North–south-striking and west-dipping Basin and Range province normal faults form the western edge of the Sierra Madre Occidental plateau in northeastern Sonora. These faults and associated half-grabens extend over a distance of more than 300 km between the San Bernardino basin in the north and the Sahuaripa basin in the south. An earthquake in 1887 ruptured three neighboring segments of this major fault zone. Our field mapping in this region indicates that the surface rupture of the 1887 earthquake extends farther to the south and is considerably longer (101.4 km end-to-end length) than previously reported. A compilation of the seismicity in the epicentral region of the 1887 earthquake shows the epicenters to be distributed in well-defined clusters at the northern end of the 1887 surface rupture, in the step-overs between the three rupture segments, on a neighboring fault in the west (Fronteras fault), and on fault segments farther south along the same fault zone (Granados region). The distribution of seismicity correlates well with calculated changes in Coulomb failure stress resulting from the 1887 earthquake.

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