Central Mexico Intermediate-Depth Seismicity and its Possible Relationship to the Regional Geotectonic Structure

Although all convergent regions have the same basic characteristics, they are highly variable features. Each controlled by the type of crustal material involved and the tectonic setting. The development of the Plate Tectonics theory give us a general framework to understand the relationship between earthquakes and subduction process. Details, however, have to be treated case by case since in many regions of the Earth they are still unknown. Explanation to the unusual position of the Trans-Mexican Volcanic Belt with respect to the Middle America Trench, and its origin itself, is one of those open questions. The present report provides an overview of the intermediate-depth seismic activity throughout Central Mexico. The discussion is based on the characteristics of damaging earthquakes of moderate magnitude that have occurred in the past 76 years in a narrow belt transversal to the Trench axis at 18øª N, in an approximately E-W direction. Even though is not an easy task to link the origin of intermediate-depth earthquakes to a brittle fracture or frictional sliding, the lineament of foci and the similarity of the focal mechanisms suggest that intermediate-depth earthquakes in Central Mexico occur along a pre-existing deep-seated normal fault system, probably in the upper part of the subducting Cocos plate. The main faults as suggested by the data, would run parallel to the southern border of the Trans-Mexican Volcanic Belt. Moreover, I speculate that these pre-existing faults are, probably, an extension of the Rivera Fracture zone observed to the west. The existence of a deep fracture system would facilitate the uprising of Mantle material and would explain why the active volcanoes chain of Central Mexico runs in an E-W direction instead of parallel to the coast as in other subduction regions of the world. It would also help to understand the genesis of intermediate-depth earthquakes in Mexico.