evidence of the Wadati-Benioff zone below 50 km.

This geometry of the Wadati-Benioff zone and other tectonic features related to the subduction of the Cocos plate beneath the Caribbean plate have been integrated into a model that correlates them with along-trench variations in age of the subducted Cocos plate. These tectonic features are (1) the shallowing of the Middle America Trench bathymetry from northwest to southeast, (2) differences in coupling between Cocos and Caribbean plates, (3) the termination of the Central America Volcanic Chain in central Costa Rica, and (4) distinct stress field variations on the overriding Caribbean plate.

The subduction of the Cocos Ridge under southern Costa Rica is partially responsible for some of these features (shallowing of the Middle America Trench in southern Costa Rica) and for the high uplift rates of the outer arc. However, since the presence of the Panama Fracture Zone limits the subducted extension of the Cocos Ridge to less than 100 km from the trench, we propose that the overall geometry of the Wadati-Benioff zone is controlled by abrupt along-trench changes in the age of the subducted Cocos plate.

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