Instrumental Evidence of the Seismic Swarm of July–September 1912 Within the City of Guadalajara, Mexico

Seismograms from a seismic station installed in the city of Guadalajara during the intense and unusual seismic swarm that occurred from 8 May to late September were recently recovered in the national historical seismograms archive of the Mexican Seismological Service. These seismograms show earthquakes recorded in August and early September 1912 by two Wiechert mechanical instruments installed with the purpose of observing this unusual sequence and as one of the permanent seismic stations of the budding Mexican Seismic Network. The earthquakes recorded in these seismograms range from barely visible signals to larger magnitude earthquakes that were reported as having been felt as moderate or strong. In some cases, it is possible to identify the arrival of the Pand S phases. The difference in arrival times suggests that the epicenter for some of these recorded events was approximately 8–12 km from the site where the Wiechert instruments were installed. This epicentral distance from the seismic station is consistent with the spatial distribution of the more intense shaking observed during the swarm. The shape and duration of the seismic traces also confirm that these events correspond to shallow earthquakes located near the seismic station. A coda-wave duration relation calibrated for crustal earthquakes in the Mexico City basin was used to estimate the magnitude of selected events. The largest estimated magnitude, interpreted as a conservative lower-bound value, was Mw 4.1, for the largest event recorded on 31 August 1912. In a classification of the intensity of shaking for earthquakes of the swarm reported instrumentally, this earthquake is classified as strong. Thus, those events classified as very strong and causing the more intense shaking most probably have a larger magnitude, compatible with the lower bound of the magnitude range inferred from macroseismic data for the largest earthquakes of the sequence.