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Structural Configuration of the Otates Fault (Southern Basin and Range Province) and Its Rupture in the 3 May 1887 M_W 7.5 Sonora, Mexico, Earthquake

Instituto de Geología , Universidad Nacional Autónoma de México, Estación Regional del Noroeste, Hermosillo, Sonora, Mexico SuterMax{at}aol.com

Online Material: Color-coded elevation model, satellite image of major Basin and Range normal faults in the study area, color version of geologic map, and additional photographs.

During the great 3 May 1887 Sonoran earthquake (surface-rupture end-to-end length: 101.8 km; M_W 7.5±0.3), an array of three north–south-striking Basin and Range Province faults (from north to south: Pitáycachi, Teras, and Otates) slipped along the western margin of the Sierra Madre Occidental Plateau of northern Mexico. This detailed field survey of the previously undocumented 1887 earthquake rupture zone along the Otates fault includes mapping the rupture scarp and measurements of surface deformation at 25 sites. The surface rupture coincides with the mapped trace of the Otates fault, dips 68° W, and has an end-point-to-end-point length of 18.2 km. The lateral rupture terminations are controlled by cross faults. Striation measurements and the style of rupture indicate extensional dip slip without significant lateral displacement. The average shortening (P) and extension (T) kinematic axes inferred from the striations are similar to published composite focal mechanisms of the microseismicity recorded in the epicentral region of the 1887 earthquake. Based on surface deformation, maximum slip along the Otates fault in the 1887 earthquake was 270 cm, and the average slip was 189 cm. A quadratic best fit to the along-rupture surface offsets indicates an asymmetric distribution with the maximum offset near the northern end of the rupture segment, which can be explained by the mechanical interaction of the Teras and Otates surface-rupture segments through their stress fields. The average recurrence interval of 1887-sized earthquakes on the Otates fault is 30–42 kyr based on the long-term geologic dip-slip rate of the fault (0.06 mm/yr) and its amount of slip in the 1887 earthquake.