Seismotectonics of North-Central Mexico (Basin and Range Province) and the 3 April 1925 MI 4.9 Chalchihuites, Zacatecas, Earthquake

Abstract North-central Mexico, which belongs to the Basin and Range Province, is characterized by numerous, generally north–south-striking map-scale faults that are concentrated toward the Gulf of California and along the western margin of the Sierra Madre Oriental physiographic province. The focal mechanisms of recent earthquakes in this region all indicate normal faulting at mid-to-lower crustal depth on approximately north–south-striking planes. Focal mechanisms, drillhole elongations, linear arrays of Quaternary volcanic vents, and the inversion of fault structural data indicate a normal-fault-type tectonic stress field with a generally east–west-oriented least horizontal stress. A previously undocumented 3 April 1925 earthquake within this province, in the border region of the Mexican States Durango and Zacatecas, caused moderate-to-severe construction damage in the historical mining town of Chalchihuites. The macroseismic observations of the mainshock, obtained for 13 population centers, range in intensity between two and eight on the modified Mercalli intensity scale. Estimates of the intensity-based magnitude of the mainshock range between 4.8 and 5.1, with an average value of 4.9 ± 0.5. Site effects likely amplified the ground shaking at Chalchihuites and Súchil, which are located on alluvial fan deposits. The town of Chalchihuites is located in a half-graben at the base of the mountain range forming the eastern half-graben shoulder. The half-graben is delimited by the north-northwest–south-southeast-striking Chalchihuites fault with a maximum topographic relief of 770 m, which displaces Cretaceous sediments against the Neogene basin fill. The local historical seismicity, the fresh morphology of the Chalchihuites fault escarpment, and the thermal springs at the base of the scarp mentioned in historical physiographic descriptions suggest that the Chalchihuites fault is active. The macroseismic intensity distribution of the 1925 mainshock and the numerous foreshocks felt locally at Chalchihuites suggest that the Chalchihuites fault was its source.