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