We have examined teleseismic earthquake locations reported by the International Seismological Centre (ISC) for the Middle America region and selected 220 as the most reliable. These hypocenters and other data are used to delineate the deep structure of the subducted Cocos Plate. The results indicate that the subducted plate consists of three major segments: Segment I extends from the Panama Fracture Zone to the Nicoya Peninsula. The structure of this segment is poorly defined. Segment II is the largest and best‐defined segment. This segment consists of two parts, IIA and IIB. Part IIA extends from the Nicoya Peninsula to western Guatemala and is very well defined and continuous in structure. Its strike follows the curvature of the trench and dips at about 60°. Part IIB extends from western Guatemala to Orizaba, Mexico. The dip of this part of the segment decreases slightly toward the northwest, and its strike is more northward than that of the trench. Segment III extends from Orizaba to the Rivera Fracture Zone, and is not well defined due to a lack of earthquake activity beneath about 100 km. Its orientation differs markedly from segment II and strikes somewhat more westward than the trench. Between parts IIA and IIB of segment II the subducted plate seems to be continuous, bending smoothly to accommodate the change in geometry. Local network data from Costa Rica suggest there may be a tear between segments I and II. Between segments II and III there is a gap in the hypocenters which makes it difficult to define the boundary. The change in geometry between these two segments indicates that there may be a tear, and two strike‐slip focal mechanisms in the region support this conclusion. We find no convincing evidence supporting the existence of segments smaller than the three described above. If there is smaller‐scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths. There is no evidence for any major tear in the subducted plate associated directly with either the Tehuantepec Ridge or the Orozco Fracture zone. The shallow subduction at the northwestern end of segment II may be related to the bouyancy of the Tehuantepec Ridge. The Cocos Ridge is probably directly responsible for the change in geometry between segments I and II and may even be slowing or stopping subduction in segment I. The structure of the subducted plate in segment II and the changes in the character of volcanism along the arc can be related to the relative motion of the North American and Caribbean Plates. The present geometry of part IIB of segment II is more consistent with the probable configuration of the trench about 7 Ma ago than with the present configuration, indicating that the North American plate is overriding the subduction zone.
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The triple junction of the North America, Cocos, and Caribbean plates is ambiguously defined, mainly because the North America-Caribbean plate boundary does not clearly continue beyond its known surface trace (the Motagua fault zone) in western Guatemala to intersect the Middle America trench. Well-located regional shallow earthquakes (h_<70 km) show that there is no intermediate or large-magnitude seismic activity associated with a presumed extension of the North America-Caribbean plate boundary to the west, beyond its well-defined surface trace. There is, however, a clear zone of shallow seismic activity from the western section of the fault system through southern Mexico. Fault plane solutions for these events indicate a left-lateral strike-slip displacement, which is in good agreement with surface faulting. We suggest that these strike-slip faults, together with the Salina Cruz fault in the isthmus of Tehuantepec, mark the boundaries of a broad zone of deformation in southern Mexico and northern central America which takes up the interactions of the three plates. In this sense, no single point constitutes the triple junction. The geologic record suggests that the Motagua fault zone developed because the westernmost portion of the Caribbean plate became locked against North America.
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