THE RUPTURE OF THE PITAYCACHI FAULT IN THE 1887 MW 7.5 SONORA, MEXICO EARTHQUAKE (SOUTHERN BASIN-AND-RANGE PROVINCE)

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 sequentially along the western margin of the Sierra Madre Occidental Plateau. This detailed field survey of the 1887 earthquake rupture zone along the Pitáycachi fault includes mapping the rupture scarp and measurements of surface deformation. The surface rupture has an endpoint-to-endpoint length of 41.0 km, dips between 60º and 86ºW, and is characterized by E-W extension, perpendicular to the fault trace. The maximum surface offset is 487 cm and the mean offset 260 cm. In the north, the rupture terminates against a major cross fault, whereas in the south, a 2.5-km-wide unbreached right step-over separates the Pitáycachi from the Teras segment. The 1887 along-strike surface offsets are proportional to the one order-of-magnitude larger along-strike offsets of a distinct Pleistocene alluvial fan surface, which suggests that the 1887 rupture dimensions are characteristic for ruptures along the Pitáycachi fault.

Whereas the Teras and Otates segments have the typical 15–20 km length of normal fault segments, the Pitáycachi fault is unusually long because of being a composite of formerly independent fault segments. The former segment boundaries are linked by en échelon rupture scarp arrays with a local horizontal slip component. However, the former segmentation is not expressed in the 1887 along-rupture surface-offset profile, which indicates that the former segments are now linked at depth into a single coherent fault surface.

The Pitáycachi surface rupture shows a well-developed bipolar branching pattern suggesting that the rupture originated in its central part, where the polarity of the rupture bifurcations changes. Most likely the rupture first propagated bilaterally along the Pitáycachi fault, from where the southern rupture front jumped across a step-over to the Teras fault and from there across a major relay zone to the Otates fault. Branching probably resulted from the lateral propagation of the rupture due to the unusual length of the fault, given that the much shorter ruptures of the Otates and Teras segments did not develop branches.