La Nacion fault system is a series of moderate- to high-angle normal faults striking north roughly parallel to the coast line and is traceable by surface features for 33 km north of the Mexico–United States border. The fault system occurs in locally folded Tertiary and Quaternary sedimentary rocks. Vertical offset of Pleistocene sediments is as much as 85 m on a given fault branch, and for the fault system may be as much as 120 m. Geophysical evidence suggests the total offset for the fault system is 500 m. Offset of alluvium has been proved at two localities, and unexplained seismic activity may be associated with La Nacion fault system.

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