Seismotectonics of the Querétaro Region (Central Mexico) and the 1934 MI 4.8 Earthquake North of Celaya

The Querétaro region (central Mexico) is located in the trans-Mexican volcanic belt, an active volcanic arc related to the subduction of oceanic plates along the Pacific margin of Mexico. It is characterized by north–south-striking normal faults of the southern Basin and Range Province, up to 40 km long and with morphologically pronounced scarps, such as the San Miguel de Allende fault and the faults forming the Querétaro graben. These faults are located directly north of a major regional-scale system of east–west striking, seismically active intra-arc normal faults that are oriented parallel to the axis of the volcanic arc. Where the two orthogonal normal fault systems interfere, the outcrop-scale observations show that the east–west intra-arc fault system overprints the Basin and Range Province structures. Here we document a 1934 earthquake in a region previously not known for seismic activity. Our study is mostly based on an unpublished contemporary dossier preserved at Archivo Histórico del Instituto de Geología de la Universidad Nacional Autónoma de México, a recently inventoried archive that also preserves several unpublished macroseismic and instrumental studies of major Mexican subduction zone earthquakes between 1911 and 1954. A mainshock–aftershock sequence that initiated 14 July 1934 is documented by instrumental recordings at the Tacubaya observatory and by macroseismic observations at ten population centers, ranging in intensity between five and seven on the modified Mercalli scale. Based on the size of the damage area, the intensity magnitude of the mainshock is estimated at 4.8 ± 0.5. Based on the intensity distribution, the epicenter was located in the Laja River valley north-northeast of the town of Celaya, in the south-southwestern extrapolated continuation of the San Miguel de Allende normal fault scarp, which suggests that this fault extends to the epicentral region of the 1934 earthquake and is characterized by recurrent Quaternary tectonic activity.