GEOMETRY OF SUBDUCTION AND DEPTH OF THE SEISMOGENIC ZONE IN THE GUERRERO GAP, MEXICO

THE western coast of Guerrero in southern Mexico has been identified as a seismic gap on the Middle American Trench in which no large earthquakes have occurred at least since 1908. It has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake. A permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region. The seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast. The resulting geometry of the subduction zone shows that the Cocos plate dips at a shallow angle beneath the North American plate to a depth of ∼40 km; from there the subducted slab is bent upward, following a subhorizontal trajectory extending inland at a depth of ∼50 km. This plate geometry is reminiscent of that found in Peru and central Argentina, two other regions where a young oceanic plate is being subducted. In Mexico, however, the slab underplates an overriding plate which is only half as thick as that observed in South America. A possible oceanic origin of the allochtonous terranes comprising southern Mexico may explain the presence of the anomalously thin lithosphère in this region. © 1990 Nature Publishing Group.