The 1887 Sonoran earthquake, in northeastern Sonora, Mexico, has an 80-km rupture length, with an average 3-m displacement, and an assumed 16-km depth of rupture. This corresponds to a seismic moment of 1.27 × 1027 dyne-cm and an Ms of 7.4. Seven to ten portable seismographs with an average spacing of 10 km were operated in a 2800-km2 region surrounding the 1887 fault scarp. Thirty-three earthquakes were detected during 30 days of usable recording. For all events, −1.3 ≦ M < 2. The most accurate locations (horizontal error ≦ 5 km) define a west-dipping, normal, range-bounding fault. All events occurred within 15 km of the surface. The name Pitaycachi fault (after the most prominent peak of the horst block) is proposed for this zone of active faulting. Seismicity along the zone terminates abruptly 13 km north of the fault scarp and extends to the south for at least 32 km beyond the southern. A composite focal mechanism from events at the southern end of the 1887 scarp indicates a normal fault striking N35°E and dipping 72° to the northwest.