Abstract

We study local and regional body-wave arrival times from several seismic networks to better define the active regional fault pattern in the epicentral region of the 3 May 1887 Mw 7.5 Sonora, Mexico (southern Basin and Range Province) earthquake. We determine hypocenter coordinates of earthquakes that originated between 2003 and 2007 from arrival times recorded by the local network RESNES (Red Sísmica del Noreste de Sonora) and stations of the Network of Autonomously Recording Seismographs (NARS)–Baja array. For events between April and December 2007, we also incorporated arrival times from USArray stations located within 150 km of the United States–Mexico border. We first obtained preliminary earthquake locations with the Hypoinverse program (Klein, 2002) and then relocated these initial hypocenter coordinates with the source-specific station term (SSST) method (Lin and Shearer, 2005). Most relocated epicenters cluster in the upper crust near the faults that ruptured during the 1887 earthquake and can be interpreted to be part of its long-lasting series of aftershocks. The region of aftershock activity extends, along the same fault zone, 40–50 km south of the documented southern tip of the 1887 rupture and includes faults in the epicentral region of the 17 May 1913 (Imax VIII, MI 5.0–0.4) and 18 December 1923 (Imax IX, MI 5.7–0.4) Granados–Huásabas, Sonora, earthquakes, which themselves are likely to be aftershocks of the 1887 event. The long aftershock duration can be explained by the unusually large magnitude of the mainshock and by the low slip rates and long mainshock recurrence times of the faults that ruptured in 1887.

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