Abstract The residents of the Guadalajara, Mexico metropolitan area awakened on 8 May 1912 to a strong earthquake. In the next two months, ∼1500 shocks followed this first event. Two additional clusters of microearthquakes occurred in July and September. Although no significant structural damage was reported, several churches, public buildings, and private dwellings in the cities of Guadalajara and Zapopan suffered damage. Based on the macroseismic data, the intensity of the larger earthquakes was modified Mercalli intensity (MMI) VI–VII. The area that strongly felt the earthquakes and where the damage was concentrated is contained within a radius of ∼5 km. We assume that the swarm took place within this circle. Based on the relations of MMI versus peak ground acceleration (PGA), it is estimated that the PGA during the swarm was ∼190 gals. The magnitude of the largest earthquakes in the swarm was initially estimated as Mw 5.7, using a published PGA-to-Mw relation. However, this relation was based on earthquakes located 20 km away from downtown Guadalajara. Considering that the distance of the swarm from the city was ∼5 km, the magnitude of the largest events in the swarm was recalibrated as Mw 4.7. A second estimate of the magnitude was based on ground-motion models to calculate the PGA resulting from an earthquake located 5 km away, at a depth of 1 km. The resulting magnitude is Mw 5.3 ± 0.5. Thus, we suggest that the largest earthquakes of the swarm had an approximate magnitude of Mw 5.0. The location of the swarm suggests the presence of active faults underlying the metropolitan area of Guadalajara, where no seismicity has been recorded. This should be carefully considered to assess the seismic hazard of this important city of over 5 million people.