Structural configuration of the Teras fault (southern Basin and Range Province) and its rupture in the 3 May 1887 MW 7.5 Sonora, Mexico earthquake

Abstract

During the great 3 May 1887 Sonoran earthquake (surface rupture end-to-end length: 101.8 km; MW = 7.5±0.3), an array of three north-south striking Basin and Range Province faults (from north to south Pitáycachi, Teras, and Otates) snapped along the western margin of the Sierra Madre Occidental plateau. My detailed field survey of the Teras fault and the 1887 earthquake rupture zone along this fault included mapping the rupture scarp and measuring surface deformation at 27 sites.

The Teras fault is ~20 km long, strikes N12°E, and has an average dip of 62°W; its maximum throw is >1,640 m and its long-term slip rate >0.08 mm/yr. The fault is structurally simple; it is not internally segmented along strike and does not branch. Striation measurements and the style of faulting indicate extensional dip-slip without significant lateral displacement. In the north, a 2.5-km wide, unbreached right step-over separates the Pitáycachi and Teras faults, whereas the southern limit of the Teras fault is a partly breached relay ramp, where the fault trace shows a 60° bend and jogs to the west. Here, the displacement of the Teras fault is transferred onto the Otates fault and the normal faults bounding the Iglesitas horst. The 1887 surface rupture along the Teras fault (end-to-end length 19.9 km) generally coincides with the mapped trace of the Teras Basin and Range Province fault. Based on 27 measurements, the maximum surface offset is 184 cm and the mean offset 112 cm. The along-rupture surface offset distribution is asymmetric, with the maximum near the southern end of the segment. This suggests that the Teras and Otates segments could be part of a single continuous 1887 rupture that stepped across the structurally complex basement ridge between them. A rough estimate of the average recurrence interval of 1887-size earthquakes on the Teras fault can be obtained from the average slip rate since 23 Ma and the amount of slip on this segment during the 1887 earthquake; the resulting values are 15 to 26 kyr.